IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
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Spatial/Spectral Endmember Extraction by
Multidimensional Morphological Operations
Antonio Plaza, Pablo Martínez, Rosa Pérez, and Javier Plaza
Abstract—Spectral mixture analysis provides an efficient
mechanism for the interpretation and classification of remotely
sensed multidimensional imagery. It aims to identify a set of
reference signatures (also known as endmembers) that can be used
to model the reflectance spectrum at each pixel of the original
image. Thus, the modeling is carried out as a linear combination
of a finite number of ground components. Although spectral
mixture models have proved to be appropriate for the purpose
of large hyperspectral dataset subpixel analysis, few methods
are available in the literature for the extraction of appropriate
endmembers in spectral unmixing. Most approaches have been
designed from a spectroscopic viewpoint and, thus, tend to neglect
the existing spatial correlation between pixels. This paper presents
a new automated method that performs unsupervised pixel purity
determination and endmember extraction from multidimensional
datasets; this is achieved by using both spatial and spectral
information in a combined manner. The method is based on
mathematical morphology, a classic image processing technique
that can be applied to the spectral domain while being able to
keep its spatial characteristics. The proposed methodology is
evaluated through a specifically designed framework that uses
both simulated and real hyperspectral data.
near future, the use of these types of sensors on satellite platforms will produce a nearly continual stream of high-dimensional data, and this expected high data volume will require fast
and efficient means for storage, transmission, and analysis [9],
[10].
Linear spectral unmixing [11]–[13] is one of the most important approaches for the analysis and classification of multi/hyperspectral datasets. This approach involves two steps: to find
spectrally unique signatures of pure ground components (usually referred to as endmembers [14]) and to express individual
pixels as a linear combination of endmembers [15]. Let
be a spectrum of values obtained at the sensor for a certain pixel
in a multispectral image. This
with spatial coordinates
spectrum can be considered as an -dimensional ( -D) vector
(where is the number of spectral bands) and may be modeled
in terms of a linear combination of several endmember vectors
,
according to the equations and constraints [16]
(1)
Index Terms—Automated endmember extraction, mathematical
morphology, morphological eccentricity index, multidimensional
analysis, spatial/spectral integration, spectral mixture model.
(2)
I. INTRODUCTION
I
MAGING spectroscopy, also known as hyperspectral remote
sensing, allows a sensor on a moving platform to gather reflected radiation from a ground target so that a detector system
consisting of CCD devices can record a great deal (typically
tens) of spectral channels simultaneously. With such detail, the
ability to detect and identify individual materials or classes is
expected to be greatly improved [1].
During the last several years, a great deal of new airborne
and spaceborne sensors have been improved for hyperspectral
remote sensing applications. A chief hyperspectral sensor is the
NASA/JPL Airborne Visible and Infra-Red Imaging Spectrometer (AVIRIS) system, which currently covers the wavelength
region from 0.38–2.50 m [2]. Other sensors that offer hyperspectral capabilities have been available as research instruments
for the last ten years, including DAIS/ROSIS [3], CASI-2 [4],
AISA [5], MIVIS [6], HyMap [7], or HYPERION [8]. In the
Manuscript received January 11, 2002; revised May 22, 2002. This
work was supported in part by the Spanish Government under Grant
TIC-2000-0739-C04-03 and by Junta de Extremadura (local Government)
Grant 2PR01A085.
The authors are with the Neural Networks and Signal Processing Group
(GRNPS), Computer Science Department, University of Extremadura,
Escuela Politécnica, 10.071 Cáceres, Spain (e-mail: aplaza@unex.es;
pablomar@unex.es; rosapere@unex.es).
Digital Object Identifier 10.1109/TGRS.2002.802494
is the number of endmembers needed to accurately
where
model the original spectrum, and is a scalar value representing
.
the fractional coverage of endmember vector in pixel
The ideal case is that the coefficients in the linear combination
are nonnegative and sum to 1, being, therefore, interpretable as
cover fractions or abundances [13].
A set of endmembers and cover fractions is called a mixture model [16]. One of the new perspectives opened by this
approach, together with the improved spectral resolution of sensors, is the possibility of subpixel analysis of scenes, which aims
to quantify the abundance of different materials in a single pixel.
Like principal components, endmembers provide a basis set in
whose terms the rest of the data can be described [17], [18].
However, unlike principal components, the endmembers are expected to provide a more “physical” description of the data with
no orthogonalization restrictions. For instance, a simple mixture
model based on three endmembers has the geometrical interpretation of a triangle whose vertices are the endmembers. Cover
fractions are determined by the position of spectra within the
triangle and can be considered relative coordinates in a new reference system given by the endmembers, as Fig. 1 shows.
Although the development of effective devices with new and
improved technical capabilities stands out, the evolution in the
design of algorithms for data processing is not as positive. Many
of the techniques currently under way for hyperspectral data
0196-2892/02$17.00 © 2002 IEEE
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IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
Fig. 1. Scatterplot of two-dimensional spectral data illustrating the physical
interpretation of a mixture model based on endmembers.
analysis and endmember extraction have their origins in classic
spectroscopy and, thus, focus exclusively on the spectral nature
of the data. Available analysis procedures do not usually take
into account information related to the spatial context, which
can be useful to improve the obtained classification [10].
In this paper, we introduce a novel approach to endmember
extraction and multidimensional data analysis: the integration
of spatial data with spectral information. Our research is structured as follows. Section II provides a detailed overview of
existing endmember extraction techniques. Section III describes
fundamental principles in the proposed approach. Section IV
illustrates our method with examples and simulation results.
Section V presents both an example of experimental data and a
comparison of our method with other well-known endmember
extraction approaches. Finally, Section VI includes some concluding statements and comments on plausible future research.
II. BACKGROUND ON ENDMEMBER EXTRACTION TECHNIQUES
A number of algorithms based on the notion of spectral
mixture modeling have been proposed over the past decade to
accomplish the complex task of finding appropriate endmembers for spectral unmixing in multi/hyperspectral data. One of
the most successful approaches has been the pixel purity index
(PPI) [15], which is based on the geometry of convex sets [19].
PPI considers spectral pixels as vectors in an -D space. A
dimensionality reduction is first applied to the original data
cube by using the minimum noise fraction [15]. The algorithm
proceeds by generating a large number of random -D vectors,
also called “skewers” [20], through the dataset. Every data
point is projected onto each skewer, along which position is
pointed out. The data points that correspond to extrema in the
direction of a skewer are identified and placed on a list. As
more skewers are generated the list grows, and the number of
times a given pixel is placed on this list is also tallied. The
pixels with the highest tallies are considered the purest ones,
since a pixel count provides a “pixel purity index.”
