[poppler] Implementing overprint in Splash

[Thomas Freitag]

Hi all!

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, 
please read 
http://www.adobe.com/content/dam/Adobe/en/devnet/pdf/pdfs/PDF32000_2008.pdf, 
chapter 8.6.7.

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?

Thomas