It turns out that cairo_image_surface_create_for_data with the image format CAIRO_FORMAT_ARGB32 returns pixels with the alpha premultiplied, but ImageMagick was treating them as if they weren't premultiplied.
Here's a fix. Not sure if it's the cleanest, but if it's using MAGICKCORE_CAIRO_DELEGATE it will unpremultiply the pixels. It's made from trunk. I'd hope to get this or something to the same effect upstream because it would fix a lot of issues we're having.
Code: Select all
diff --git a/coders/svg.c b/coders/svg.c
index a68dec0..8a5eca5 100644
--- a/coders/svg.c
+++ b/coders/svg.c
@@ -2852,6 +2852,17 @@ static Image *ReadSVGImage(const ImageInfo *image_info,ExceptionInfo *exception)
fill_color.blue=ScaleCharToQuantum(*p++);
#endif
fill_color.opacity=QuantumRange-ScaleCharToQuantum(*p++);
+
+/*
+ * If we're using Cairo, we've already gotten back an image that has premultiplied alpha
+ * (see CAIRO_FORMAT_ARGB32 http://library.gnome.org/devel/cairo/stable/cairo-image-surface.html#cairo-format-t)
+ */
+#if defined(MAGICKCORE_CAIRO_DELEGATE)
+ fill_color.blue /= 1.0-QuantumScale*fill_color.opacity;
+ fill_color.green /= 1.0-QuantumScale*fill_color.opacity;
+ fill_color.red /= 1.0-QuantumScale*fill_color.opacity;
+#endif
+
MagickCompositeOver(&fill_color,fill_color.opacity,q,(MagickRealType)
q->opacity,q);
q++;
Thanks,
Mike Lewis
