Hello IMers.
I have a 32x32 pixel RGBA image (in TGA format). It contains 4 colors,
namely:
Colors: 4
Histogram:
528: ( 74, 0, 61,212) #4A003DD4 rgba(74,0,61,0.831373)
445: ( 74, 0, 61,211) #4A003DD3 rgba(74,0,61,0.827451)
48: ( 74, 0, 61,210) #4A003DD2 rgba(74,0,61,0.823529)
3: ( 74, 0, 61,209) #4A003DD1 rgba(74,0,61,0.819608)
If I do "convert -colors 256 -type TrueColorMatte input.tga output.tga",
then I'd expect output.tga to look at least very similar to input.tga.
In a perfect world they'd be pixel-perfect copies. As it is, I'm
getting some unexpected behaviour.
On IM 6.3.4 06/14/07 Q16 (Windows), it zeroed the RGB (but not A) parts
of the first color, leaving me with:
Colors: 4
Histogram:
528: ( 0, 0, 0,212) #000000D4 rgba(0,0,0,0.831373)
445: ( 74, 0, 61,211) #4A003DD3 rgba(74,0,61,0.827451)
48: ( 74, 0, 61,210) #4A003DD2 rgba(74,0,61,0.823529)
3: ( 74, 0, 61,209) #4A003DD1 rgba(74,0,61,0.819608)
I downloaded the latest IM (6.3.5 07/28/07), and interestingly I got a
different result. It did the same as 6.3.4, and also threw away one of
the other colors:
Colors: 3
Histogram:
528: ( 74, 0, 61,211) #4A003DD3 rgba(74,0,61,0.827451)
489: ( 0, 0, 0,212) #000000D4 rgba(0,0,0,0.831373)
7: ( 74, 0, 61,209) #4A003DD1 rgba(74,0,61,0.819608)
Here are some relevant points:
I've previously read and understood the information at
http://www.imagemagick.org/Usage/quantize/#quantize_not_exact, about
'pure' colors not being preserved, but I don't think my image falls into
that category.
I know that the command I'm running looks a bit silly in this context.
However, the image I'm dealing with is just one of a huge set that I'm
iterating over. Some (most) of them have more than 256 colors.
I don't think the file format is relevant. I tried converting the file
to a PNG beforehand and got the same result.
I can send the image to anybody who is interested. It's only 4K, but I
know that attachments to mailing lists can be unpopular.
Is the behaviour I'm seeing a bug, or an unavoidable part of the
quantization process?
Thanks in advance for any pointers,
Peter Pimley
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