Legacy ImageMagick Discussions Archive

I have a PNG image that has only 4 colors and one is a white background. If I add an alpha channel to it, the histogram shows transparent white. If I use -unique colors, it shows transparent black.

Input:
http://www.fmwconcepts.com/misc_tests/u ... esign4.png

Code: Select all

convert design4.png -format "%c" histogram:info:

181054: ( 16, 3, 78) #10034E srgb(16,3,78)
51286: (178, 32, 24) #B22018 srgb(178,32,24)
1277: (178,211,239) #B2D3EF srgb(178,211,239)
565759: (255,255,255) #FFFFFF white

Code: Select all

convert design4.png -unique-colors txt:

0,0: (16,3,78) #10034E srgb(16,3,78)
1,0: (178,32,24) #B22018 srgb(178,32,24)
2,0: (178,211,239) #B2D3EF srgb(178,211,239)
3,0: (255,255,255) #FFFFFF white

Now if I add a binary alpha channel so that the white background is transparent, I get:

Code: Select all

convert design4.png \
\( -clone 0 -fill black +opaque white -negate \) \
-alpha off -compose copy_opacity -composite \
design4a.png

http://www.fmwconcepts.com/misc_tests/u ... sign4a.png

Code: Select all

convert design4a.png -format "%c" histogram:info:

181054: ( 16, 3, 78,255) #10034E srgba(16,3,78,1)
51286: (178, 32, 24,255) #B22018 srgba(178,32,24,1)
1277: (178,211,239,255) #B2D3EF srgba(178,211,239,1)
565759: (255,255,255, 0) #FFFFFF00 srgba(255,255,255,0)

Code: Select all

convert design4a.png -unique-colors txt:

0,0: (16,3,78,1) #10034E srgba(16,3,78,1)
1,0: (178,32,24,1) #B22018 srgba(178,32,24,1)
2,0: (178,211,239,1) #B2D3EF srgba(178,211,239,1)
3,0: (0,0,0,0) #00000000 none

[snip]

Forget my previous post, let me see if I can figure out whats wrong.

I think "-unique-colors" regards all fully-transparent pixels as the same colour, and equivalent to transparent black. However, it then doesn't correctly group these together. For example:

f:\web\im>%IM%convert xc:red xc:blue +append -alpha transparent txt:
# ImageMagick pixel enumeration: 2,1,65535,srgba
0,0: (100%,0%,0%,0) #FFFF000000000000 srgba(255,0,0,0)
1,0: (0%,0%,100%,0) #00000000FFFF0000 srgba(0,0,255,0)

f:\web\im>%IM%convert xc:red xc:blue +append -alpha transparent -unique-colors txt:
# ImageMagick pixel enumeration: 2,1,65535,srgba
0,0: (0%,0%,0%,0) #0000000000000000 none
1,0: (0%,0%,0%,0) #0000000000000000 none

I would expect the second result to have one pixel, not two.

We can reproduce the problem you posted and have a patch in ImageMagick 6.9.0-4 Beta, available by sometime tomorrow. Thanks.

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