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Tutorial Image Pyramids
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‎doc/tutorials/imgproc/pyramids/pyramids.markdown

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Image Pyramids {#tutorial_pyramids}
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==============
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@prev_tutorial{tutorial_morph_lines_detection}
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@next_tutorial{tutorial_threshold}
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Goal
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----
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In this tutorial you will learn how to:
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- Use the OpenCV functions @ref cv::pyrUp and @ref cv::pyrDown to downsample or upsample a given
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- Use the OpenCV functions **pyrUp()** and **pyrDown()** to downsample or upsample a given
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image.
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Theory
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-# *Upsize* the image (zoom in) or
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-# *Downsize* it (zoom out).
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- Although there is a *geometric transformation* function in OpenCV that -literally- resize an
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image (@ref cv::resize , which we will show in a future tutorial), in this section we analyze
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image (**resize** , which we will show in a future tutorial), in this section we analyze
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first the use of **Image Pyramids**, which are widely applied in a huge range of vision
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applications.
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predecessor. Iterating this process on the input image \f$G_{0}\f$ (original image) produces the
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entire pyramid.
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- The procedure above was useful to downsample an image. What if we want to make it bigger?:
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columns filled with zeros (\f$0\f$)
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columns filled with zeros (\f$0 \f$)
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- First, upsize the image to twice the original in each dimension, wit the new even rows and
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- Perform a convolution with the same kernel shown above (multiplied by 4) to approximate the
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values of the "missing pixels"
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- These two procedures (downsampling and upsampling as explained above) are implemented by the
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OpenCV functions @ref cv::pyrUp and @ref cv::pyrDown , as we will see in an example with the
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OpenCV functions **pyrUp()** and **pyrDown()** , as we will see in an example with the
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code below:
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@note When we reduce the size of an image, we are actually *losing* information of the image.
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Code
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----
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This tutorial code's is shown lines below. You can also download it from
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[here](https://github.com/opencv/opencv/tree/master/samples/cpp/tutorial_code/ImgProc/Pyramids.cpp)
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This tutorial code's is shown lines below.
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@add_toggle_cpp
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You can also download it from
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[here](https://raw.githubusercontent.com/opencv/opencv/master/samples/cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp)
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@include samples/cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp
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@end_toggle
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@add_toggle_java
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You can also download it from
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[here](https://raw.githubusercontent.com/opencv/opencv/master/samples/java/tutorial_code/ImgProc/Pyramids/Pyramids.java)
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@include samples/java/tutorial_code/ImgProc/Pyramids/Pyramids.java
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@end_toggle
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@include samples/cpp/tutorial_code/ImgProc/Pyramids.cpp
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@add_toggle_python
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You can also download it from
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[here](https://raw.githubusercontent.com/opencv/opencv/master/samples/python/tutorial_code/imgProc/Pyramids/pyramids.py)
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@include samples/python/tutorial_code/imgProc/Pyramids/pyramids.py
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@end_toggle
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Explanation
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-----------
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Let's check the general structure of the program:
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- Load an image (in this case it is defined in the program, the user does not have to enter it
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as an argument)
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp load
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#### Load an image
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@add_toggle_cpp
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@snippet cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp load
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@end_toggle
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@add_toggle_java
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@snippet java/tutorial_code/ImgProc/Pyramids/Pyramids.java load
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@end_toggle
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@add_toggle_python
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@snippet python/tutorial_code/imgProc/Pyramids/pyramids.py load
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@end_toggle
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#### Create window
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@add_toggle_cpp
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@snippet cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp show_image
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@end_toggle
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@add_toggle_java
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@snippet java/tutorial_code/ImgProc/Pyramids/Pyramids.java show_image
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@end_toggle
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@add_toggle_python
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@snippet python/tutorial_code/imgProc/Pyramids/pyramids.py show_image
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@end_toggle
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- Create a Mat object to store the result of the operations (*dst*) and one to save temporal
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results (*tmp*).
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@code{.cpp}
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Mat src, dst, tmp;
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/* ... */
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tmp = src;
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dst = tmp;
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@endcode
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#### Loop
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- Create a window to display the result
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp create_window
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@add_toggle_cpp
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@snippet cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp loop
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@end_toggle
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- Perform an infinite loop waiting for user input.
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp infinite_loop
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@add_toggle_java
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@snippet java/tutorial_code/ImgProc/Pyramids/Pyramids.java loop
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@end_toggle
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Our program exits if the user presses *ESC*. Besides, it has two options:
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@add_toggle_python
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@snippet python/tutorial_code/imgProc/Pyramids/pyramids.py loop
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@end_toggle
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- **Perform upsampling (after pressing 'u')**
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp pyrup
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We use the function @ref cv::pyrUp with three arguments:
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Perform an infinite loop waiting for user input.
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Our program exits if the user presses **ESC**. Besides, it has two options:
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- *tmp*: The current image, it is initialized with the *src* original image.
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- *dst*: The destination image (to be shown on screen, supposedly the double of the
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- **Perform upsampling - Zoom 'i'n (after pressing 'i')**
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We use the function **pyrUp()** with three arguments:
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- *src*: The current and destination image (to be shown on screen, supposedly the double of the
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input image)
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- *Size( tmp.cols*2, tmp.rows\*2 )\* : The destination size. Since we are upsampling,
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@ref cv::pyrUp expects a size double than the input image (in this case *tmp*).
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- **Perform downsampling (after pressing 'd')**
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp pyrdown
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Similarly as with @ref cv::pyrUp , we use the function @ref cv::pyrDown with three arguments:
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- *tmp*: The current image, it is initialized with the *src* original image.
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- *dst*: The destination image (to be shown on screen, supposedly half the input
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image)
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- *Size( tmp.cols/2, tmp.rows/2 )* : The destination size. Since we are upsampling,
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@ref cv::pyrDown expects half the size the input image (in this case *tmp*).
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- Notice that it is important that the input image can be divided by a factor of two (in
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both dimensions). Otherwise, an error will be shown.
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- Finally, we update the input image **tmp** with the current image displayed, so the
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subsequent operations are performed on it.
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@snippet cpp/tutorial_code/ImgProc/Pyramids.cpp update_tmp
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- *Size( tmp.cols*2, tmp.rows\*2 )* : The destination size. Since we are upsampling,
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**pyrUp()** expects a size double than the input image (in this case *src*).
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@add_toggle_cpp
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@snippet cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp pyrup
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@end_toggle
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@add_toggle_java
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@snippet java/tutorial_code/ImgProc/Pyramids/Pyramids.java pyrup
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@end_toggle
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@add_toggle_python
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@snippet python/tutorial_code/imgProc/Pyramids/pyramids.py pyrup
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@end_toggle
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- **Perform downsampling - Zoom 'o'ut (after pressing 'o')**
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We use the function **pyrDown()** with three arguments (similarly to **pyrUp()**):
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- *src*: The current and destination image (to be shown on screen, supposedly half the input
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image)
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- *Size( tmp.cols/2, tmp.rows/2 )* : The destination size. Since we are upsampling,
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**pyrDown()** expects half the size the input image (in this case *src*).
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@add_toggle_cpp
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@snippet cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp pyrdown
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@end_toggle
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@add_toggle_java
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@snippet java/tutorial_code/ImgProc/Pyramids/Pyramids.java pyrdown
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@end_toggle
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@add_toggle_python
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@snippet python/tutorial_code/imgProc/Pyramids/pyramids.py pyrdown
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@end_toggle
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Notice that it is important that the input image can be divided by a factor of two (in both dimensions).
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Otherwise, an error will be shown.
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Results
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-------
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- After compiling the code above we can test it. The program calls an image **chicky_512.jpg**
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that comes in the *samples/data* folder. Notice that this image is \f$512 \times 512\f$,
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- The program calls by default an image [chicky_512.png](https://raw.githubusercontent.com/opencv/opencv/master/samples/data/chicky_512.png)
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that comes in the `samples/data` folder. Notice that this image is \f$512 \times 512\f$,
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hence a downsample won't generate any error (\f$512 = 2^{9}\f$). The original image is shown below:
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![](images/Pyramids_Tutorial_Original_Image.jpg)
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- First we apply two successive @ref cv::pyrDown operations by pressing 'd'. Our output is:
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- First we apply two successive **pyrDown()** operations by pressing 'd'. Our output is:
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![](images/Pyramids_Tutorial_PyrDown_Result.jpg)
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- Note that we should have lost some resolution due to the fact that we are diminishing the size
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of the image. This is evident after we apply @ref cv::pyrUp twice (by pressing 'u'). Our output
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of the image. This is evident after we apply **pyrUp()** twice (by pressing 'u'). Our output
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is now:
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![](images/Pyramids_Tutorial_PyrUp_Result.jpg)

