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from GIMP documentation I understand that GIMP processes internally all in 3x32b floating point no matter what bit depth is used.

What puzzles me most - sRGB is 8bit gamma corrected and if I import 3x16b TIFF, what happens to image originally exported with ProPhoto RGB???

is sRGB just for preview and all available data 3x16b are used for processing anyway, please?

I do not know how to explain it, hopefully you understand.

sRGB only tells how to interpret the RGB coding in bytes of the JPEG/PNG. When Gimp converts to floating point, the data is usually put in "Linear light", so the Gamma-correction is removed. It is of course re-instated when you export to PNG/JPEG.

Note that internally, Gimp is always in FP-linear, the "precision" is only for storage (32-bit FP uses 4 times more RAM than 8-bit). This also means that unless you are in 32-bit FP there will be additional conversions, so 32-bit FP is normally faster than the other precisions.

Thanks for reply,

if I understand correctly ICC profiles only shows how it looks like and doesn't alter any data in source file.

I usually import TIFF, 16b because my camera has got real 14b DAC.

I let most setting default in GIMP so it is imported in 16b linear and built-in sRGB

Exporting JPEG, sRGB, Q100 for printing since not all labs take TIFF 16b and because there is no printer capable of 16b color depth anyway