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[Octave-bug-tracker] [bug #41584] .bmp / .png display error for saturate

From: Carnë Draug
Subject: [Octave-bug-tracker] [bug #41584] .bmp / .png display error for saturated RGB image.
Date: Thu, 13 Feb 2014 23:39:33 +0000
User-agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20131030 Firefox/17.0 Iceweasel/17.0.10

Follow-up Comment #2, bug #41584 (project octave):

Mike, I can replicate the problem on my system.

imread() returns a 128x128x3 logical matrix on my system which imshow() fails
to display.

I don't know if the quantum depth used with GraphicsMagick (GM) could make a
difference (I have it built with quantum-depth 32).

The gm application identifies the image correctly has 8bit

$ gm identify test.bmp
test.bmp BMP 128x128+0+0 DirectClass 8-bit 48.1K 0.000u 0:01

I believe the problem may be with get_depth() in __magick_read__.cc which
tries to workaround some wrong results of GM (there's a big block of comments
there trying to explain the issue).

The issue is that GM is not in fault here. What they return as depth,
channelDepth, classType, etc is not meant to be interpreted as the way the
image was represented in the file, but the way GM is keeping it in memory. If
an image has few colors, GM may decide to interpret it as indexed, if all
channels are equal, it may decide to keep it grayscale, etc. They don't have
methods that will reliable tell us what was in the file.


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