‎doc/tutorials/imgproc/table_of_content_imgproc.markdown

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- @subpage tutorial_pyramids
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*Languages:* C++, Java, Python
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*Compatibility:* \> OpenCV 2.0
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*Author:* Ana Huamán

‎samples/cpp/tutorial_code/ImgProc/Pyramids.cpp

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‎samples/cpp/tutorial_code/ImgProc/Pyramids/Pyramids.cpp

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/**
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* @file Pyramids.cpp
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* @brief Sample code of image pyramids (pyrDown and pyrUp)
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* @author OpenCV team
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*/
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#include "iostream"
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#include "opencv2/imgproc.hpp"
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#include "opencv2/imgcodecs.hpp"
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#include "opencv2/highgui.hpp"
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using namespace std;
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using namespace cv;
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const char* window_name = "Pyramids Demo";
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/**
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* @function main
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*/
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int main( int argc, char** argv )
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{
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/// General instructions
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cout << "\n Zoom In-Out demo \n "
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"------------------ \n"
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" * [i] -> Zoom in \n"
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" * [o] -> Zoom out \n"
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" * [ESC] -> Close program \n" << endl;
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//![load]
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const char* filename = argc >=2 ? argv[1] : "../data/chicky_512.png";
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// Loads an image
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Mat src = imread( filename );
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// Check if image is loaded fine
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if(src.empty()){
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printf(" Error opening image\n");
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printf(" Program Arguments: [image_name -- default ../data/chicky_512.png] \n");
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return -1;
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}
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//![load]
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//![loop]
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for(;;)
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{
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//![show_image]
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imshow( window_name, src );
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//![show_image]
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char c = (char)waitKey(0);
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if( c == 27 )
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{ break; }
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//![pyrup]
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else if( c == 'i' )
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{ pyrUp( src, src, Size( src.cols*2, src.rows*2 ) );
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printf( "** Zoom In: Image x 2 \n" );
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}
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//![pyrup]
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//![pyrdown]
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else if( c == 'o' )
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{ pyrDown( src, src, Size( src.cols/2, src.rows/2 ) );
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printf( "** Zoom Out: Image / 2 \n" );
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}
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//![pyrdown]
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}
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//![loop]
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return 0;
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}

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