[gs-bugs] [Bug 689961] New: Radial gradation issue

[bugs.ghostscript.com-bugzilla-daemon at ghostscript.com]

http://bugs.ghostscript.com/show_bug.cgi?id=689961

           Summary: Radial gradation issue
           Product: Ghostscript
           Version: 8.60
          Platform: All
        OS/Version: All
            Status: NEW
          Severity: normal
          Priority: P4
         Component: Color
        AssignedTo: michael.vrhel at artifex.com
        ReportedBy: marcos.woehrmann at artifex.com
         QAContact: gs-bugs at ghostscript.com


I have a strange problem with gradations in certain cases.

The attached PDF file contains a single radial gradation, going from a pure magenta to a pure white. 
When I rip this file at 60 dpi with my CMYK device (and with my custom version of GS), and with Color 
Management ON (meaning that CMYK colors are converted to an output ICC profile, in the procedure 
registered by get_color_mapping_procs), the result is wrong. I  get a kind of triangulation effect, see 
the attached file "bad.tif".

Now, when I disable Color management (i.e. when the CMYK mapping proc does not modify the colors), 
then the result is correct (see the attached file "noicc.tif") - and identical to what tiff32nc produces.

This problems happens with both GS8.53 and GS8.60, but fortunately it's very rare. It depends of the 
file, of the resolution, and of the profiles used (e.g. if I  rip the same file at 600 dpi instead of 60, then 
there's no problem). I've seen it 3 or 4 times in the last months.

Of course, that could be a problem in my ICC color conversions, but for many reasons I think that's not 
the case. For example, when I apply the same ICC conversions in post-processing (i.e to the file 
"noicc.tif"), I obtain the correct result (attached file "ok.tif"). This is the one I should get directly from 
GS, normally.

Now, I have tried to reproduce the problem with tiff32nc in the standard GS, by modifying it's mapping 
proc procedures, and making them return the same CMYK values than in my test case. But that doesn't 
work. I have no idea why, but with tiff32nc the CMYK mapping proc is not called for the colors of the 
gradation (not even for the pure magenta). I've even tried to modify the default function 
(cmyk_cs_to_cmyk_cm(), in gxcmap.c), but with no more success.

So I hope you'll have an idea of what the problem could be by examining the attached files. Otherwise, I 
can retry with tiff32nc if you explain me why the color conversions are not called.



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