Re: PNG image files with alpha channel

[Chong Yidong <cyd@stupidchicken.com>]

Miles Bader <miles@gnu.org> 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.
>
> I did step through the code, BTW, and as far as I can tell, Emacs is
> doing the correct things.
>
> So the problem may in fact be something with the libpng 16-to-8-bit
> conversion code...

One possibility is that libpng's png_set_strip_16 function only strips
the RGB channels down to 8 bits, while keeping the alpha channel at 16
bits.