Switch agg backend to use pixfmt_rgba32_plain instead of pixfmt_rgba32, which fixes a problem with alpha-blending partially transparent backgrounds.

Author: Westacular

doc/users/whats_new.rst

@@ -148,6 +148,13 @@ default quality is 95; previously, the default quality was 75.  This change
 minimizes the artifacting inherent in JPEG images, particularly with images
 that have sharp changes in color as plots often do.
 
+Full control of the background color
+------------------------------------
+Wes Campaigne and 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-1-2-2:
 
 new in matplotlib 1.2.2
@@ -166,11 +173,6 @@ 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/tests/baseline_images/test_figure/alpha_background.png

@@ -15,12 +15,12 @@
 
 @cleanup
 def test_repeated_save_with_alpha():
-    # We want an image which has a background color of blue, with an
+    # We want an image which has a background color of bluish green, with an
     # alpha of 0.25.
 
     fig = Figure([1, 0.4])
     canvas = FigureCanvas(fig)
-    fig.set_facecolor((0, 0, 0.4))
+    fig.set_facecolor((0, 1, 0.4))
     fig.patch.set_alpha(0.25)
 
     # The target color is fig.patch.get_facecolor()
@@ -38,9 +38,9 @@ def test_repeated_save_with_alpha():
                     edgecolor='none')
 
         # Check the first pixel has the desired color & alpha
-        # (approx: 0, 0, 0.1, 0.25)
+        # (approx: 0, 1.0, 0.4, 0.25)
         assert_array_almost_equal(tuple(imread(img_fname)[0, 0]),
-                                  (0.0, 0.0, 0.098, 0.250),
+                                  (0.0, 1.0, 0.4, 0.250),
                                   decimal=3)
     finally:
         os.remove(img_fname)

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

@@ -75,18 +75,21 @@ def test_suptitle():
 
 
 @image_comparison(baseline_images=['alpha_background'],
-                  extensions=['png', 'svg'], # PDF is wrong
-                  savefig_kwarg={'facecolor': 'black', 'edgecolor': 'none'})
+                  # only test png and svg. The PDF output appears correct,
+                  # but Ghostscript does not preserve the background color.
+                  extensions=['png', 'svg'],
+                  savefig_kwarg={'facecolor': (0, 1, 0.4), 'edgecolor': 'none'})
 def test_alpha():
-    # We want an image which has a background color of black, with an
-    # alpha of 0.3.
+    # We want an image which has a background color and an
+    # alpha of 0.4.
     fig = plt.figure(figsize=[2, 1])
-    fig.set_facecolor('black')
-    fig.patch.set_alpha(0.3)
+    fig.set_facecolor((0, 1, 0.4))
+    fig.patch.set_alpha(0.4)
 
     import matplotlib.patches as mpatches
     fig.patches.append(mpatches.CirclePolygon([20, 20],
                                               radius=15,
+                                              alpha=0.6,
                                               facecolor='red'))
 
 

lib/matplotlib/tests/test_agg.py

@@ -1108,7 +1108,7 @@ RendererAgg::draw_image(const Py::Tuple& args)
         inv_mtx.invert();
 
         typedef agg::span_allocator<agg::rgba8> color_span_alloc_type;
-        typedef agg::image_accessor_clip<agg::pixfmt_rgba32>
+        typedef agg::image_accessor_clip<agg::pixfmt_rgba32_plain>
             image_accessor_type;
         typedef agg::span_interpolator_linear<> interpolator_type;
         typedef agg::span_image_filter_rgba_nn<image_accessor_type,

lib/matplotlib/tests/test_figure.py

@@ -53,7 +53,7 @@
 
 const size_t NUM_VERTICES[] = { 1, 1, 1, 2, 3, 1 };
 
-typedef agg::pixfmt_rgba32 pixfmt;
+typedef agg::pixfmt_rgba32_plain pixfmt;
 typedef agg::renderer_base<pixfmt> renderer_base;
 typedef agg::renderer_scanline_aa_solid<renderer_base> renderer_aa;
 typedef agg::renderer_scanline_bin_solid<renderer_base> renderer_bin;

src/_backend_agg.cpp

@@ -33,7 +33,7 @@
 #include "mplutils.h"
 
 
-typedef agg::pixfmt_rgba32 pixfmt;
+typedef agg::pixfmt_rgba32_plain pixfmt;
 typedef agg::pixfmt_rgba32_pre pixfmt_pre;
 typedef agg::renderer_base<pixfmt> renderer_base;
 typedef agg::span_interpolator_linear<> interpolator_type;