backport image-filters/alpha fix to 2.3.x branch - refs #1313

include/mapnik/image_filter.hpp

@@ -394,8 +394,12 @@ void apply_convolution_3x3(Src const& src_view, Dst & dst_view, Filter const& fi
 template <typename Src, typename Filter>
 void apply_filter(Src & src, Filter const& filter)
 {
-    double_buffer<Src> tb(src);
-    apply_convolution_3x3(tb.src_view, tb.dst_view, filter);
+    {
+        src.demultiply();
+        double_buffer<Src> tb(src);
+        apply_convolution_3x3(tb.src_view, tb.dst_view, filter);
+    } // ensure ~double_buffer() is called before premultiplying
+    src.premultiply();
 }
 
 template <typename Src>

tests/python_tests/image_filters_test.py

@@ -49,18 +49,18 @@ def test_style_level_image_filter():
             mapnik.render(m, im)
             actual = '/tmp/mapnik-style-image-filter-' + filename + '.png'
             expected = 'images/style-image-filter/' + filename + '.png'
-            im.save(actual)
+            im.save(actual,"png32")
             if not os.path.exists(expected):
                 print 'generating expected test image: %s' % expected
-                im.save(expected)
+                im.save(expected,'png32')
             expected_im = mapnik.Image.open(expected)
             # compare them
-            if im.tostring() == expected_im.tostring():
+            if im.tostring('png32') == expected_im.tostring('png32'):
                 successes.append(name)
             else:
                 fails.append('failed comparing actual (%s) and expected(%s)' % (actual,'tests/python_tests/'+ expected))
                 fail_im = side_by_side_image(expected_im, im)
-                fail_im.save('/tmp/mapnik-style-image-filter-' + filename + '.fail.png')
+                fail_im.save('/tmp/mapnik-style-image-filter-' + filename + '.fail.png','png32')
         eq_(len(fails), 0, '\n'+'\n'.join(fails))
 
 if __name__ == "__main__":