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Re: PNG image files with alpha channel
From: |
Chong Yidong |
Subject: |
Re: PNG image files with alpha channel |
Date: |
Thu, 08 May 2008 14:19:56 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.0.60 (gnu/linux) |
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.
>
> 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.
- PNG image files with alpha channel, Miles Bader, 2008/05/08
- Re: PNG image files with alpha channel, Chong Yidong, 2008/05/08
- Re: PNG image files with alpha channel, Jason Rumney, 2008/05/08
- Re: PNG image files with alpha channel, Miles Bader, 2008/05/08
- Re: PNG image files with alpha channel, Jason Rumney, 2008/05/08
- Re: PNG image files with alpha channel, Miles Bader, 2008/05/08
- Re: PNG image files with alpha channel, Jason Rumney, 2008/05/08
- Re: PNG image files with alpha channel,
Chong Yidong <=
- Re: PNG image files with alpha channel, Chong Yidong, 2008/05/08
- Re: PNG image files with alpha channel, Miles Bader, 2008/05/08
- Re: PNG image files with alpha channel, Chong Yidong, 2008/05/08
- Re: PNG image files with alpha channel, James Cloos, 2008/05/08
- Re: PNG image files with alpha channel, Miles Bader, 2008/05/08