Grayscale PNGs are rendered incorrectly

[narfg]

Servo renders grayscale PNGs incorrectly.
Both images with and without alpha channel are broken.
Test e.g. with the German wikipedia logo:
./servo http://upload.wikimedia.org/wikipedia/commons/e/ec/Wikipedia-logo-v2-de.png

The problem doesn't occur with RGB images.
I experience this problem with CPU and GPU rendering and the skia backend.

I tried to find the source of this problem and found these locations which may be helpful for someone trying to fix the bug.

The bug may be in src/components/gfx/render_context.rs draw_image
let azure_surface = draw_target_ref.create_source_surface_from_data(image.pixels.as_slice(), size, stride as i32, B8G8R8A8);
where it might be possible to create a grayscale surface instead of B8G8R8A8 (I don't know if this is supported).

Another possible location may be

src/support/azure/rust-azure/src/gfx/2d/SourceSurfaceSkia.cpp
SourceSurfaceSkia::InitFromData

where it seems to be assumed that the input image and the surface have the same color format.

Of course there are other ways to fix this, like converting the image to RGB in the decoding step but this may be wasteful in terms of memory.
Maybe someone with experience in Azure can take a look at this or file a bug report in Azure if this is not servo's fault.

[SimonSapin]

So the problem apparently is that we’re accessing the pixels without checking the image’s color type. Forcing us to do so would help prevent this kind of bug. Enums are designed to do just that: servo/rust-png#42

[mbrubeck]

Fixed by servo/rust-png#45.