matplotlib · efiring · Feb 22, 2015 · Nov 3, 2014 · pelson · Dec 3, 2014
diff --git a/CHANGELOG b/CHANGELOG
@@ -1,14 +1,18 @@
+2015-02-19 Rewrite of C++ code that calculates contours to add support for
+           corner masking.  This is controlled by the 'corner_mask' keyword
+           in plotting commands 'contour' and 'contourf'. - IMT
+
 2015-01-23 Text bounding boxes are now computed with advance width rather than
-  	   ink area.  This may result in slightly different placement of text.
+           ink area.  This may result in slightly different placement of text.
 
 2014-10-27 Allowed selection of the backend using the `MPLBACKEND` environment
            variable. Added documentation on backend selection methods.
 
 2014-09-27 Overhauled `colors.LightSource`.  Added `LightSource.hillshade` to
-	   allow the independent generation of illumination maps. Added new
-	   types of blending for creating more visually appealing shaded relief
-	   plots (e.g.  `blend_mode="overlay"`, etc, in addition to the legacy
-	   "hsv" mode).
+           allow the independent generation of illumination maps. Added new
+           types of blending for creating more visually appealing shaded relief
+           plots (e.g.  `blend_mode="overlay"`, etc, in addition to the legacy
+           "hsv" mode).
 
 2014-06-10 Added Colorbar.remove()
 
@@ -64,7 +68,7 @@
            of a generator at line 263.
 
 2014-04-02 Added `clipon=False` to patch creation of wedges and shadows
-	   in `pie`.
+           in `pie`.
 
 2014-02-25 In backend_qt4agg changed from using update -> repaint under
            windows.  See comment in source near `self._priv_update` for

diff --git a/doc/api/api_changes/2015-02-19-IMT.rst b/doc/api/api_changes/2015-02-19-IMT.rst
@@ -0,0 +1,25 @@
+Changed behaviour of contour plots
+``````````````````````````````````
+
+The default behaviour of :func:`~matplotlib.pyplot.contour` and
+:func:`~matplotlib.pyplot.contourf` when using a masked array is now determined
+by the new keyword argument `corner_mask`, or if this is not specified then
+the new rcParam `contour.corner_mask` instead.  The new default behaviour is
+equivalent to using `corner_mask=True`; the previous behaviour can be obtained
+using `corner_mask=False` or by changing the rcParam.  The example
+http://matplotlib.org/examples/pylab_examples/contour_corner_mask.py
+demonstrates the difference.  Use of the old contouring algorithm, which is
+obtained with `corner_mask='legacy'`, is now deprecated.
+
+Contour labels may now appear in different places than in earlier versions of
+matplotlib.
+
+In addition, the keyword argument `nchunk` now applies to
+:func:`~matplotlib.pyplot.contour` as well as
+:func:`~matplotlib.pyplot.contourf`, and it subdivides the domain into
+subdomains of exactly `nchunk` by `nchunk` quads, whereas previously it was
+only roughly `nchunk` by `nchunk` quads.
+
+The C/C++ object that performs contour calculations used to be stored in the
+public attribute QuadContourSet.Cntr, but is now stored in a private attribute
+and should not be accessed by end users.
diff --git a/doc/users/whats_new/plotting.rst b/doc/users/whats_new/plotting.rst
@@ -3,3 +3,19 @@ Added center and frame kwargs to pie
 
