First, let me say that having more PDF renderers support Overprint Preview (since it's really a simulation and not the real thing) will be GREAT! Especially so in light of the "conforming reader" requirements in ISO 32000-2 that are forthcoming.
Now, that said, I think you may be simplifying things for #3 - the actual implementation. It's not as simple as modifying the code that puts the CMYK "bits" down (based on the OP/OPM values) BECAUSE of the way that Poppler's colorspaces work. You will actually need (IIRC) to do a bunch of work in there in order to handle the differences for Separation and DeviceN colorants (and DeviceN will be the most tricky to get correct). It's clearly doable, but it's not going to be trivial... Also, I STRONGLY recommend that Poppler do the same thing that Adobe Acrobat/Reader does and which is now (partially) codified in the forthcoming ISO 32000-2....If you detect that the PDF claims compliance with one of the PDF/X standards, you should ALWAYS render with Overprint Preview enabled. Leonard -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Thomas Freitag Sent: Monday, March 21, 2011 1:59 AM To: [email protected] Subject: [poppler] Implementing overprint in Splash 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 _______________________________________________ poppler mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/poppler _______________________________________________ poppler mailing list [email protected] http://lists.freedesktop.org/mailman/listinfo/poppler