It is important to emphasize that the PPI algorithm does
not identify a final list of endmembers. PPI was conceived
not as a solution, but as a guide; in fact, the author proposed
comparing the pure pixels with target spectra from a library
and successively projecting the data toward lower dimensional
spaces while endmembers were identified [15]. There are several
interactive software tools oriented to perform this task, but
the supervised nature of such tools leads to the fact that the
analysis of a scene usually requires the intervention of a highly
trained analyst. Then, interactive solutions may not be effective
in situations where large scenes must be analyzed quickly as a
matter of routine. In addition, randomness in the selection of
skewers has been identified by some authors as a shortcoming
of the algorithm [20]. The original implementation of PPI
proposes the use of unitary vectors as skewers in random
directions of the -D space. This implementation may be
improved by careful selection of existing vectors to skew the
dataset. Intelligent selection of skewers may result in a more
efficient behavior of the algorithm.
Some tools based on variations of PPI concepts have been
proposed. The manual endmember selection tool (MEST) [21]
is a computer display for interactive derivation of spectral endmembers. The first step in the selection process is the determination of the number of mixture components via a principal component analysis (PCA). If the PCA finds that most variance in
orthogonal directions (referred
the data is accounted for by
to as eigenvectors), then the number of endmembers is fixed to
, and MEST displays the spectral data mean corrected and
projected onto the -D space, which is determined by the first
eigenvectors. MEST provides the user with a means of exspectra that
ploration in the mixing space to search for
are acceptable as the spectral signatures of ground components;
likewise, these spectra contain the spectral data in the simplex
that they span in the mixing space. Again, the supervised nature
of the approach may become a major drawback.
There are several recent efforts in the literature that pursue
the fully automated identification of endmembers from multidimensional data cubes. The N-FINDR algorithm [22] finds the
set of pixels with the largest possible volume by “inflating”
a simplex within the data. The input for the algorithm is the
full image cube, with no previous dimensionality reduction. A
random set of vectors is initially selected. In order to refine the
initial estimate of endmembers, every pixel in the image must
be evaluated in terms of pixel purity likelihood or nearly pure
statehood. To achieve this, the volume is calculated for every
pixel in the place of each endmember. A trial volume is calculated for every pixel in each endmember position by replacing
that endmember and finding the volume. If the replacement results in a volume increase, the pixel replaces the endmember.
This procedure is repeated until there are no more replacements
of endmembers. Once the pure pixels are found, their spectra
can be used to unmix the original image.This produces a set of
images that show the abundance of each endmember. While the
endmember determination step of N-FINDR has been optimized
and can be executed rapidly, the computational performance of
the algorithm depends on the accuracy of the initial random selection of endmembers.
The ORASIS (optical real-time adaptative spectral identification system) algorithm [23] uses a process, called exemplar
selection, to trim the dataset. This procedure rejects redundant
spectra by calculating the angle between spectral vectors. Any
vector that is not separated by a certain threshold angle is
removed from the data. The procedure then finds a basis set
PLAZA et al.: SPATIAL/SPECTRAL ENDMEMBER EXTRACTION
of much lower dimension than the original data by a modified
Gram–Schmidt process. The exemplar spectra are then projected onto this basis subspace, and a simplex is found through
a minimum volume transform. Even though ORASIS is tuned
for rapid execution, there is no dimensionality reduction as
part of this algorithm. The whole process is very sensitive to
the threshold angle parameter, related to sensor noise. This
parameter must be selected cautiously in order to avoid the loss
of important endmembers.
The iterative error analysis (IEA) algorithm [24] is also
executed directly on the data with neither the dimensionality
reduction nor the data thinning. An initial vector (usually the
mean spectrum of the data) is chosen to start the process. A
constrained unmixing is then performed, and an error image is
formed. The average score of vectors with higher degrees of
error (distance from the initial vector) is assumed to be the first
endmember. Another constrained unmixing is then performed,
and the error image is formed. The average score of vectors with
greater errors (distance from the first endmember) is assumed
to be the second endmember. This process is continued until a
predetermined number of endmembers is found, and fractional
abundance maps are found by the final constrained unmixing.
There are no published computer time estimates for the IEA
algorithm, but the repeated constrained unmixings involved
should take significant time.
The use of multiple endmembers at a pixel level has also been
a main topic of interest to account for endmember variability.
Multiple endmember spectral mixture analysis (MESMA) [25]
is a spectral unmixing approach in which many possible mixture models are analyzed simultaneously in order to produce the
best fit. The weighting coefficients (fractions) for each model
and each pixel are determined so that the linear combination
of the endmember spectra produces the lowest margin of RMS
error when compared to the apparent surface reflectance for the
pixel. Weighting coefficients are constrained to lie between zero
and one, and a valid fit is restricted to a maximum preset RMS
error. This approach requires an extensive library of field, laboratory, and image spectra, where each plausible component is
represented at least once. Since the need for spectral libraries
turns out to be a demanding constraint (intensive ground work
is required in their development), this fact may be the major
limitation of this approach.
Another attempt to incorporate endmember variability into
spectral mixture analysis [16] is based on the representation of
a landscape component type, not with one endmember spectrum
but with a set or bundle of spectra, where each spectrum is feasible as an instance of the component (e.g., in the case of a tree
component, each spectrum could be the spectral reflectance of
a tree canopy). A pixel may have an infinite number of mixture
models with endmembers selected from the bundles. Authors in
this approach [16] use a simulated annealing algorithm to derive bundles of endmembers. This task is carried out by constructing a simplex from a partition of facets in a convex hull
of a point data cloud. Bundle unmixing finds the maximum and
minimum endmember fractions in those models, i.e., the range
of endmember fraction values possible for a pixel.
Finally, self-organizing neural network algorithms have been
applied also to the unsupervised classification of multi/hy-
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TABLE I
A COMPARISON OF SEVERAL ENDMEMBER EXTRACTION APPROACHES
perspectral datasets through endmember extraction [26].
Kohonen’s self-organizing maps are suitable to perform competitive endmember learning. Hence, clusters are formed in the
spectral space. The topology of the network and the training
algorithm are key parameters that must be fixed with caution in
order to obtain satisfactory results.
Table I shows a comparison between the above-mentioned
approaches, emphasizing some of their key operational features.
As can be seen in Table I, most endmember extraction methods
rely on spectral properties of the scene alone, neglecting the
information related to the spatial correlation of pixels when
deriving endmembers.
It is well known that many multispectral remote sensing
datasets have the unique feature of being representations of
collections of images, where a particular pixel is characterized
by spectral and spatial properties. Spectral information deals
with pixel value in an independent manner from the neighboring pixel values, whereas spatial information concerns the
relationship between the pixel value and those of its neighbors.
Despite the inherent spatial–spectral duality that resides in
a multidimensional data cube, most available processing
techniques tend to focus exclusively on the spectral domain,
handling the data not as an image but as an unordered listing of
spectral measurements. These gauges can be shuffled arbitrarily
without affecting analysis [10]. The incorporation of spatial
information results from the notion of image processing, which
is not yet fully exploited in the field of multidimensional data
analysis, due to the likely fact that most analysis techniques
originate from classic spectroscopy.
