[Printing-architecture] [Openicc] [Gimp-print-devel] Colour

Chris Murphy lists at colorremedies.com
Tue Nov 17 00:20:47 PST 2009

On Nov 16, 2009, at 8:08 PM, Gerhard Fuernkranz wrote:
>> Can the effect profiles be applied in a late binding style - remotely after spooling?
> Well, where and when in the pipeline is the rasterizing is done? Once
> the document has been half-toned, it's rather impossible to apply color
> transformations any more (i.e. in case the final raster data were spooled).

I agree it's a bit of a challenge to make this happen with screened data, but I'd wonder why it's screened if it's not meant for immediate output?

If it's normalized to output space but NOT screened, an "effects profile" or "abstract" profile is relatively easy:

devicespace >PCS > abstract profile > PCS > devicespace

If the supported precision is at least 16bpc I'd consider it pretty much a non-issue. (ACE is 32bpc float precision.)

If it is screened, well, it's still absolutely doable but it's basically a 1-bit TIFF style workflow. I think there are probably other priorities, but I'm open to discussing this.

> And applying an additional device -> PCS -> abstract -> PCS -> device
> transformation in a *separate* post-processing step (after converting
> the object colors to the color space associated with the printer's
> process color model, but before half-toning) will certainly not help to
> improve the overall accuracy of the color transformations,

I think if the precision level of the conversion is high enough, quantization is a non-issue. But there might be gamut issues...including abstract profile induced posterization.

> and some
> colors may already have been clipped, which cannot be undone any more
> afterwards by a de-saturation effect (though this clipping would have
> been avoided if the de-saturation would have been applied before
> applying the output profile). So I think the effects should be rather
> incorporated into the color transformation chain from object colors to
> the printer's process color model. Or alternatively, the intermediate
> color space before applying the effects would need to be rather PCS.

A soft proof that incorporates the abstract profile effect in the chain, as well as the output device profile, has clear benefits in avoiding undesirable gamut mapping problems.

> Applying effects without integrating them into the profile seems also
> difficult to me, if one intends to use pre-computed device link profiles
> for the conversion from object colors to the printer's process color model.

Devicelink workflows are inherently extremely fragile, as presented by the ICC spec. Only a creative solution, that is not obviously prescribed by the ICC spec, can work around such problems. This may very well include *mandatory* PSID support (although I have yet to see explicitly documentation for this tag), or some mechanism for referencing at least the destination profile for the purpose of tagging the resulting data correctly once converted. The effect of an abstract profile would occur before this, but interestingly enough the aggressiveness of the abstract profile in terms of a saturation boost very possibly will affect the rendering intent.

i.e. with a null abstract profile, and use selects RelCol+BPC, but then decides that an abstract profile boosting saturation is preferred, might necessitate the perceptual intent in order to better manager the larger percentage of out of gamut colors as a result of applying the abstract profile that boosts chroma.

*shrug* It's an interesting challenge. The more advanced and imaginative the workflow gets, the more we run into problems with the ICC spec.

Chris Murphy

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