[Poppler-bugs] [Bug 68986] pdftoppm doesn't render eci_altona-test-suite-v2_technical2_x4.pdf correctly

[bugzilla-daemon at freedesktop.org]

https://bugs.freedesktop.org/show_bug.cgi?id=68986

--- Comment #1 from Thomas Freitag <Thomas.Freitag at alfa.de> ---
I found the reason for the regression in test "C" (patch "C" in altona
notation): The transparency group with knockout has two subgroups with blend
mode multiply, the first one draws "TEXT TO", the second one draws "KNOCK OUT".
The blend mode multiply says that we have to multiply the current color with
the backdrop colour, and my implementation of knockout in bug 12185 multiplies
in case of "KNOCK OUT" the blue current color with the darkened blue of "TEXT
TO" where it overlaps, but the enclosing knockout transparency group forces
that the "TEXT TO" has to be removed first where it overlaps and the current
color has to be multiplied with the former pink background color again.

After days of working and thinking about it I find now a solution, it was
really complicate to solve it.

BTW, also the cairo backend has big problems with it: it ignores the blend mode
completely and in the overlapping region the resulting color becomes white. But
I myself can only give additional hints, I'm not able to solve it in the cairo
backend.

The patch I'll upload in a few minutes will unfortunately only solve the
knockout problem, all the other tests in the altona PDF still fails :-(

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