The importance of analyzing spatial and spectral patterns
simultaneously has been identified as a desired goal by many
scientists who are devoted to multidimensional data analysis
[27]–[29]. This type of processing can be approached from
various points of view. In [30], a possibility is discussed for the
refinement of results obtained by spectral techniques through
a second step based on spatial context. The techniques proposed by these authors do not focus on endmember extraction,
but yield some insightful feedback on plausible spatial and
spectral response combinations. One of the most interesting
approaches proposed is contextual classification, which accounts for the tendency of certain ground cover classes to
occur more frequently in some contexts than in others. Once
an initial clustering based on spectral information alone has
been performed, a postclassification technique is applied. This
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IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
method consists of two main parts: the definition of a pixel
neighborhood (surrounding each pixel of the scene) and the
performance of a local operation so that the pixel may be
changed into the label mostly represented in the window that
defines the neighborhood. This simple operation entirely separates spatial information from spectral information, and thus
the two types of information are not treated simultaneously;
quite the opposite, spectral information is first extracted, and
a spatial context is then imposed.
In this paper, we present the possibility of obtaining a method
that integrates both spatial and spectral responses in a simultaneous manner. Mathematical morphology [31], [32] is a classic
nonlinear spatial processing technique that provides a remarkable framework to achieve the desired integration. Morphology
was originally defined for binary images and has been extended
to the grayscale [33] and color [34] image cases, but it has
been seldom used to process multi/hyperspectral imagery [35].
We propose a new application of mathematical morphology
focused on the automated extraction of endmembers from multidimensional data. In addition, we list key advantages in the
use of morphology to perform this task.
A first major point is that endmember extraction is basically
a nonlinear task. Furthermore, morphology allows for the introduction of a local-to-global approach in the search for endmembers by using spatial kernels (structuring elements in the
morphology jargon). These items define local neighborhoods
around each pixel. This type of processing can be applied to
the search of convexities or extremities within a data cloud of
spectral points and to the use of the spatial correlation between
the data. Endmembers can be identified by following an iterative process where pixels in close proximity in the spatial domain compete against each other in terms of their convexity or
spectral purity. As a result, they allow for the determination of
a local representative in a neighborhood that will be compared
to other locally selected pixels. The adoption of this approach
leads to the incorporation of spatial information into the endmember determination procedure. As a final main step, morphological operations are implemented by replacing a pixel with a
neighbor that satisfies a certain condition. In binary/grayscale
morphology, the condition is usually related to the digital value
of the pixel, and the two basic morphological operations (dilation and erosion) are, respectively, based on the replacement of
a pixel by the neighbor with the maximum and minimum value.
Since an endmember is defined as a spectrally pure pixel that
describes several mixed pixels in the scene, extended morphological operations can obviously contribute to locating suitable
pixels that replace others in the scene according to some desired
particularity of the pixel, e.g., its spectral purity.
Therefore, we propose a mathematical framework to extend
mathematical morphology to the multidimensional domain,
which results in the definition of a set of spatial/spectral operations that can be used to extract reference spectral signatures.
Our objective in this research is to analyze the viability of
extended morphological operations to analyze multidimensional remote sensing data. It is important to emphasize that
the combination of spatial and spectral information leads to a
new interpretation of the endmember concept, related to the
capacity of a particular signature in order to describe various
mixed pixels coherently in both spectral and spatial terms.
III. METHODS
This section is organized as follows. Section III-A provides
an overview of classic mathematical morphology. Section III-B
focuses on the proposed framework to extend classic morphological operations to a multidimensional domain. Some
simple examples that illustrate the behavior of basic extended
morphological operations are included. Finally, Section III-C
describes the proposed approach for endmember extraction:
the general algorithm, parameters, and implementation options
are also discussed.
A. Classic Mathematical Morphology
Mathematical morphology is the science of shape and
structure, based on set-theoretical, topological, and geometrical
concepts [31], [32]. Over the past few years, mathematical
morphology has grown as a powerful method for image
processing and has become a solid mathematical theory. It
has been successfully applied to various disciplines, such
as mineralogy and medical diagnostics, finding increasing
applications in digital image processing, computer vision, and
pattern recognition [36].
The two basic operations of mathematical morphology are
dilation and erosion. These operations were originally defined
for binary images [31], but have been extended to the grayscale
image case [33]. In grayscale morphology, images are handled
be a gray-level function
as continuous valued sets. Let
representing an image, and let represent another set that comprises a finite number of pixels. This set is usually referred to
as a kernel or structuring element in morphology terminology.
by structuring element
Erosion and dilation of image
are respectively written as follows:
Inf
(3)
Sup
(4)
denotes the weight associated with the different
where
elements of the kernel. The expressions for grayscale dilation
and erosion bear a marked similarity to the convolution integral often encountered in digital image processing, with sums
and differences replacing multiplication and minimum and maximum replacing summation. The main computational task in
dealing with grayscale morphological operations is the query
for local maxima or minima in a local search area around each
image pixel. This area is determined by the size and shape of
structuring element .
In order to simplify the algorithms developed, we refer only
to convex and plain structuring elements: a special class of struc[37].
turing elements that results when
This is not a general requirement of morphological operations,
and other types of kernels can be used in the future developments of the method. For illustrative purposes, let be a plain
3 3 structuring element. If a dilation operation using
is
applied over a grayscale image, then the local effect of the operation is the selection of the brightest pixel in a 3 3-pixel
search area around the target pixel. The constraints imposed on
PLAZA et al.: SPATIAL/SPECTRAL ENDMEMBER EXTRACTION
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the kernel definition causes all pixels lying within the kernel to
be handled equally, i.e., no weight is associated with the pixels
according to their position along the kernel, and the pixel with
maximum digital value is selected. The previous operation is repeated with all the pixels of the image, leading to a new image
(with the same dimensions as the original) where lighter zones
are developed concerning the size and shape of the structuring
element. In contrast, grayscale erosion has the global effect of
shrinking the lighter zones of the image. The effect obtained by
these operations is illustrated in Fig. 2, where a simple grayscale
synthetic image is eroded and dilated using a 3 3 structuring
element.
By applying these basic operations, one can construct more
advanced tools [31], [32]. For example, a classical morphological edge detector may be obtained by subtracting the eroded
image from the dilated image; opening (erosion followed by dilation) is used to detect peaks or local maxima; closing (dilation
followed by erosion) is applied to detect valleys or local minima,
etc.
Fig. 2. Local and global effect of applying grayscale dilation and erosion
operations to a simple grayscale image.
B. Extending Mathematical Morphology to Multispectral
Images
In grayscale morphology, the calculation of maximum and
minimum gray values relies on the definition of a simple ordering relation given by the digital value of pixels. In the case
of multispectral datasets, this natural assumption is not straightforward, since there is no natural means for the total ordering of
multivariate pixels. Hence, the greatest challenge in the task of
extending morphology operations to multispectral images is the
definition of a vector ordering relation that allows for the determination of the maximum and minimum elements in any family
of -D vectors [34]. In order to obtain the desired performance,
we have tested two possibilities that follow a similar strategy.
First, a lattice structure is imposed onto the spectral space by
the definition of a distance measure, and, second, morphological operators are defined by extension. These possibilities are
described in the next section. Instead, we finish this section by
including a simple example of the effect of multispectral dilation and erosion as compared to grayscale counterparts.
