[gs-bugs] [Bug 691900] New: pdf14 compositor should detect transparent colours better

[bugzilla-daemon at ghostscript.com]

http://bugs.ghostscript.com/show_bug.cgi?id=691900

           Summary: pdf14 compositor should detect transparent colours
                    better
           Product: Ghostscript
           Version: HEAD
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: enhancement
          Priority: P4
         Component: Graphics Library
        AssignedTo: support at artifex.com
        ReportedBy: robin.watts at artifex.com
         QAContact: gs-bugs at ghostscript.com
   Estimated Hours: 0.0


To fix Bug 691897, revision 12038 was committed; a small tweak to ensure that
BLEND_MODE_Compatible and BLEND_MODE_Normal were always treated the same. This
exposed a bug in the handling of a quality logic file
PDF_1.7_FTS/fts_08_0815.pdf.

A shading that included transparency (a softmask, I believe) was used to fill a
path. Because this shading was plotted as a solid colour using the Normal blend
mode, update_lop_for_pdf14 decided that it was safe not to set the pdf14 lop
bit. This meant that the stroke code stroked each segment of the path
separately, resulting in overrendering where the segments overlap on non solid
sections of the path.

The fix (committed as revision 12039) is to modify update_lop_for_pdf14 so that
it checks the current color for transparency, and sets the bit if any is found.

This works well for type 1 patterns, but the test is rather blunt for type 2
patterns; currently all type 2 patterns are held to be transparent.

It was suggested that I look at the 'has_transparency' field of the saved
pattern instance, but this is (almost) always set to true before
update_lop_for_pdf14 is called, so wouldn't help.

The code now gives the correct result, but could be enhanced with a better test
to allow the fast code to be used in more instances.

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