From: Miles Bader <miles@gnu.org>
To: Jason Rumney <jasonr@gnu.org>
Cc: Chong Yidong <cyd@stupidchicken.com>, emacs-devel@gnu.org
Subject: Re: PNG image files with alpha channel
Date: Fri, 09 May 2008 00:39:27 +0900 [thread overview]
Message-ID: <87tzh8yfdc.fsf@catnip.gol.com> (raw)
In-Reply-To: <482319D6.1020308@gnu.org> (Jason Rumney's message of "Thu, 08 May 2008 16:18:46 +0100")
Jason Rumney <jasonr@gnu.org> writes:
>> png_set_IHDR (libpng_struct, libpng_info, width, height,
>> 8, PNG_COLOR_TYPE_RGB_ALPHA, PNG_INTERLACE_NONE,
>> PNG_COMPRESSION_TYPE_DEFAULT, PNG_FILTER_TYPE_DEFAULT);
>> png_set_gAMA (libpng_struct, libpng_info, gamma_correction);
>
> Is there any program that can dump a PNG header in some readable format,
> so we can compare yours with the image that Gimp saves?
>
> Or maybe someone needs to step through the PNG display code in Emacs,
> and see where it (or libpng) makes the wrong decision.
Actually I realized that while my software wrote the original image,
those "*-256.png", etc, images are decimated versions that I created
using imagemagick, with "convert -resize ...".
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...
[I have no idea why imagemagick creates 16-bit-per-channel output images
from 8-bit-per-channel input images; offhand it seems like a bad idea as
16-bit support is probably less widespread...]
-Miles
--
Idiot, n. A member of a large and powerful tribe whose influence in human
affairs has always been dominant and controlling.
next prev parent reply other threads:[~2008-05-08 15:39 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 [this message]
2008-05-08 15:51 ` Jason Rumney
2008-05-08 18:19 ` Chong Yidong
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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