1) Cumulative Distance for Pixel Ordering in Terms of Spectral Purity: In order to impose an ordering relation in terms
of spectral purity in a set of multispectral pixels lying within a
structuring element or kernel , we propose defining a metric
that calculates the cumulative distance between one particular
of the kernel, where
denotes a vector of
pixel
dimensions at spatial coordinates
and every other pixel
. The metric is denoted by and is defined
as follows:
(5)
is a pointwise linear distance measure between two
where
-D vectors, i.e., the spectral angle distance. This cumulative
distance can order the vectors of a kernel in terms of their spectral purity. The computational time from calculating may be
expressed in terms of the number of dot calculations as
Fig. 3. Calculation of the most spectrally singular (a) and most spectrally
mixed (b) pixels within a kernel by using cumulative distance .
K
D
, where
is the number of elements in the kernel. The
dot operations, and
distance between two vectors involves
is calculated between every vector and all the others in the
kernel, resulting in a cubic complexity of
in asymptotical
notation. Multispectral dilation and erosion can be defined after
the expressions for grayscale dilation and erosion in (3) and (4),
respectively, as follows:
(6)
(7)
and
, respectively, denote the pixel
where
vector that maximizes and minimizes cumulative distance .
is a set of
The ordering relation is illustrated in Fig. 3. If
spatial neighboring pixels defined by a plain kernel, then
is the most spectrally singular pixel [Fig. 3(a)], and
is the
most highly mixed element [Fig. 3(b)].
2) Distance to the Centroid of the Data Cloud: The computational cost of calculating the cumulative distance is cubic,
resulting in a very high complexity if the kernel has a large size.
We have checked another possibility that relies on the calculation of the distance between every spectral point in the kernel
and the centroid of the kernel data cloud, defined as
(8)
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where
is the number of elements in . Then, a local eccentricity measure for a particular -D point of the kernel
can be obtained by calculating the distance in relation to the centroid as
(9)
is a pointwise linear distance function (spectral
where
may
angle). In this case, computational time to obtain
, i.e., the distance between two
be expressed as
dot operations and is calculated between
vectors involves
every vector of the kernel and the centroid. This results in a
(in asymptotical
quadratic computational complexity
defines an ordering relation in the data
notation). Distance
cloud that allows for the identification of the maximum as that
element that is most “eccentrical” or distant from the centroid
[Fig. 4(a)], while the minimum is the closest element to the
centroid [Fig. 4(b)]. Multispectral dilation and erosion can be
defined respectively as
Fig. 4. Calculation of the most spectrally singular (a) and most spectrally
mixed (b) pixels within a kernel
by using the distance to the centroid of the
data cloud ( ).
D
K
(10)
(11)
and
, respectively, denote the pixel
where
vector that maximizes and minimizes the distance to the centroid . In this paper, only the two distances ( and ) are
considered. We have demonstrated that the selection of the maximum and minimum through both methods is equivalent in the
case of a linear distance measure as the spectral angle. Yet,
this is not necessarily so. For example, if we take a nonlinear
distance measure as the spectral information divergence (SID),
defined in [38], the sum of the distances to each pixel in the
kernel would not be equivalent, in general terms, to the distance
to the centroid. Our research only focuses on a simple linear
distance in order to simplify algorithm design. A comparative
effort including nonlinear distances as well as distances that use
second-order statistics [39], [40] deserves future attention.
3) Example of Multispectral Erosion and Dilation: The result of applying an erosion/dilation operation to a multispectral
dataset is a new data cube, with exactly the same dimensions
as the original, where each pixel has been replaced by the maximum/minimum of the kernel neighborhood considered. For illustrative purposes, let us consider a simple synthetic hyperspectral image with pure and mixed spectral signatures and a
plain 3 3 kernel. The kernel is moved, pixel by pixel, through
the image, defining a local spatial search area around each target
pixel. The maximum (dilation) or minimum (erosion) pixel in
the search area is calculated according to the schema described
above, yielding the results depicted in Fig. 5.
As can be examined in Fig. 5, dilation has the effect of
expanding zones with pure spectral signatures according to
the spatial properties of the structuring element, while erosion
shrinks zones with pure signatures. Both operations use spatial
and spectral information in a combined fashion. From this
simple example, it is straightforward that extended morphological operations can be used to identify which pixels of the
Fig. 5. Local and global effect of applying spatial/spectral dilation and erosion
operations to a simple hyperspectral image.
image scene are pure and which ones are formed by a mixture
of components.
C. Automated Morphological Endmember Extraction
As explained above, the physical idea of an endmember is
that of a spectrally pure signature (generally corresponding to
a pure ground component) that can be used to describe several
nonpure pixels through a mixture model based on linear combinations of endmembers [11]. The notion behind this idea is the
determination of a spectral purity index at a kernel level. Two
different possibilities have been explored to determine the purity index. Thus, we focus on advantages and disadvantages for
both possibilities. This section ends with a detailed description
of our spatial/spectral algorithm, which we have named automated morphological endmember extraction (AMEE).
1) “Voting” Approach to Endmember Selection: Multispectral dilation can extract the most particular pixel in a
certain kernel neighborhood by the definition of a purity
distance measure between pixels. Two distance measures have
been described in Section III-B for this purpose (see Figs. 3
and 4). The identification of the purest pixel by the dilation
operation is done as follows. As a first step, one particular
pixel of the neighborhood is selected (candidate pixel). This
pixel receives a score from all its neighbors (voters). The score
is the spectral angle distance between the candidate and the
neighboring pixel. Once all the neighbors have voted, the
individual scores are summed, and the accumulated value is
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assigned to the candidate pixel. The operation is repeated with
every pixel of the kernel, so that all the pixels assume the
role of candidates once. In the end, the pixel with the highest
accumulated score (the winner in the vote) is selected as the
neighborhood endmember, and a counter associated with the
pixel is increased in one unit.
The major limitation of this approach, used by other existing
endmember extraction methodologies, is the fact that such a
voting system does not provide detailed information about the
degree of purity in a certain pixel. For instance, the PPI algorithm uses a very similar schema. Every pixel of the scene is
projected to random unitary vectors, and the most extreme pixel
in the direction of the unitary vector increases its purity counter
in one unit. A pixel is labeled as “spectrally pure” if it is repeatedly selected as extreme in several random projections. Nevertheless, an important question like “how extreme is the pixel in
the data cloud” is neglected. The “eccentricity” of a pixel, i.e.,
its distance from the least spectrally pure or most highly mixed
element, may be an important issue in the determination of the
pixel purity degree. The incorporation of this information may
result in a much more complete evaluation of the capacity of
pure pixels to describe other mixed pixels and, thus, improve
the whole endmember selection process.
2) “Evaluation” Approach to Endmember Selection: The
voting system was characterized by a single multispectral dilation operation. In order to account for the “eccentricity” of the
maximum pixel, we propose the combination of the output provided by dilation and erosion operations. While dilation selects
the purest pixel (maximum), erosion selects the most highly
mixed pixel (minimum) in a certain spatial neighborhood. The
distance between the maximum and the minimum (a widely
used operation in classic mathematical morphology) provides
an eccentricity value for the selected pixel that allows for an
evaluation of the quality of the pixel in terms of its spectral purity. In order to incorporate this idea, a new quality measure must
be introduced. We define the morphological eccentricity index
(MEI) as the distance between the maximum element (obtained
by a dilation) and minimum element (obtained by an erosion) in
the kernel.
