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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 21:00:28 -0400	[thread overview]
Message-ID: <87ej8cnvf7.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.

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.




  parent reply	other threads:[~2008-05-09  1:00 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
2008-05-09  1:00           ` Chong Yidong [this message]
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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