Re: PNG image files with alpha channel

[Chong Yidong]

Miles Bader <address@hidden> writes:

> Is Emacs supposed to properly handle an alpha channel in a PNG file?

Yeah.  See the png_load function in image.c.

> I vaguely recall that it used to do something halfway reasonable (even
> if just merging in the default bg color), but now it seem to simply
> ignore an alpha channel.

It merges in the :background spec if there is one; otherwise, it uses
the frame background color.

> If you don't have an alpha-channel-bearing image, try this:
>
>    http://download.savannah.gnu.org/releases/emacs/icons/emacs5-256.png
>
> The "background" around the sphere has an alpha channel that's mostly zero.

Hmm, I'm not sure what's wrong with this image.  For an example of a png
image with alpha background rendered correctly in Emacs, see

http://www.w3.org/Graphics/PNG/alphatest.png