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

Michael Sweet msweet at msweet.org
Sat May 8 00:04:09 UTC 2021


> On May 7, 2021, at 6:11 PM, Till Kamppeter <till.kamppeter at gmail.com> wrote:
> On 07/05/2021 21:53, Michael Sweet wrote:
>>> TL;DR: Some doubts about supporting extended color depth and color spaces:
>>> - How to discover if a page in a PDF is 8/16-bit color/grayscale
>>>   sRGB/Adobe RGB?
>> So there are ways to do this, but none are pretty... :/
> Are they somewhere documented? Is there sample code? Is it worthwhile to implement it? Does implementing it take less new lines in cups-filters than cups-filters already has?

You have to parse the PDF file and look at the images.

> If it is too complicated, I think the most intuitive solution, at least for the high color depth is to trigger it by print-quality=high.
> WDYT, would it also make sense to also switch to Adobe RGB (if available on the printer) if the user sets print-quality=high?

That would be a reasonable approach.

>>> - How important is Adobe RGB for printing? If a printer advertises that
>>>   it supports it, is it worthwhile for us to support jobs in Adobe RGB?
>> Very important, since a lot of cameras use AdobeRGB (or provide an option for it) for an expanded color gamut.  And if you are going from Display P3 (which is another one you'll see a lot of from iPhones/iPads) it is even more important since sRGB is a really small color space.
> For printing photos from photo applications the incoming PDF is probably raster-only and could be treated with pclmtoraster and so an easy way to determine whether it is AdobeRGB, but if it is an actual vector PDF it gets more difficult. And what does one do with Display P3? Is it mentioned in PWG/IPP/Apple Raster/PWG Raster/Mopria? Is it well known, is there a free solution to convert it to Adobe RGB?

It is a standard colorspace backed by an ICC profile.

>>> - How is all this influenced by the "print-content-optimize" setting?
>> Two different things - certainly the 'photo' value might be hint that AdobeRGB/deep color is useful/important, but I'd also say that "print-quality" can also feature in this decision, e.g., 'high' (5) print quality might indicate deep color is preferred.
> OK, probably I will trigger it by the high quality setting. Important is to know whether I can simply switch to Adobe RGB (only if printer has Adobe RGB) regardless of the incoming job and the printer does always the right thing (and not try to print sRGB unconverted as Adobe RGB or uses a conversion with significant quality loss).
>> So print-color-mode and pwg-raster-document-type-supported/urf-support do two different things.  "print-color-mode" is a rendering intent (color, grayscale, bi-level, etc.) while the others list the actual color spaces and bit depths that are supported.
> OK.
>   Till

Michael Sweet

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