Fig. 6 illustrates how the MEI index is calculated. Following
be the spatial coordinates of the maximum
(6) and (7), let
at the kernel neighborhood of a certain pixel with
pixel
spatial coordinates
. Let
be the minimum pixel of
the same kernel neighborhood. Then, the MEI index is given
is a pointwise distance
by the following expression, where
function (in this paper, the spectral angle distance).
(12)
The MEI index is associated with a certain pixel in order to
determine its capacity to describe other pixels in the kernel.
It is important to emphasize that the MEI value is assigned to
the maximum pixel in the kernel neighborhood. An illustrative example of the improvements produced by the evaluation
system based on the MEI index is shown in Fig. 7. The maximum element within the kernel is selected in Fig. 7(a) (voting
system), but no statement can be made about its spectral purity.
Fig. 6. Calculation of the MEI index by combining multispectral dilation and
erosion.
In Fig. 7(b) (evaluation system), the maximum element is not
only selected but also evaluated using its distance to the minimum, which is particularly important when all the pixels in the
kernel neighborhood have similar spectral purity.
The proposed evaluation approach presents some limitations
(illustrated in Fig. 8) related to the kernel size under consideration. Determining the quality of a local endmember by the MEI
index is only appropriate if the kernel size is sufficiently large
to include sample pixels with different degrees of purity. If the
kernel used is not large enough, then there is no sufficient information to determine wether a pixel is a reasonable candidate
to be used as an endmember. The main reason for that is the
unavailability of different samples to perform the comparison.
There may exist situations where a low MEI value is assigned
to a pure pixel [as depicted in Fig. 8(a)]. In order to solve this
problem, we propose the application of an iterative process with
different kernel sizes. All the pixels of the scene are evaluated
in each iteration, but increasingly larger neighborhoods around
each pixel are considered in subsequent iterations in order to obtain detailed spatial/spectral information at a pixel level.
The use of increasingly larger kernel sizes may lead to another issue. Since only one pixel per kernel neighborhood is
selected, there may be situations where important endmembers
are lost [as depicted in Fig. 8(b)]. This shortcoming is alleviated by the fact that the kernel is moved through all the pixels of
the image, but may still produce undesired effects. Therefore, it
is straightforward that the MEI index is sensitive to kernel size
and shape, which must be adjusted so that a satisfactory result
of the algorithm is obtained. The spatial/spectral nature of the
proposed method introduces, in fact, a dependence between the
structuring element and the algorithm output that is also present
in classic mathematical morphology. This dependence allows
the tuning of the method in order to improve the identification of
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Fig. 7.
IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
Evaluation of endmembers improves the results obtained by voting of endmembers through the definition of a morphological eccentricity index.
Fig. 8. Limitations of the evaluation approach.
endmembers associated with targets with a certain size, shape,
and spatial distribution in the scene.
As a final concern, a minor limitation of the current MEI definition is that it tends to exaggerate the purity of a pixel in certain
cases: those in which the kernel is located over an area where
all the pixels are relatively pure but correspond to different endmembers. We have proved by experimenting with real data that
this circumstance does not significantly affect the performance
of the algorithm due to the fact that all the pixels are good candidates to be selected as endmembers. With these issues in mind,
we describe the proposed endmember extraction algorithm.
3) Algorithm Description: A general block diagram of the
AMEE algorithm is shown in Fig. 9. The input to the method
is the full data cube, with no previous dimensionality reduction or preprocessing. Parameter refers to the number of iterations that the algorithm performs. Parameter
and
,
respectively, denote the minimum and maximum kernel sizes
that will be considered in the iterative process. These parameters are interrelated, as the number of iterations depends on the
minimum and maximum kernel sizes under scrutiny.
is considered. Then, the
First, the minimum kernel size
kernel is moved through all the pixels of the image. The spectrally purest pixel and the spectrally most highly mixed pixel
are obtained at each kernel neighborhood by multispectral dilation and erosion operations. An MEI value is associated with
the purest pixel by comparing the result of the dilation to the result of erosion. The previously described operation is repeated
Fig. 9. Schematic representation of the proposed automated morphological
endmember extraction method through a block diagram.
by using structuring elements of progressively increased size,
and the algorithm performs as many iterations as needed until
is achieved. The associated MEI
the maximum kernel size
value of selected pixels at subsequent iterations is updated by
means of newly obtained values, as a larger spatial context is
considered, and a final MEI image is generated after iterations [41], [42]. This approach ensures a complete spatial/spectral description of the image and provides an efficient tool to
integrate both types of information simultaneously. However, if
the size of the maximum kernel used is large, which can become a need in order to obtain a correct description (see Fig. 7),
this results in a considerable increase in morphological operation computational cost, mainly because of the high number
of pixels involved in the calculation of the distances. In this
case, the kernel size increases exponentially, which results in an
in asymptotical notation). We
exponential complexity (
have tested some variations of the previous implementation in
order to reduce the computational complexity of the proposed
approach, and a constrained implementation through cascade dilation operations and nonoverlapping kernel sizes [43] proved
to be considerably faster than the previous approach. The constraints imposed result in a computational complexity (
in asymptotical notation).
PLAZA et al.: SPATIAL/SPECTRAL ENDMEMBER EXTRACTION
2033
Fig. 10. (a) Reflectance spectra used to generate simulated scenes. (b) Radiance spectra used to generate simulated scenes. (c) Simulated Scene_1. (d) Simulated
Scene_2.
Automated endmember selection is performed from the MEI
image obtained in the competitive endmember selection step
by a threshold value . There are many available techniques
for automated thresholding of grayscale images [44]. We have
found appropriate results by using the multilevel Otsu method,
an approach based on the minimization of the weighted sum
of group variances [45]. The final selection is refined by a
region-growing procedure that incorporates neighboring pixels
that are sufficiently similar (according to a certain threshold
angle parameter ) to the regions obtained after thresholding.
Mean spectra are obtained for the resulting regions after the
region-growing process. As a consequence, a final set of
endmembers is obtained. This approach has been developed by
following the ideas presented in [16]. Traditional methodologies
such as the spectral angle mapper (SAM) or linear spectral
unmixing (LSU) can be used for the purpose of mapping the
obtained endmembers over the original image and obtaining
a final classification result.
IV. EVALUATION WITH SIMULATED DATA
In this section we evaluate the performance of the proposed
algorithm by using simulated data. For the sake of simplicity,
we will only consider scenes with two and three different components. Section IV-A describes how synthetic data are gener-
ated. Both radiance and reflectance data are used in order to
analyze the effect of these two options in our endmember extraction algorithm. In Section IV-B, a preliminary experiment is
conducted to study the accuracy of AMEE algorithms to classify
pixels as “pure” or “not pure” and the influence of the kernel size
parameter in this particular task. Finally, Section IV-C presents
a set of experiments that allow us to test the accuracy of AMEE
in the task of estimating the abundances of constituents. Scenes
with subtle and abrupt changes in the abundance of materials
are used in the simulations.
