From: Chong Yidong <cyd@stupidchicken.com>
To: Miles Bader <miles@gnu.org>
Cc: emacs-devel@gnu.org, Jason Rumney <jasonr@gnu.org>
Subject: Re: PNG image files with alpha channel
Date: Thu, 08 May 2008 14:19:56 -0400 [thread overview]
Message-ID: <87od7gu08j.fsf@stupidchicken.com> (raw)
In-Reply-To: <87tzh8yfdc.fsf@catnip.gol.com> (Miles Bader's message of "Fri, 09 May 2008 00:39:27 +0900")
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.
next prev parent reply other threads:[~2008-05-08 18:19 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2008-05-08 4:46 PNG image files with alpha channel Miles Bader
2008-05-08 11:48 ` Chong Yidong
2008-05-08 12:05 ` Jason Rumney
2008-05-08 15:02 ` Miles Bader
2008-05-08 15:18 ` Jason Rumney
2008-05-08 15:39 ` Miles Bader
2008-05-08 15:51 ` Jason Rumney
2008-05-08 18:19 ` Chong Yidong [this message]
2008-05-09 1:00 ` Chong Yidong
2008-05-09 1:10 ` Miles Bader
2008-05-09 2:35 ` Chong Yidong
2008-05-08 23:47 ` James Cloos
2008-05-09 0:43 ` Miles Bader
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