I exported some TIFFs from Lightroom with ProPhoto RGB profile. But the embedded v2.1 ICC profile (as visible with Mac ColorSync or node.js exifr ICC) only contains the fixed gamma value 1.8 and the color matrix. I miss a hint about the gamma decomposition (similar to sRGB’s) as given in the following gamma formulas for ProPhoto RGB:
c <= 0.001953 ? 16 * c : Math.pow(c, 1/1.8)
c <= 16 * .001953 ? c / 16 : Math.pow(c, 1.8)
It seems that the check for the lower values belongs to a regular ProPhoto RGB interpretation, but I don’t find any hint about it in the embedded profile (as provided by Lightroom 5.6). Besides this, I searched for “gamma” in the ICC.1:2001-04 specification. The only result is in chapter 6.5.3 (“curveType”):
when count is 1, then the data entry is interpreted as a simple gamma value encoded as a u8Fixed8Number. Gamma is interpreted canonically and not as an inverse.
Can you explain to me what this means? Can I somehow derive the
c <= 0.001953 ? 16 * c part from the ICC profile and this sentence about the “canonical interpretation”?