A. Generation of Simulated Data
A set of four spectral signatures directly extracted from a
1997 AVIRIS image scene of the Jasper Ridge, CA region [46]
will be used in computer simulations to illustrate the proposed
algorithm. This dataset, which has extensively been studied in
the literature, is available in radiance and reflectance units. This
fact allows us to study the performance of our algorithm in both
cases. The selected signatures [see Fig. 10(a) and (b)] correspond to soil, vegetation, buildings, and roads.
Two simple 60 60-pixel hyperspectral scenes have been
generated using the above-mentioned spectra. The first scene,
called Scene_1 [Fig. 10(c)], was created by using artificially
generated mixtures between two selected signatures (soil and
vegetation) with different abundance fractions. The scene is
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B. Identification of Pure Pixels
TABLE II
ABUNDANCE ASSIGNMENT FOR REGIONS IN SIMULATED
SCENES SCENE_1 AND SCENE_2
formed by six regions of ten-pixels width, and the abundances
have been assigned to the regions so that the linear mixture
between the two components is progressive and adds to one (as
shown in Table II).
The synthetic hyperspectral scene in Fig. 10(d) (called
Scene_2) simulates homogeneous targets (a road and a
building) in a soil background. The simple simulated “objects”
are a 10 10-pixel building and a road ten pixels wide. This
image is characterized by abrupt changes in the abundances of
materials. There is no linear mixture among components in the
scene. Instead, there is a situation where 1) three different pure
constituents are present and 2) each pixel is characterized by
having an abundance of one for one particular endmember and
zero for the others (see Table II).
Both Scene_1 and Scene_2 have been generated by using radiance and reflectance spectra. We will refer to them from now
on as Scene_1_radiance, Scene_1_reflectance, Scene_2_radiance, and Scene_2_reflectance. Random noise was used to
simulate contributions from ambient (clutter) and instrumental
sources. Noise was created by using numbers with a standard
normal distribution obtained from a pseudorandom number
generator and added to each pixel to generate a signal-to-noise
ratio (SNR) of 30:1. For the simulations, we will define the
SNR for each band as the ratio of the 50% signal level to the
standard deviation of the noise [47]. Thus, the simulated data
are created, based on a simple linear model, by the following
expression [19]:
(13)
is a vector containing the simulated discrete
where
spectrum,; is the number of endmembers used; is a scalar
value representing the fractional abundance of endmember
at pixel
(Table II contains a description of the abundance
is the noise factor.
of endmembers in each region); and
The signal is scaled by 50% of the SNR, which is equivalent
to reducing the noise standard deviation by the inverse factor
), so that the simulated data meet the SNR definition
(
[19]. The vector terms in the parentheses are multiplied element
by element.
One of the properties of synthetic images is simplicity in
the generation of entirely reliable ground truth from data.
Receiver operating characteristic (ROC) [48] is a classic
approach to evaluate the performance of image classification algorithms when ground-truth information is available.
A preliminary experiment has been performed by using
Scene_1_Reflectance [see Fig. 10(c)] to analyze the influence
of the kernel size parameter on the accuracy of our method in
the task of determining whether or not a certain pixel is “pure.”
The goal of ROC analysis is to estimate a curve that describes
the tradeoff between sensitivity (true positive rate) and specificity (false alarm rate) in the tested algorithm at different decision compromises [48]. In our case, the decision is related
to the threshold value that determines which pixels of an MEI
image, provided by the AMEE algorithm, are labeled as “pure”
and which are labeled as “mixed.” Since there are many possible thresholds, the ROC curve is constructed by taking a set
of equidistant threshold values between the minimum and the
maximum purity index of the MEI image. Each threshold value
leads to the generation of a binary classification image (BCI),
where pixels with a purity index higher than the threshold are
marked as “pure,” and pixels with a purity index lower than the
threshold are labeled as “not pure.” Each BCI is compared to a
ground-truth purity index image (GTPI) directly obtained from
the original image, where only pure pixels are marked. If a certain pixel is classified as “pure” in both the BCI and GTPI, then
a true positive is checked. On other hand, if a pixel is labeled as
“pure” in the BCI and as “not pure” in the GTPI, then a false
positive is counted. Each point of the ROC curve is obtained by
plotting the average true positives against average false positives
per BCI image.
Fig. 11 shows an example of the construction of an ROC
curve for an MEI image obtained after applying the AMEE algorithm to Scene_1_Reflectance, using a square-shaped 11 11
kernels (note that this kernel is one pixel wider than the regions in the scene). In this particular image, pure pixels are
those included in regions R1 and R6. Fig. 11(a) shows the MEI
image produced by the algorithm. Fig. 11(b)–(e) shows the resulting images after thresholding the MEI image with several
equidistant threshold values in the dynamic range of the image.
Fig. 11(f) shows an example of ROC curve construction by
using the previous images. Each point at the curve is obtained
by calculating the average number of true and false positives per
image. As can be pinpointed in the graph, the extreme points of
an ROC curve are (0, 0) and (1, 1). These values indicate that
the price of complete success in the algorithm for recognizing
all existing targets is complete failure, i.e., detection of all false
alarms as true positives. Then, as the performance of a method
increases, the ROC curve shifts toward the upper left corner,
which represents perfect performance. Then, the quality of an
MEI image may be determined by calculating the area below its
associated ROC curve. From detection theory definitions [48],
the greater the area below the ROC curve, the better the algorithm performance.
In order to analyze the impact of the kernel size parameter on
the classification, we have performed the following experiment.
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2
Fig. 11. (a) MEI image obtained after applying the AMEE algorithm to Scene_1_Reflectance by using a square-shaped 11 11 kernel. (b)–(e) Resulting images
after thresholding the MEI image with four equidistant threshold values. (f) Construction of an ROC curve for the MEI image (the area under the curve is also
addressed). (g) Area under ROC curves associated with classification images obtained by using voting and evaluation of endmembers with different kernel sizes.
The kernel size parameter is initially set to 3 3 pixels and is
progressively increased to a maximum value of 21 21 pixels.
For each considered size, an MEI image is obtained. An ROC
curve is then constructed for each MEI image, and the area under
the curve is estimated. The resulting area estimations are plotted
against the correspondent values of the parameter, obtaining
a curve that indicates which values of the parameter provide
better results. Fig. 11(g) shows the results of this experiment
after applying the AMEE algorithm to Scene_1_Reflectance.
For comparative purposes, we have considered the voting and
evaluation systems for endmember selection (refer to Section III
for a detailed description of each approach). The results shown
in Fig. 11(g) reveal that the purity determination process is
sensitive to the kernel size parameter, which must be large
enough to contain samples with different spectral purity. In
both cases (voting and evaluation), performance is high when
the kernel size is 11 11 or larger (the width of spatial
patterns in Scene_1 is ten pixels). In contrast, this experiment
reveals that voting is much more dependent on kernel size
than evaluation, which provides acceptable results for small
kernel sizes.
