Re: PDF: Disable converting color to "Device Gray"

[Craig Ringer <cr...@postnewspapers.com.au>]

On 30/08/10 22:47, Jeremias Maerki wrote:
> FOP doesn't have such an option. The PDF 1.4 spec says in chapter 6.2.1:
> "A gray level is equivalent to an RGB value with all three components
> the same."
> 
> And therefore we render gray values in the DeviceGray color space. That
> this doesn't work in your case is a bit surprising. But I've heard
> something similar just two weeks ago from a company that does photo
> books with FOP/Batik. But they had to do strange things like using CMYK
> and adding Magenta in addition to the K=1.0 value. They still just get
> an approximate black. I guess that's an issue of the printer they use. I
> suspect your case may be similar: a printer issue.

It's often an issue with the RIP or preflight tools, rather than the
printer/press its self.

ICC-aware RIPs will use ICC color space transforms to convert RGB
colours in a given space to the output device space - usually a custom
CMYK profile for the printer/press. /DeviceGray is a different colour
space to /DeviceRGB and the /DeviceGray to CMYK conversion is a
different mapping to the RGB one, potentially done with different
profiles. Some RIPs just convert /DeviceGray to /DeviceRGB then do the
transform from RGB, but that's not necessarily correct, especially in a
tagged document where there are mulitiple colour spaces in the same
co-ordinate system, like sRGB and AdobeRGB.

A proper configuration will use a greyscale profile that matches the RGB
profile. That's not very common, though, and really only works out where
the assumed greyscale curve happens to match the chosen or assumed RGB
profile.

If the RGB profile being used to interpret input RGB values has neutral
greys on the same curve as the profile being used for DeviceGray you'll
get the same CMYK result (barring colour manager quirks) - but if the
RGB profile doesn't have neutral grays, has a different gamma, etc then
you'll get different results. This is particularly the case where GCR or
UCR settings are involved on the output side; it's not uncommon to see
"rich" CMYK blacks produced for RGB input, but pure-K channel blacks
produced for greyscale input. Unsurprisingly, this looks horrid.

While proper RIP configuration - in particular, supplying a greyscale
profile that matches the RGB profile - will fix this, in practice it's
often best to stick to using one colour space in your document,
generally tagged RGB.

So, honestly, I don't think it's at all ideal to emit /DeviceGray in an
otherwise colour document. I'd certainly want the option to specify the
colour co-ordinate system(s) used - /DeviceGray only,
/DeviceGray+/DeviceRGB, or /DeviceRGB only.

I wasn't aware that fop did RGB to greyscale replacement, and I'm mildly
concerned about it, as I'm working on software that'll be producing PDFs
for pre-press consumption. I can always fix it up with some PitStop
rules in preflight, but I'd rather not need to.

Some time later I'd like to go head down in the FOP code to see if I can
extend it to support multiple colour spaces and CMYK profiles. I'm just
a bit wary about that, given the rather hostile reception I've seen to
contributed changes here; I can't afford to throw that much time away
and can't afford to maintain a parallel branch.

-- 
Craig Ringer

Tech-related writing: http://soapyfrogs.blogspot.com/

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Re: PDF: Disable converting color to "Device Gray"

[Frangoo <fr...@yahoo.de>]

Hi,
first: thanks a LOT to all of your replies!!
And sorry for my late answer, I was busy with other stuff. But I was
following the answers and trying out your tips.
As I am a digital-web-person, I'm not that familiar with all the
color-profile-stuff so I didn't understood everything 100%.
I "solved" the problem now for my special situation, but I wanted to let you
know, maybe my information helps someone to go one with that problem (or to
ignore it in case everything was just ignorance on my side *argh*).

Because by a fluke, one of two images of the PDF-Page wasn't included in the
document, and than the background-color came out as real black :-) There was
something wrong with the image, so I changed it (was an transparent PNG)
with an JPG, and the background-color was still black. Great! BUT: Only in
the PDF-Reader. By printing, depending on the printer, the background-color
came out again not in a 100% black color.
So I made an ugly hack and put a black JPG as a background-image in the
page.

Regards

Frangoo
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