[ProgramMax]

PNG previously supported ICC v2. That was updated to ICC v4. However, neither of these are capable of HDR.

Maybe iccMAX supports HDR. I'm not sure. In either case, that isn't what PNG supported.

So something new was required for HDR.

[LegionMammal978]

> However, neither of these are capable of HDR.

How so? As far as I can tell, the ICCv2 spec is very agnostic as to the gamut and dynamic range of the output medium. It doesn't say anything to the extent of "thou shalt not produce any colors outside the sRGB gamut, nor make the white point too bright".

Unless HDR support is supposed to be something other than just the primaries, white point, and transfer function. All the breathless blogspam about HDR doesn't make it very clear what it means in terms of colorspaces.

[spider-mario]

> How so? As far as I can tell, the ICCv2 spec is very agnostic as to the gamut and dynamic range of the output medium. It doesn't say anything to the extent of "thou shalt not produce any colors outside the sRGB gamut, nor make the white point too bright".

That’s precisely what makes it unsuitable for HDR. With PQ, (1, 1, 1) means 10 000 cd/m² – if you simply create an ICC profile with the PQ transfer function, an image that looks right on a hypothetical 10 000 cd/m² monitor will look way too dim when naïvely scaled down (as ICC-type colour management would have you do) to the 300 cd/m² of a typical monitor. HLG, meanwhile, has a transfer function that depends on the peak luminance, which is not possible to do with ICC (the profile would have to assume a specific peak luminance), and the reason that it does that is to preserve the subjective perception of the image.

So, sure, you can prepare an HDR image so that it looks right on a monitor with a 1000 cd/m² peak luminance, describe the colorspace in relative terms using an ICC profile, and you will have “done HDR using ICC”, but that’s arguably a very low bar for “supporting HDR”.

[ProgramMax]

IIRC (been a while), the reason was ICCv2/v4 still requires a gamma function. And PQ is not a gamma function. Maybe they can cover HLG, but if we want to represent any given HDR content, we needed something more than ICCv2/v4.