5 ms·

Reading the linked blog post on the new cICP chunk type [0], it looks like the "proper HDR support" isn't something that you couldn't already do with an embedde

Reading the linked blog post on the new cICP chunk type [0], it looks like the "proper HDR support" isn't something that you couldn't already do with an embedded ICC profile, but instead a much-abbreviated form of the colorspace information suitable for small image files. [0] https://svgees.us/blog/cICP.html https://svgees.us/blog/cICP.html

cormorant 1y ago
"common but not representable RGB spaces like Adobe 1998 RGB or ProPhoto RGB cannot use CICP and have to be identified with ICC profiles instead." cICP is 16 bytes for identifying one out of a "list of known spaces" but they chose not to include a couple of the most common ones. Off to a great start... I wonder if it's some kind of legal issue with Adobe. That would also explain why EXIF / DCF refer to Adobe RGB only by the euphemism "optional color space" or "option file". [1] [1] https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_system#DCF_objects https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_sy...
- spider-mario 1y ago
  CICP (H.273) originates from the video world, where Adobe RGB and ProPhoto RGB really aren’t common.
ProgramMax 1y ago
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 1y ago
  > 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.
  - ProgramMax 1y ago
    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.
    - LegionMammal978 1y ago
      That doesn't sound quite right to me. ICCv2's 'curveType' gives the option of a full lookup table instead of a simple gamma function. Maybe it has to do with ICCv2 saying that the reference viewing condition has an illumination level of 500 lx for the perceptual intent? (But how does that apply to non-reflective media?) I don't doubt that there's lots of problems in the chain from RGB samples to display output, but I'm finding this whole thing horribly confusing. Wikipedia tries to distinguish 'HDR' transfer functions like PQ [0] from 'SDR' transfer functions in terms of their absolute luminance, but the ICC specs are just filled with relative values all the way down. (Not to mention how much these things get fiddled with in practice. Once, I had the idea of writing a JPEG decoder, so I looked into how exactly to convert between sRGB and Rec. 601 YCbCr coordinates. I thought, "I know, I'll just use the standard-defined XYZ conversions to bridge between them!" But psych, the ICC sRGB profile has its own black point scaling that the standards don't tell you about. I'm still not sure what the correct answer is for "these sRGB coordinates represent the exact same color as these Rec. 601 YCbCr coordinates".) [0] https://en.wikipedia.org/wiki/Perceptual_quantizer https://en.wikipedia.org/wiki/Perceptual_quantizer
      - ProgramMax 1y ago
        Agreed that it gets confusing. That's a piece of why I'm unable to give you a solid answer. This isn't my area of expertise. Here is what I can tell you confidently: The original plan was to provide an ICC profile that approximates PQ as best as we could. But it wasn't enough. So the proposal was to force the profile name to be a special string. When a PNG decoder saw that name, it would ignore the ICC profile and do actual PQ. Here is that original proposal: https://w3c.github.io/png-hdr-pq/ https://w3c.github.io/png-hdr-pq/ Possibly more context (I just found this) from Apple. I'm not sure of date: https://www.color.org/hdr/02-Luke_Wallis.pdf https://www.color.org/hdr/02-Luke_Wallis.pdf Slide 29: "HDR parametric transfer functions not in ICC spec Parametric 3D tone mapping functions not in ICC spec - Neither can be approximated by 1-D or 3-D LUTs" I'm not sure why they cannot be approximated by LUT. Maybe because of the inversion problem?
account42 1y ago
Unfortunately that seems to mean that the backwards compatibility here is washed out preview instead of limited-to-sRGB.

cormorant 1y ago

"common but not representable RGB spaces like Adobe 1998 RGB or ProPhoto RGB cannot use CICP and have to be identified with ICC profiles instead." cICP is 16 bytes for identifying one out of a "list of known spaces" but they chose not to include a couple of the most common ones. Off to a great start... I wonder if it's some kind of legal issue with Adobe. That would also explain why EXIF / DCF refer to Adobe RGB only by the euphemism "optional color space" or "option file". [1] [1] https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_system#DCF_objects https://en.wikipedia.org/wiki/Design_rule_for_Camera_File_sy...

spider-mario 1y ago
CICP (H.273) originates from the video world, where Adobe RGB and ProPhoto RGB really aren’t common.

ProgramMax 1y ago

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 1y ago
> 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.
- ProgramMax 1y ago
  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.
  - LegionMammal978 1y ago
    That doesn't sound quite right to me. ICCv2's 'curveType' gives the option of a full lookup table instead of a simple gamma function. Maybe it has to do with ICCv2 saying that the reference viewing condition has an illumination level of 500 lx for the perceptual intent? (But how does that apply to non-reflective media?) I don't doubt that there's lots of problems in the chain from RGB samples to display output, but I'm finding this whole thing horribly confusing. Wikipedia tries to distinguish 'HDR' transfer functions like PQ [0] from 'SDR' transfer functions in terms of their absolute luminance, but the ICC specs are just filled with relative values all the way down. (Not to mention how much these things get fiddled with in practice. Once, I had the idea of writing a JPEG decoder, so I looked into how exactly to convert between sRGB and Rec. 601 YCbCr coordinates. I thought, "I know, I'll just use the standard-defined XYZ conversions to bridge between them!" But psych, the ICC sRGB profile has its own black point scaling that the standards don't tell you about. I'm still not sure what the correct answer is for "these sRGB coordinates represent the exact same color as these Rec. 601 YCbCr coordinates".) [0] https://en.wikipedia.org/wiki/Perceptual_quantizer https://en.wikipedia.org/wiki/Perceptual_quantizer
    - ProgramMax 1y ago
      Agreed that it gets confusing. That's a piece of why I'm unable to give you a solid answer. This isn't my area of expertise. Here is what I can tell you confidently: The original plan was to provide an ICC profile that approximates PQ as best as we could. But it wasn't enough. So the proposal was to force the profile name to be a special string. When a PNG decoder saw that name, it would ignore the ICC profile and do actual PQ. Here is that original proposal: https://w3c.github.io/png-hdr-pq/ https://w3c.github.io/png-hdr-pq/ Possibly more context (I just found this) from Apple. I'm not sure of date: https://www.color.org/hdr/02-Luke_Wallis.pdf https://www.color.org/hdr/02-Luke_Wallis.pdf Slide 29: "HDR parametric transfer functions not in ICC spec Parametric 3D tone mapping functions not in ICC spec - Neither can be approximated by 1-D or 3-D LUTs" I'm not sure why they cannot be approximated by LUT. Maybe because of the inversion problem?

account42 1y ago

Unfortunately that seems to mean that the backwards compatibility here is washed out preview instead of limited-to-sRGB.