Re: [sdl.web@gmail.com: Re: problem with transparent PNG image display]

[Jason Rumney <jasonr@gnu.org>]

Richard Stallman wrote:
> [I sent this message a few weeks ago but did not get a response.]
>
> Would someone please debug this, then ack?
>   
> For example the attached icon has transparent background but shows a
> white background in Emacs.
>   
Someone investigated before the release, and decided that the image
caching code needed overhauling, so this should be done in the trunk.

Because we cache images in their uncompressed and rendered form, we
cannot support full transparency for png images. So we use the
background color that is saved in the image itself to render the image.
When no background color is saved in the image, we render using the
background color of the face the image is first drawn with, so the
problem is not immediately apparent with those images, but if you draw
the same image object onto a different colored background, you will see
the same problem occur.

I don't know which platforms will let us cache image objects with
transparency, but the solution would be to do that, and draw the
background separately each time the image is drawn.