[gs-bugs] [Bug 691494] New: Clipped glyphs on display when TextAlphaBits = 2 or 4

bugzilla-daemon at ghostscript.com bugzilla-daemon at ghostscript.com
Sat Jul 24 12:52:54 UTC 2010


http://bugs.ghostscript.com/show_bug.cgi?id=691494

           Summary: Clipped glyphs on display when TextAlphaBits = 2 or 4
           Product: Ghostscript
           Version: HEAD
          Platform: PC
        OS/Version: Windows XP
            Status: NEW
          Severity: normal
          Priority: P4
         Component: Color
        AssignedTo: michael.vrhel at artifex.com
        ReportedBy: sags5495 at hotmail.com
         QAContact: gs-bugs at ghostscript.com
   Estimated Hours: 0.0


When running GS to display a PostScript file on screen with antialiased 
text (-dTextAlphaBits=2 or 4), glyphs are clipped so that only their 
leftmost part is shown. The resolution used makes a difference. I used a 
debug build of the current HEAD (rev 11540); COMPILE_INITS=0, FT_BRIDGE=0 
or 1; physical display set to 32bpp.

when using Ghostscript via GSView things are even worse, in that glyph 
fragments are show with some reddish/ yellowish pixels at their right.

I’ll attach a sample file, screenshots, commandline, and a suggested patch.

I found the first revision to exhibit this bug to be 11362 
<http://svn.ghostscript.com/viewvc?view=rev&revision=11362>. While I do 
consider that patch to be wrong (bug #691328 comment #2), it does not seem 
to be the root cause of the problem, it merely uncovered a very old bug.

As I see it, the root cause is in gdevdbit.c::gx_default_copy_alpha(). The 
depth of the source bitmap does not necessarily equal the one of the target 
device. The function handles this case, but when outputting a scanline it 
passes ‘raster’ as the scanline width in bytes. This value is correct for 
the source colour depth, but not for the same number of pixels at the 
device’s/ scanline’s colour depth.

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