Fixed background colour of PNGs saved with a non-zero opacity.

Author: pelson

doc/users/whats_new.rst

@@ -166,6 +166,11 @@ Multiple images on same axes are correctly transparent
 When putting multiple images onto the same axes, the background color
 of the axes will now show through correctly.
 
+Full control of the background color
+------------------------------------
+Phil Elson fixed the Agg backend such that PNGs are now saved with the correct
+background color when :meth:`fig.patch.get_alpha` is not 1.
+
 .. _whats-new-1-2:
 
 new in matplotlib-1.2

lib/matplotlib/backends/backend_agg.py

@@ -16,9 +16,8 @@
 
 TODO:
 
-  * allow save to file handle
-
   * integrate screen dpi w/ ppi and text
+
 """
 from __future__ import division
 import threading
@@ -438,7 +437,7 @@ def draw(self):
         """
         if __debug__: verbose.report('FigureCanvasAgg.draw', 'debug-annoying')
 
-        self.renderer = self.get_renderer()
+        self.renderer = self.get_renderer(cleared=True)
         # acquire a lock on the shared font cache
         RendererAgg.lock.acquire()
 
@@ -449,7 +448,7 @@ def draw(self):
 
 
 
-    def get_renderer(self):
+    def get_renderer(self, cleared=False):
         l, b, w, h = self.figure.bbox.bounds
         key = w, h, self.figure.dpi
         try: self._lastKey, self.renderer
@@ -459,6 +458,8 @@ def get_renderer(self):
         if need_new_renderer:
             self.renderer = RendererAgg(w, h, self.figure.dpi)
             self._lastKey = key
+        elif cleared:
+            self.renderer.clear()
         return self.renderer
 
     def tostring_rgb(self):

lib/matplotlib/backends/backend_pdf.py

@@ -436,7 +436,7 @@ def __init__(self, filename):
 
         revision = ''
         self.infoDict = {
-            'Creator': 'matplotlib %s, http://matplotlib.sf.net' % __version__,
+            'Creator': 'matplotlib %s, http://matplotlib.org' % __version__,
             'Producer': 'matplotlib pdf backend%s' % revision,
             'CreationDate': datetime.today()
             }

lib/matplotlib/tests/baseline_images/test_figure/alpha_background.png

@@ -1,6 +1,50 @@
 from __future__ import print_function
 
 import os
+import tempfile
+
+
+from numpy.testing import assert_array_almost_equal
+
+
+from matplotlib.image import imread
+from matplotlib.backends.backend_agg import FigureCanvasAgg as FigureCanvas
+from matplotlib.figure import Figure
+from matplotlib.testing.decorators import cleanup
+
+
+@cleanup
+def test_repeated_save_with_alpha():
+    # We want an image which has a background color of blue, with an
+    # alpha of 0.25.
+
+    fig = Figure([1, 0.4])
+    canvas = FigureCanvas(fig)
+    fig.set_facecolor((0, 0, 0.4))
+    fig.patch.set_alpha(0.25)
+
+    # The target color is fig.patch.get_facecolor()
+
+    _, img_fname = tempfile.mkstemp(suffix='.png')
+    try:
+        fig.savefig(img_fname,
+                    facecolor=fig.get_facecolor(),
+                    edgecolor='none')
+
+        # Save the figure again to check that the
+        # colors don't bleed from the previous renderer.
+        fig.savefig(img_fname,
+                    facecolor=fig.get_facecolor(),
+                    edgecolor='none')
+
+        # Check the first pixel has the desired color & alpha
+        # (approx: 0, 0, 0.1, 0.25)
+        assert_array_almost_equal(tuple(imread(img_fname)[0, 0]),
+                                  (0.0, 0.0, 0.098, 0.250),
+                                  decimal=3)
+    finally:
+        os.remove(img_fname)
+
 
 def report_memory(i):
     pid = os.getpid()
@@ -64,3 +108,8 @@ def report_memory(i):
 ##     # w/o text and w/o write_png: Average memory consumed per loop: 0.02
 ##     # w/o text and w/ write_png : Average memory consumed per loop: 0.3400
 ##     # w/ text and w/ write_png  : Average memory consumed per loop: 0.32
+
+
+if __name__ == "__main__":
+    import nose
+    nose.runmodule(argv=['-s', '--with-doctest'], exit=False)

lib/matplotlib/tests/baseline_images/test_figure/alpha_background.svg

@@ -74,6 +74,22 @@ def test_suptitle():
     fig.suptitle('title', color='g', rotation='30')
 
 
+@image_comparison(baseline_images=['alpha_background'],
+                  extensions=['png', 'svg'], # PDF is wrong
+                  savefig_kwarg={'facecolor': 'black', 'edgecolor': 'none'})
+def test_alpha():
+    # We want an image which has a background color of black, with an
+    # alpha of 0.3.
+    fig = plt.figure(figsize=[2, 1])
+    fig.set_facecolor('black')
+    fig.patch.set_alpha(0.3)
+
+    import matplotlib.patches as mpatches
+    fig.patches.append(mpatches.CirclePolygon([20, 20],
+                                              radius=15,
+                                              facecolor='red'))
+
+
 if __name__ == "__main__":
     import nose
     nose.runmodule(argv=['-s', '--with-doctest'], exit=False)

lib/matplotlib/tests/test_agg.py

@@ -411,7 +411,7 @@ RendererAgg::RendererAgg(unsigned int width, unsigned int height, double dpi,
     renderingBuffer.attach(pixBuffer, width, height, stride);
     pixFmt.attach(renderingBuffer);
     rendererBase.attach(pixFmt);
-    rendererBase.clear(agg::rgba(1, 1, 1, 0));
+    rendererBase.clear(agg::rgba(0, 0, 0, 0));
     rendererAA.attach(rendererBase);
     rendererBin.attach(rendererBase);
     hatchRenderingBuffer.attach(hatchBuffer, HATCH_SIZE, HATCH_SIZE,
@@ -2388,7 +2388,7 @@ RendererAgg::clear(const Py::Tuple& args)
     _VERBOSE("RendererAgg::clear");
 
     args.verify_length(0);
-    rendererBase.clear(agg::rgba(1, 1, 1, 0));
+    rendererBase.clear(agg::rgba(0, 0, 0, 0));
 
     return Py::Object();
 }