Merge pull request matplotlib#4481 from efiring/rasterize_colorbar

Rasterize colorbar when it has many colors; closes matplotlib#4480

Author: WeatherGod

doc/users/whats_new/rasterized_colorbar.rst

@@ -0,0 +1,10 @@
+Dense colorbars are rasterized
+``````````````````````````````
+Vector file formats (pdf, ps, svg) are efficient for
+many types of plot element, but for some they can yield
+excessive file size and even rendering artifacts, depending
+on the renderer used for screen display.  This is a problem
+for colorbars that show a large number of shades, as is
+most commonly the case.  Now, if a colorbar is showing
+50 or more colors, it will be rasterized in vector
+backends.

lib/matplotlib/colorbar.py

@@ -256,6 +256,8 @@ class ColorbarBase(cm.ScalarMappable):
                    'min': slice(1, None),
                    'max': slice(0, -1)}
 
+    n_rasterize = 50  # rasterize solids if number of colors >= n_rasterize
+
     def __init__(self, ax, cmap=None,
                  norm=None,
                  alpha=None,
@@ -517,6 +519,8 @@ def _add_solids(self, X, Y, C):
                                     colors=(mpl.rcParams['axes.edgecolor'],),
                                     linewidths=linewidths)
             self.ax.add_collection(self.dividers)
+        elif len(self._y) >= self.n_rasterize:
+            self.solids.set_rasterized(True)
 
     def add_lines(self, levels, colors, linewidths, erase=True):
         '''