 These control where the center of the pie graph are and if
 the Axes frame is shown.
+
+Contour plot corner masking
+```````````````````````````
+
+Ian Thomas rewrote the C++ code that calculates contours to add support for
+corner masking.  This is controlled by a new keyword argument
+``corner_mask`` in the functions :func:`~matplotlib.pyplot.contour` and
+:func:`~matplotlib.pyplot.contourf`.  The previous behaviour, which is now
+obtained using ``corner_mask=False``, was for a single masked point to
+completely mask out all four quads touching that point.  The new behaviour,
+obtained using ``corner_mask=True``, only masks the corners of those
+quads touching the point; any triangular corners comprising three unmasked
+points are contoured as usual.  If the ``corner_mask`` keyword argument is not
+specified, the default value is taken from rcParams.
+
+.. plot:: mpl_examples/pylab_examples/contour_corner_mask.py
diff --git a/examples/pylab_examples/contour_corner_mask.py b/examples/pylab_examples/contour_corner_mask.py
@@ -0,0 +1,35 @@
+#!/usr/bin/env python
+"""
+Illustrate the difference between corner_mask=False and corner_mask=True
+for masked contour plots.
+"""
+import matplotlib.pyplot as plt
+import numpy as np
+
+# Data to plot.
+x, y = np.meshgrid(np.arange(7), np.arange(10))
+z = np.sin(0.5*x)*np.cos(0.52*y)
+
+# Mask various z values.
+mask = np.zeros_like(z, dtype=np.bool)
+mask[2, 3:5] = True
+mask[3:5, 4] = True
+mask[7, 2] = True
+mask[5, 0] = True
+mask[0, 6] = True
+z = np.ma.array(z, mask=mask)
+
+corner_masks = [False, True]
+for i, corner_mask in enumerate(corner_masks):
+    plt.subplot(1, 2, i+1)
+    cs = plt.contourf(x, y, z, corner_mask=corner_mask)
+    plt.contour(cs, colors='k')
+    plt.title('corner_mask = {}'.format(corner_mask))
+
+    # Plot grid.
+    plt.grid(c='k', ls='-', alpha=0.3)
+
+    # Indicate masked points with red circles.
+    plt.plot(np.ma.array(x, mask=~mask), y, 'ro')
+
+plt.show()
diff --git a/lib/matplotlib/contour.py b/lib/matplotlib/contour.py
@@ -13,6 +13,7 @@
 import numpy as np
 from numpy import ma
 import matplotlib._cntr as _cntr
+import matplotlib._contour as _contour
 import matplotlib.path as mpath
 import matplotlib.ticker as ticker
 import matplotlib.cm as cm
@@ -989,9 +990,10 @@ def get_transform(self):
 
     def __getstate__(self):
         state = self.__dict__.copy()
-        # the C object Cntr cannot currently be pickled. This isn't a big issue
-        # as it is not actually used once the contour has been calculated
-        state['Cntr'] = None
+        # the C object _contour_generator cannot currently be pickled. This
+        # isn't a big issue as it is not actually used once the contour has
+        # been calculated.
+        state['_contour_generator'] = None
         return state
 
     def legend_elements(self, variable_name='x', str_format=str):
@@ -1433,18 +1435,34 @@ def _process_args(self, *args, **kwargs):
         Process args and kwargs.
         """
         if isinstance(args[0], QuadContourSet):
-            C = args[0].Cntr
             if self.levels is None:
                 self.levels = args[0].levels
             self.zmin = args[0].zmin
             self.zmax = args[0].zmax
+            self._corner_mask = args[0]._corner_mask
+            if self._corner_mask == 'legacy':
+                contour_generator = args[0].Cntr
+            else:
+                contour_generator = args[0]._contour_generator
         else:
             x, y, z = self._contour_args(args, kwargs)
 
             _mask = ma.getmask(z)
-            if _mask is ma.nomask:
+            if _mask is ma.nomask or not _mask.any():
                 _mask = None
-            C = _cntr.Cntr(x, y, z.filled(), _mask)
+
+            self._corner_mask = kwargs.get('corner_mask', None)
+            if self._corner_mask is None:
+                self._corner_mask = mpl.rcParams['contour.corner_mask']
+
+            if self._corner_mask == 'legacy':
+                cbook.warn_deprecated('1.5',
+                                      name="corner_mask='legacy'",
+                                      alternative='corner_mask=False or True')
+                contour_generator = _cntr.Cntr(x, y, z.filled(), _mask)
+            else:
+                contour_generator = _contour.QuadContourGenerator(
+                    x, y, z.filled(), _mask, self._corner_mask, self.nchunk)
 
             t = self.get_transform()
 
@@ -1465,7 +1483,10 @@ def _process_args(self, *args, **kwargs):
             self.ax.update_datalim([(x0, y0), (x1, y1)])
             self.ax.autoscale_view(tight=True)
 
-        self.Cntr = C
+        if self._corner_mask == 'legacy':
+            self.Cntr = contour_generator
+        else:
+            self._contour_generator = contour_generator
 
     def _get_allsegs_and_allkinds(self):
         """
@@ -1476,20 +1497,28 @@ def _get_allsegs_and_allkinds(self):
             lowers, uppers = self._get_lowers_and_uppers()
             allkinds = []
             for level, level_upper in zip(lowers, uppers):
-                nlist = self.Cntr.trace(level, level_upper,
-                                        nchunk=self.nchunk)
-                nseg = len(nlist) // 2
-                segs = nlist[:nseg]
-                kinds = nlist[nseg:]
-                allsegs.append(segs)
+                if self._corner_mask == 'legacy':
+                    nlist = self.Cntr.trace(level, level_upper,
+                                            nchunk=self.nchunk)
+                    nseg = len(nlist) // 2
+                    vertices = nlist[:nseg]
+                    kinds = nlist[nseg:]
+                else:
+                    vertices, kinds = \
+                        self._contour_generator.create_filled_contour(
+                                                           level, level_upper)
+                allsegs.append(vertices)
                 allkinds.append(kinds)
         else:
             allkinds = None
             for level in self.levels:
-                nlist = self.Cntr.trace(level)
-                nseg = len(nlist) // 2
-                segs = nlist[:nseg]
-                allsegs.append(segs)
+                if self._corner_mask == 'legacy':
+                    nlist = self.Cntr.trace(level)
+                    nseg = len(nlist) // 2
+                    vertices = nlist[:nseg]
+                else:
+                    vertices = self._contour_generator.create_contour(level)
+                allsegs.append(vertices)
         return allsegs, allkinds
 
