Bug #68508 for Imager: error diffusion dither confusing with grey maps

# Fri May 27 08:44:38 2011 TONYC [...] cpan.org - Ticket created

Subject:	error diffusion dither broken
Date:	Fri, 27 May 2011 22:44:27 +1000
To:	bug-Imager [...] rt.cpan.org
From:	tonyc [...] cpan.org

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Translating an image with the error diffusion dither doesn't appear to be handled correctly. perl -MImager -MImager::Test=test_image -e '$im = test_image(); $im->write(file => "mono.gif", make_colors => "mono", translate => "errdiff") or die $im->errstr' produces a mostly black image, but there should be some shading over the green/red/blue filled areas.

# Mon May 30 04:07:16 2011 TONYC [...] cpan.org - Subject changed from 'error diffusion dither broken' to 'error diffusion dither confusing with grey maps'

# Mon May 30 04:07:16 2011 TONYC [...] cpan.org - Severity Normal added

# Mon May 30 04:07:16 2011 TONYC [...] cpan.org - Broken in 0.83 added

# Mon May 30 04:21:09 2011 TONYC [...] cpan.org - Correspondence added

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The problem is the error only accumulates on the red channel, and since the channel is capped at 255, the black entry continues to match for the entire image. As a partial solution we can process the image as grayscale when the supplied palette is grayscale. This isn't a perfect solution, since the caller can still provide a palette that doesn't cover the gamut used in the image, but it does handle a common case.

# Mon May 30 04:21:10 2011 TONYC [...] cpan.org - Status changed from 'new' to 'open'

# Mon May 30 04:26:56 2011 TONYC [...] cpan.org - Correspondence added

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Note: the conversion to greyscale isn't done for the other colour translators since they do no error propagation, so the current algorithm is enough.

# Fri Jun 03 02:22:45 2011 TONYC [...] cpan.org - Status changed from 'open' to 'patched'

# Tue Jun 21 09:44:13 2011 TONYC [...] cpan.org - Correspondence added

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Fixed about as well as it can be in 0.84.

# Tue Jun 21 09:44:13 2011 The RT System itself - Status changed from 'patched' to 'open'

# Tue Jun 21 09:44:13 2011 TONYC [...] cpan.org - Status changed from 'open' to 'resolved'

# Tue Jun 21 09:44:27 2011 TONYC [...] cpan.org - Fixed in 0.84 added