[Printing-architecture] Make use of extended color spaces on IPP printers

Michael Sweet msweet at msweet.org
Mon May 31 03:12:27 UTC 2021


> On May 30, 2021, at 5:50 PM, Till Kamppeter <till.kamppeter at gmail.com> wrote:
> On 30/05/2021 22:53, Michael Sweet wrote:
>> Yes, and there is a mechanism for that - the printer will add the document-unprintable-error value in job-state-reasons or return client-error-document-unprintable-error from the Print-Job/Send-Document request. Then CUPS will re-print as raster.
> Is this already implemented in CUPS?

Yes, it has been in CUPS for many years now...

> So if I send a JPEG to CUPS and the printer which supports JPEG chokes on my JPEG then CUPS sees the error and retries the job in raster (which probably lets the imagetoraster filter being used)?
> Is this only implemented for JPEG? Or does CUPS go from one *cupsfilter(2): line in the PPD to the next until the printer does not error any more?

This happens for PDF, JPEG, and PNG, and it gets retried once as PWG/Apple raster.

>> ...
>> You will never see "AdobeRGB" or "sRGB" in the PDF file, you will see an ICC based color space or a calibrated color space (/CalRGB or /CalGray) with suitable parameters.
> Which parameters? Ids it documented somewhere how I can identify PDFs (or elements in them) as sRGB or AdobeRGB?

The PDF will contain color resources.  AdobeRGB:

/CalRGB<</Gamma[2.2 2.2 2.2]/Matrix[0.7667 0.29734 0.02703 0.18556 0.62736 0.07069 0.18823 0.07529 0.99134]/WhitePoint[0.9505 1 1.089]>>]

Display P3:

[/CalRGB<</Gamma[2.2 2.2 2.2]/Matrix[0.48657 0.2297 0 0.26567 0.69174 0.04511 0.19822 0.07929 1.04394]/WhitePoint[0.9505 1 1.089>>]


[/CalRGB<</Gamma[2.2 2.2 2.2]/Matrix[0.4124 0.2126 0.0193 0.3576 0.715 0.1192 0.1805 0.0722 0.9505]/WhitePoint[0.9505 1 1.089]>>]

As you can see, the only difference is the Matrix value.

>>> ...
>>> Do you know how I can get appropriate PDF files for testing? Are there somewhere sample files for download? Is there a recipe how to turn an AdobeRGB JPEG into an AdobeRGB PDF under Linux?
>> I will shortly have the necessary changes in pdfio (https://www.msweet.org/pdfio) to be able to embed a JPEG or PNG file in a PDF file with the expected calibrated color spaces.
> This is new as of today? What are the differences to QPDF? QPDF I use heavily in cups-filters and I have got some students adding features to it.

They can do similar things, but PDFio is both general-purpose (reading, "rewriting", and writing) and also won't be implementing useless writing features such as linearization and encryption.  I'm also targeting PDFio for enbedded usage.

Michael Sweet

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