[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 #18 from Thomas Freitag <Thomas.Freitag at alfa.de> ---
Created attachment 97905
  --> https://bugs.freedesktop.org/attachment.cgi?id=97905&action=edit
wrong output for patch "N"

The complete wrong output for patch "N" if compiled with SPLASH_CMYK and using
the overprint parameter for pdftoppm is caused by the fact that in this case
DeviceN is used and the blend functions completely erase background with spot
colors.
BTW, if You run pdftoppm without the overprint parameter, You'll get a better
output, BUT: the output is shown in 
http://www.eci.org/_media/de/eci_altona-test-suite-v2_technical2_documentation_eng-4.pdf
as a sample for incorrect output. The reason for the incorrect output is that
the spot color orange is first converted to its RGB components and then the
blend function is applied. This is definitely wrong on separable blend modes.
The PDF spec says:

When performing blending operations in subtractive colour spaces (DeviceCMYK,
Separation, and DeviceN), the colour component values shall be complemented
(subtracted from 1.0) before the blend function is applied and the results of
the function shall then be complemented back before being used.

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