[poppler] Implementing overprint in Splash

[Thomas Freitag]

Hi Leonard!

First, thank You for Your response.

Am 21.03.2011 14:10, schrieb Leonard Rosenthol:
> 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).
Yes, I propably simplified. Unnecessary to say, that it is really easy 
to put the overprint values on the splash state stack, so we have it 
everywhere where it is needed. And I found the central place, where 
blending and computing the result pixel values is done. BUT: when we 
expect we are in splashCMYK8, I have at this central place any source 
color already converted to CMYK, and the destination color in CMYK. So 
what I mean it is here quite easy to implement overprint. If the 
painting colorspace is any other the Separation or DeviceN, I can easily 
implement table 148 and 149 from ISO 32000-1. Unfortunately, what has to 
be done when it is Separation or DeviceN, it is not really clear for me. 
I thought, if I have there CMYK values I could handle them as if they 
CMYK values from a DeviceCMYK. I made some tests with PDFs claimed to be 
in DeviceN and use overprint, and the result seems to be simular to what 
Acrobat Reader shows...
I also found no samples in ISO 32000-1 ( seems to be it has samples for 
nearly everything, but not for overprinting :-) ). So any help, hints to 
other documents which could clearify it for me are appreciated.

But nevertheless, Your answer makes implementing overprinting for me 
even more interesting :-)

> 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.
Okay, but because in poppler You have to specify the output colorspace 
BEFORE starting render the page, and overprinting will be implemented 
only in splashCMYK8, it would be up to the application which uses 
poppler to detect that overprinting is used and use splashCMYK8 then.

Thomas
> Leonard
>
> -----Original Message-----
> From: poppler-bounces+leonardr=adobe.com at lists.freedesktop.org [mailto:poppler-bounces+leonardr=adobe.com at lists.freedesktop.org] On Behalf Of Thomas Freitag
> Sent: Monday, March 21, 2011 1:59 AM
> To: poppler at lists.freedesktop.org
> 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
>
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