[gs-bugs] [Bug 691937] PDF fts_15_1517.pdf Shading different to Adobe

bugzilla-daemon at ghostscript.com bugzilla-daemon at ghostscript.com
Mon Feb 7 20:45:41 UTC 2011


http://bugs.ghostscript.com/show_bug.cgi?id=691937

--- Comment #4 from Robin Watts <robin.watts at artifex.com> 2011-02-07 20:45:38 UTC ---
The short answer is that the test file is broken.

The file defines a Type 5 shading; in this case a 3x3 grid, divided into
triangles. If the points in the grid are 0 to 9, the grid might be visualised
as:


 0-----1-----2
 |    /|    /|
 |  /  |  /  |
 |/    |/    |
 3-----4-----5
 |    /|    /|
 |  /  |  /  |
 |/    |/    |
 6-----7-----8

To achieve this, they should send the grid points in the order: 0 1 2 3 4 5
(for the top row) then 3 4 5 6 7 8 (for the bottom row).

Instead, the file actually sends: 0 3 1 4 2 5 (for the top row) and 3 6 4 7 5 8
(for the bottom row).

This results in a 'twisted' grid where points on the target are covered by
multiple triangles. In this case, as there is no ordering placed upon the
triangles, it is undefined what color will be given. Thus ghostscript and
acrobat give different results.

I will attempt to construct a 'fixed' version of the file to verify that gs and
acrobat give the same results on a well formed input.

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