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

Elle Stone ellestone at
Thu Nov 16 22:06:26 GMT 2017

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

In terms of the digiKam code going forward, does this mean there will be 
a user option to disable the optimizations, such as Krita and GIMP provide?

In other words, what do you mean in practical terms by "specialized 
option for 16bit -> 16bit conversions"? What about 32-bit floating point 
images? Could there be 16-bit thumbnails as an option?

Marti Maria has made it clear that he doesn't think that linear gamma 
RGB image editing makes sense except for raw processing. But he does 
allow that there are "strong opinions" on this topic.

Except that this isn't really a matter of opinion, but a matter of how 
colors actually do mix in linear gamma RGB vs perceptually uniform RGB:

* If you like darkening between mixed colors, hue shifts in Curves, and 
other such gamma artifacts, editing in perceptually uniform RGB is 
exactly the right thing to do.

* If you want colors in the digital darkroom to blend more like color 
blend out there in the real world, then use linear gamma RGB for editing.

It's as simple as that.

Krita documentation encourages people to use linear gamma RGB precisely 
because of the color-mixing benefits (there are nice pictures 
illustrating the various points):

Quite a few image editing softwares now make it possible to edit at 32f 
in linear gamma color spaces and output high bit depth linear gamma 
images, so it's to be expected that users will be outputting images in 
linear gamma color spaces.

I think it would be nice if digiKam made it possible for users to view 
their linear gamma images without seeing the tonal distortion in the 
shadows produced by the LCMS optimizations.


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