C. Estimation of Abundances
In order to perform abundance estimation simulations, we
have applied our algorithm to the simulated scenes described
in Fig. 10. A set of endmembers is extracted for each image,
and the abundance of each endmember is estimated by using
fully constrained linear spectral unmixing [49]. In order to determine the accuracy of our method in this particular task, we compare the estimated abundances to the true abundances, shown in
Table II. Both reflectance and radiance spectra are used in the
simulations.
Fig. 12 shows some results obtained after applying the
AMEE algorithm to Scene_1_reflectance and Scene_1_radiance using a square-shaped 11
11 kernels. In both cases,
two endmembers were extracted. The reflectance endmembers
are labeled as Ref_1_1 and Ref_1_2 [Fig. 12(a)] and the
radiance endmembers as Rad_1_1 and Rad_1_2 [Fig. 12(b)]. A
visual comparison of the extracted endmembers to the original
signatures used to generate the images [Fig. 10(a) and (b)]
reveals that the extracted endmembers correspond to soil and
vegetation. Even though the resulting abundance maps are
noisy [Fig. 12(c)–(f)], the spatial zones corresponding to pure
soil and vegetation can be easily identified. A scatter plot
of true versus estimated fractional abundances is also shown
for each abundance map [Fig. 12(g)–(j)], showing that the
correlation is high even in the presence of noise. It should also
be pointed out that the algorithm produces very similar results
with radiance and reflectance data. This is not a surprising
result: the transformation of radiance data into reflectance units
can modify the spectral shape, but the spectral singularity of
different constituents remains.
Fig. 13 shows the results obtained after applying the AMEE
algorithm to Scene_2_reflectance and Scene_2_radiance. Three
endmembers were extracted for each scene [Fig. 13(a) and
(b)]. A visual comparison of the extracted endmembers to the
original signatures used to generate the images [Fig. 10(a)
and (b)] reveals that the extracted endmembers correspond
to soil, road, and building. By using the road and building
extracted endmembers, we have created two matched filters
and their correspondent output images [Fig. 13(c)–(f)]. Even
though the output is noisy, we can easily separate the two
targets from the background. These objects can be easily
obtained from the images through simple thresholding [the
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IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
Fig. 12. (a) Endmembers derived from Scene_1_Reflectance. (b) Endmembers derived from Scene_1_Radiance. (c), (d) Abundance maps for reflectance
endmembers. (e), (f) Abundance maps for radiance endmembers. (g), (h) Scatterplot of true versus estimated abundance for reflectance endmembers. (i), (j)
Scatterplot of true versus estimated abundance for radiance endmembers.
automated Otsu method [45] was used to obtain the threshold
value, producing the images shown in Fig. 13(g)–(j)]. Some
of the above-mentioned segmented images are corrupted by
“salt-and-pepper” noise due to the sensor noise simulation in
the scenes. This noise can be easily eliminated, at the expense
of some image degradation, by simple 3 3 median filters
[50].
The experiments presented in this section reveal that the
AMEE algorithm can be used for the purpose of mixture quantification and abundance determination. The algorithm is also
applicable for the purpose of identifying and separating targets
from a homogeneous background. The use of radiance/reflectance data does not significantly affect the performance of
the method. Although the method seems to work well in theory
development, further experiments with real data are desirable
in order to fully validate the previous remarks.
V. EVALUATION WITH EXPERIMENTAL DATA
In this section, a comparative analysis of AMEE to other
well-known endmember extraction algorithms is presented. The
comparison has been carried out with real AVIRIS hyperspectral data from the Cuprite mining district in Nevada. This scene
is well understood mineralogically [51] and has reliable ground
truth in several forms, including laboratory spectra [52] and
abundance maps [53]. This fact has made this region a standard
test site for comparison of classification algorithms. The scene
that we have selected for this work was acquired in 1995 and
consists of 50 spectral bands in the SWIR-2 region of the spectrum, where the differences among minerals are clear.
In order to perform this experiment, we have selected several
laboratory spectra from the U.S. Geological Survey (USGS)
Digital Spectral Library [52] to be used as ground-truth
references in the comparison. These signatures, acquired
PLAZA et al.: SPATIAL/SPECTRAL ENDMEMBER EXTRACTION
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Fig. 13. (a) Endmembers derived from Scene_2_reflectance. (b) Endmembers derived from Scene_2_radiance. (c), (d) Matched filter output for building and road
(Scene_2_reflectance). (e), (f) Matched filter output for building and road (Scene_2_radiance). (g), (h) Otsu segmentation of (c) and (d). (i), (j) Otsu segmentation
of (e) and (f).
with ground spectrometers on the Cuprite site, correspond to
minerals that have been identified in the 1995 AVIRIS scene. It
is important to emphasize that there are other minerals present
in the scene, as well as intimate mixtures between several
minerals, but we have focused our study on four particular
minerals (alunite, buddingtonite, calcite, and kaolinite) that can
be found prominently and in pure form in the area.
Two of the most well known algorithms for endmember
extraction are compared to the AMEE algorithm in this section:
PPI and N-FINDR. As described in Section II, PPI requires
close human supervision during the process of determining
the endmembers, while N-FINDR is fully automated. PPI and
N-FINDR endmembers were extracted and provided by the
developers of each method. The parameters used by the authors
in order to produce the above-mentioned results are unknown
to us. The kernel size parameter used by the AMEE algorithm
was 15 15. This parameter was determined after analyzing
the width in pixels of patterns of interest in the abundance
planes published in [53].
The compared spectra have been labeled as follows. The
USGS spectra were labeled with their correspondent names
in the library. AMEE, N-FINDR, and PPI endmembers were
labeled with the initial letter of the method plus a number that
indicates the order of identification of the signatures. Each
method has its own mechanisms to extract the endmembers,
and the order in which the endmembers are identified is not
important. Thus, AMEE endmembers are labeled as A1…A9,
N-FINDR endmembers as N1…N10, and PPI endmembers
as P1…P11. It should be taken into account that the number
of endmembers is different for each method and that “false”
endmember spectra are produced due to shade, noise, sensor
artifacts, etc. The term “false” should not be misunderstood,
since these endmembers are actually needed to accurately
unmix the scene.
In order to determine the best matching endmember for
each USGS reference signature, the confusion matrix method
[54] is used. Tables III–V show, respectively, the confusion
matrix of spectral angle distances between USGS ground-truth
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TABLE III
CONFUSION MATRIX OF SPECTRAL ANGLE DISTANCES BETWEEN AMEE ENDMEMBERS AND USGS REFERENCE SPECTRA
TABLE IV
CONFUSION MATRIX OF SPECTRAL ANGLE DISTANCES BETWEEN N-FINDR ENDMEMBERS AND USGS REFERENCE SPECTRA
TABLE V
CONFUSION MATRIX OF SPECTRAL ANGLE DISTANCES BETWEEN PPI ENDMEMBERS AND USGS REFERENCE SPECTRA
signatures and AMEE, N-FINDR, and PPI endmembers. We
have selected the spectral angle distance for the comparison due
to existing scale and ilumination variations between reference
and extracted endmembers, which are mainly due to atmospheric
transmission effects that are not present in spectra acquired
by ground spectrometers.