     def _contour_args(self, args, kwargs):
@@ -1672,6 +1701,20 @@ def _initialize_x_y(self, z):
 
         Optional keyword arguments:
 
+          *corner_mask*: [ *True* | *False* | 'legacy' ]
+            Enable/disable corner masking, which only has an effect if *Z* is
+            a masked array.  If *False*, any quad touching a masked point is
+            masked out.  If *True*, only the triangular corners of quads
+            nearest those points are always masked out, other triangular
+            corners comprising three unmasked points are contoured as usual.
+            If 'legacy', the old contouring algorithm is used, which is
+            equivalent to *False* and is deprecated, only remaining whilst the
+            new algorithm is tested fully.
+
+            If not specified, the default is taken from
+            rcParams['contour.corner_mask'], which is True unless it has
+            been modified.
+
           *colors*: [ *None* | string | (mpl_colors) ]
             If *None*, the colormap specified by cmap will be used.
 
@@ -1750,6 +1793,15 @@ def _initialize_x_y(self, z):
             filled contours, the default is *True*.  For line contours,
             it is taken from rcParams['lines.antialiased'].
 
+          *nchunk*: [ 0 | integer ]
+            If 0, no subdivision of the domain.  Specify a positive integer to
+            divide the domain into subdomains of *nchunk* by *nchunk* quads.
+            Chunking reduces the maximum length of polygons generated by the
+            contouring algorithm which reduces the rendering workload passed
+            on to the backend and also requires slightly less RAM.  It can
+            however introduce rendering artifacts at chunk boundaries depending
+            on the backend, the *antialiased* flag and value of *alpha*.
+
         contour-only keyword arguments:
 
           *linewidths*: [ *None* | number | tuple of numbers ]
@@ -1774,13 +1826,6 @@ def _initialize_x_y(self, z):
 
         contourf-only keyword arguments:
 
-          *nchunk*: [ 0 | integer ]
-            If 0, no subdivision of the domain. Specify a positive integer to
-            divide the domain into subdomains of roughly *nchunk* by *nchunk*
-            points. This may never actually be advantageous, so this option may
-            be removed. Chunking introduces artifacts at the chunk boundaries
-            unless *antialiased* is *False*.
-
           *hatches*:
             A list of cross hatch patterns to use on the filled areas.
             If None, no hatching will be added to the contour.
@@ -1802,4 +1847,6 @@ def _initialize_x_y(self, z):
         .. plot:: mpl_examples/pylab_examples/contour_demo.py
 
         .. plot:: mpl_examples/pylab_examples/contourf_demo.py
+
+        .. plot:: mpl_examples/pylab_examples/contour_corner_mask.py
         """
diff --git a/lib/matplotlib/rcsetup.py b/lib/matplotlib/rcsetup.py
@@ -389,6 +389,13 @@ def validate_negative_linestyle_legacy(s):
         return (0, dashes)  # (offset, (solid, blank))
 
 
+def validate_corner_mask(s):
+    if s == 'legacy':
+        return s
+    else:
+        return validate_bool(s)
+
+
 def validate_tkpythoninspect(s):
     # Introduced 2010/07/05
     warnings.warn("tk.pythoninspect is obsolete, and has no effect")
@@ -589,8 +596,10 @@ def __call__(self, s):
     'image.origin':        ['upper', six.text_type],  # lookup table
     'image.resample':      [False, validate_bool],
 
+    # contour props
     'contour.negative_linestyle': ['dashed',
                                     validate_negative_linestyle_legacy],
+    'contour.corner_mask':        [True, validate_corner_mask],
 
     # axes props
     'axes.axisbelow':        [False, validate_bool],

diff --git a/lib/matplotlib/tests/baseline_images/test_contour/contour_corner_mask_False.png b/lib/matplotlib/tests/baseline_images/test_contour/contour_corner_mask_False.png
diff --git a/lib/matplotlib/tests/baseline_images/test_contour/contour_corner_mask_True.png b/lib/matplotlib/tests/baseline_images/test_contour/contour_corner_mask_True.png