[poppler] Implementing overprint in Splash
Thomas.Freitag at kabelmail.de
Mon Mar 21 01:58:35 PDT 2011
After being quite close with my last bigger poppler project I now think
about implementing overprinting in Splash.
1. What is overprinting?
For those who don't know what is overprinting, please refer to
http://en.wikipedia.org/wiki/Overprinting. This is just a poor
description, what overprinting is. Who need to know more about it,
2. Actual state of overprinting in poppler.
Overprinting is implemented in Gfx, only missing the parameter overprint
mode, but the only output device which supports it in the moment, is
PSOutputDev. But even there probably the most people haven't encountered
that really: If You render the PostScript produced with PSOutputDev with
ghostscript, it is normally rendered in RGB, an additive colorspace. And
because overprinting is normally specified only in subtractive
colorspaces like CMYK, You have to specify a cmyk device like tiff32nc
to see the effects of overprinting.
3. Implementing overprint in Splash
It is quite easy to implement overprinting in Splash. Splash supports
CMYK output with the compiler switch SPLASH_CMYK. But as in PSOutputDev
I fear, that only a few people use it. And the mainly used program
pdftoppm doesn't support CMYK output. And now we are coming to the main
point why I write this email:
4. Support of overprint in pdftoppm
To support overprint in pdftoppm we have to enable SPLASH_CMYK in
pdftoppm and use it for rendering. But all output formats defined in
pdftoppm uses RGB as output colorspace, and even the main output formats
ppm and png do not support CMYK colorspace. Therefore we have to
possibilities to support overprinting in pdftoppm:
a) The easiest way would be to specify a new output format like i.e.
jpegcmyk and create a jpeg image in CMYK colorspace where overprinting
will be supported.
b) The more interesting way is to add a new parameter -overprint, when
set use splashCMYK8 as colorMode and when writing the output file
convert it to RGB. The first implementation could use the poor
colorspace conversion in Splash to convert the CMYK bitmap to RGB, but
we should think of use little cms to do that work for us, which of
course means that compiling pdftoppm will become more complex.
Any suggestions from the community to point 4?
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