Automate Stair Step Scaling

Processing
41

Darktable doesn’t has linear sRGB as output , right ?

42

Not out of the box. I use Elle’s linear srgb.

43

Weird question, is it the scaling or is the output from DT already different? Different output profiles can maybe affect filmic’s gamut mapping.

44

Yes, see filmic section in the manual, under “Background” heading -
“This gamut mapping uses the output color profile as a definition of the display color space and automatically adjusts to any output space.”

45

The image downscaled in linear gamma looks too much desaturated on my monitor

There is indeed greater saturation in the non-linear version. I don’t know how to explain it.

Interesting point. Comparing the two images of the big tree: (1) resized in non-linear sRGB space and (2) resized in linear space, then converted to non-linear sRGB, image (1) seems to be more saturated.

Why does that happen? I don’t know, and couldn’t figure it out. It turns out the answer is: the average chroma (aka “saturation”) of the two images are virtually the same. I am measuring chroma as the C channel of IM’s HCL. For some pixels (1) has the higher chroma, and for others (2) has the higher chroma.

The rule seems to be: where the input chroma is low, resizing in linear gives the highest chroma; but where the chroma is high, resizing in sRGB gives the highest chroma. However, low input chroma tends to occur where intensity is low, so disentangling the possible rule isn’t easy.

As we would expect, in areas of flat colour (1) and (2) have the same chroma.

EDIT: I should add that my findings are consistent with “resizing in sRGB seems to have the higher chroma.” Our attention is drawn to areas of high chroma, and that is where sRGB chroma is higher. Our attention ignores area of low chroma, so we don’t notice the chroma boost that linear gives there.

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46

I had a similar thought, but the exported full size tiffs from dt were identical. The output profiles may be different, but they both have srgb primaries and white point, so the gamut mapping is equal for both. This means the difference occurs in the resizing.

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