Thumbnails not color-managed + severely distorted shadow tonality for thumbs, previews

Marcel Wiesweg marcel.wiesweg at
Fri Nov 17 20:51:34 GMT 2017

I have no precise opinion, but essentially thought of a user accessible switch 
in the setup.

Performance must be optimized for the >>90% of users with the limited use 
cases images in sRGB or Adobe RGB or RAWs -> no monitor profile or one created 
with argyll cms / dispcal.

Feedback from people like you with expertise on CM is certainly appreciated.

> On 11/16/2017 03:27 PM, Marcel Wiesweg wrote:
> > Regarding the optimization flag, I find Marti Maria's definitive answer to
> > this issue raised by you here
> > 
> >
> > 
> > asserting that we should not change the flag per default for very
> > convincing performance reasons, but a specialized option for 16bit ->
> > 16bit conversions appears acceptable
> Are you suggesting some sort of internal switch - not accessible by the
> user - that detects the color space and/or bit depth and then switches
> between using and not using the LCMS optimizations, without the user
> having any say in the matter?
> If so, then I would ask you to consider instead putting in a user
> preference to allow the *user* to enable or disable the LCMS
> optimizations per their own choice. Krita has a very nice example of
> such an option.
> Regarding the "performance reasons" that you mention, this is something
> *users* should experiment with and make their own informed decisions
> instead of relying on generalities that might not even apply to their
> own particular use cases.
> Currently I'm using a relatively fast machine with a lot of RAM. But my
> last computer was ten years old before it finally stopped working around
> three years ago. That machine only had a one core processor, and by
> today's standards it was a slow processor, though the machine did have
> 8GB RAM. That's my "benchmark" machine.
> Linear gamma RGB profiles are matrix profiles. My monitor profile is a
> matrix profile made using ArgyllCMS. I'm fairly sure that the majority
> of users have matrix monitor profiles.
> For matrix-to-matrix ICC profile conversions, even on my old machine I
> never noticed any slow-down at all, regardless of whether the LCMS
> optimizations were being used or not being used.
> At least on Linux (I am not willing to make statements about what
> happens on Windows or Mac), I'm fairly sure the only time disabling the
> LCMS optimizations is likely to slow down performance to the point of
> being noticeable by the user, is when the source and/or destination
> profile is a LUT profile, such as a printer profile. And even then, "how
> slow is too slow" is surely machine-dependent and also a matter of user
> preference.
> If a user does normally use a LUT monitor profile, well, that person
> would probably want to make some experiments and decide whether or not
> to enable or disable the LCMS optimizations.
> Soft proofing using LCMS does slow down the display of the image, with
> or without the optimizations, and I'm sure this has to do with the
> double profile conversion going on, as well as with the fact that
> printer profiles are LUT profiles.
> Personally I don't use LCMS for soft proofing because the LCMS
> soft-proofing gamut checks fail rather badly when the source profile is
> a linear gamma matrix profile. So I use PhotoFlow for soft-proofing.
> PhotoFlow has internal soft proofing code that very nicely works around
> the current limitations in LCMS soft proofing. AFAIK this code is still
> only in the linear gamma branch of PhotoFlow, but will be merged with
> the main branch at least by the 0.30 release
> (,
> 6/3).
> Best,
> Elle

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