The endmember spectra for AMEE, N-FINDR, and PPI are
shown in Fig. 14 for the following minerals:
1)
2)
3)
4)
alunite;
buddingtonite;
calcite;
kaolinite.
Extracted endmembers have the same reflectance units as the
input data, since each endmember is actually the spectrum
(or an average) of real image pixels. For illustrative purposes,
ground USGS spectra for the same minerals are also plotted.
USGS spectra are expressed in scaled reflectance. It can be
noted that, although there are differences in the scales, spectral
shape is preserved in most cases. A visual comparison of
the buddingtonite USGS laboratory spectrum and derived
endmembers [Fig. 14(b)] reveals that atmospheric transmission
effects are relevant in the case of this mineral.
Although the three algorithms find endmembers by using
very different techniques, the output of the three algorithms
operating on the same dataset is very similar, as expected from
the results shown in the confusion matrices. For a quantitative
analysis of the performance of each approach, the plots in
Fig. 14 also address the spectral angle distance between each
extracted endmember and its correspondent USGS reference
spectrum. AMEE endmembers present high spectral similarity
to ground truth for the alunite, buddingtonite, and kaolinite
minerals. N-FINDR provides the best result for the calcite
mineral. The combination of spectral and spatial context
information in the pure pixel identification step of AMEE is
the main reason for the encouraging results obtained by the
algorithm in this example.
VI. CONCLUSION AND FUTURE LINES
Spectral unmixing and object classification are two important
goals of remote imaging spectroscopy. The idea of using endmembers derived from the data for classification and unmixing
has been considered before, but few methods have exploited the
existing correlation between neighboring pixels. The integration
of spatial and spectral information has been identified as a desired goal by the scientific community dedicated to multidimensional data analysis and classification.
We have described a novel approach based on mathematical
morphology to perform unsupervised endmember extraction
from multi/hyperspectral data. The method uses both spectral
and spatial information in a simultaneous manner. An evaluation
of the method by using simulated and real data has been
presented and discussed.
PLAZA et al.: SPATIAL/SPECTRAL ENDMEMBER EXTRACTION
2039
Fig. 14. Algorithm scene derived and USGS laboratory reflectance spectra for (a) alunite, (b) buddingtonite, (c) calcite, and (d) kaolinite. Extracted endmembers
have the same reflectance units as the input data, while USGS spectra are expressed in scaled reflectance. The spectral angle distance between each extracted
endmember and the correspondent USGS reference spectrum is addressed.
Results with simulated data reveal that the method can accurately model the spatial distribution of spectral patterns in the
scene by extended morphological operations that apply plain
spatial kernels. The spatial properties (size and shape) of the
kernel have a strong influence on the final result, a fact that is
consistent with classic mathematical morphology theory. The
behavior of the algorithm also depends on the relationship between the spatial properties of the kernels used and the distribution of spectral patterns in the scene. This fact allows for the
tuning of our method for a wide range of applications, from
target detection to global classification and spectral unmixing
of scenes. The use of reflectance/radiance data does not have a
significant impact on the output of the algorithm.
Results with experimental data show that the proposed
method produces results that are comparable to those found
by working with other widely accepted methodologies. The
proposed method is accurate in the task of identifying endmembers from complicated scenes as the famous AVIRIS
Cuprite dataset, which has become a standard test site for the
comparison of algorithms due to the availability of quality
ground-truth measurements.
As with any new approach, there are some unresolved issues
that may present challenges over time. In this sense, future lines
will cover some relevant developments that were not included in
the present study. An evaluation of different distance measures
(both linear and nonlinear) to be used in the extension of morphological operations is a key topic, deserving future research.
In addition, the structuring elements developed in this paper
are limited to plain square-shaped kernels. The use of kernels
with no such limitations is of great interest in order to explore,
in greater detail, the existing spatial/spectral interrelation between the kernel used and patterns in the scene. The number
of endmembers extracted per kernel neighborhood is also an
interesting issue. In the present research, some limitations of
the proposed method have been identified as a consequence of
the fact that only one pixel per kernel neighborhood is selected.
Alternative definitions of the local MEI index may overcome existing limitations. Finally, efficient hardware implementations
based on field-programmable logic arrays and systolic arrays
are being currently tested at our laboratory in order to provide
the methodology with real-time capabilities.
ACKNOWLEDGMENT
The authors acknowledge the suggestions and comments of
J. Pinzón, R. O. Green, and J. A. Gualtieri that helped to improve
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IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 40, NO. 9, SEPTEMBER 2002
the quality of this paper. Indications made by P. Gomez-Vilda
about efficient implementation in the proposed method are also
gratefully acknowledged. In addition, the authors would like
to express their gratitude to M. Winter and E. Winter for providing results of the N-FINDR algorithm for the Cuprite dataset.
Finally, we also wish to state our appreciation to A. Curado
Fuentes, from the Department of English at University of Extremadura, who reviewed the submitted version of this paper.
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Antonio Plaza received the M.S. degree in computer
science from the University of Extremadura, Caceres,
Spain, in 1997, where he is currently pursuing the
Ph.D. degree.
He is currently an Assistant Professor at the
Computer Science Department, University of
Extremadura, since 1999, where he was a Research
Associate, 1997 to 1999. He was also a Visiting
Researcher at Applied Information Sciences Branch,
Goddard Space Flight Center, Greenbelt, MD, and
the AVIRIS Data Facility, Jet Propulsion Laboratory,
Pasadena, CA. His main research interests include computer vision, image processing, pattern recognition, and the development and efficient implementation
of algorithms for multi/hyperspectral image analysis.
2041
Pablo Martínez received the M.S. and Ph.D.
degrees in physics from the University of Granada,
Andalucia, Spain, in 1980 and 1992 respectively.
He is currently the Head Scientist of the Neural
Networks and Signal Processing Group (GRNPS)
at the University of Extremadura, Caceres, Spain,
where he has also been a Professor of computer
science at University of Extremadura since 1985. He
was Visiting Researcher at the Applied Information
Sciences Branch, Goddard Space Flight Center,
Greenbelt MD in 2000. His main research interests
include remote sensing, digital image analysis, and neural networks.
Rosa Pérez received the M.S. degree in mathematics
from the University of Extremadura, Caceres, Spain,
in 1985, and the Ph.D. degree in computer science
from the Polytechnic University of Madrid, Madrid,
Spain, in 1995.
Since 1985, she has been a Professor of computer
science at University of Extremadura, Caceres,
Spain. Her main research interests include neural
networks, systolic arrays, field-programmable logic
array design, and signal processing.
Javier Plaza is currently pursuing the M.S. degree in computer science from the University of
Extremadura, Caceres, Spain.
His current research work is focused on imaging
spectrometry. He is mainly involved in quantitative
applications using real and simulated remote sensing
data. Other major research interests include telecommunications and image processing.