Re: PNG image files with alpha channel

[Chong Yidong]

Miles Bader <address@hidden> writes:

> Upon looking closer, I see on very odd attribute:  while the original
> image I wrote was RGBA, 8-bits per channel, the decimated versions
> written by imagemagick are RGBA, 16-bits per channel...  The Emacs
> PNG-reading code asks libpng to convert 16-bit-per-channel images to
> 8-bits per channel, but as such images are fairly rare, it may be that
> libpng doesn't do it correctly.
>
> If I open the original RGBA 8-bits per channel image with Emacs, indeed,
> it handles the alpha-channel correctly.

It looks like Emacs doesn't handle 16-bit RGBA png images correctly,
according to the images on this page:

http://www.fnordware.com/superpng/pnggrad16rgba.png
http://www.fnordware.com/superpng/pngtest16rgba.png

The 8-bit RGBA test image on the same page works fine:

http://www.fnordware.com/superpng/pngtest8rgba.png

The weird thing, though, is that the image emacs5-256.png which you
sent, whose alpha channel is also much up, is 8-bit!  This can be
verified by stepping through image.c in gdb, or using pngtools.