Notice of Meeting - Output Transforms Architecture VWG - February 9, 2022

Discussions - ACESNext VWG – Output Transforms
1

Output Transforms Architecture VWG Meeting

Wednesday, February 9, 2022
1pm Pacific time Add to Calendar (.ics)

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2

The recording of Meeting #41 is available.

The notes are now also available.

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3

Re-reading the meeting notes and forgot to mention that the [0, 0, 15] I was mentioning was in sRGB space gamut expanded to P3 because of purity then converted to AP1. The AP1 value would be somewhere around [0.09, 0.04, 14.41]. Realistically though, this value would be an aberration and I colour picked the graded and gamut expanded sky blue in one of our shots and it was around [1.16, 2.10, 3.56] in AP1 which is something I would still describe as light blue even though it has more green than red.

4

Can you clarify what you mean by that?

I get completely different values when converting sRGB [0, 0, 15] to AP1:

>>> colour.RGB_to_RGB([0, 0, 15],
                       RGB_COLOURSPACE_sRGB,
                       RGB_COLOURSPACE_ACESCG,
                       chromatic_adaptation_transform='Bradford')
array([  0.71125044,   0.20177286,  13.04678106])
5

It’s the unreal engine gamut expansion method. The full flow is :

  • Convert sRGB to AP1
  • Calculate lerp factor between sRGB and custom wide gamut colour space. Lerp factor is calculated based on AP1 luminance and AP1 chroma.

Primaries of custom wide gamut colour space :

// CIE 1931 chromaticities:	x		y
//				Red:		0.6965	0.3065
//				Green:		0.245	0.718
//				Blue:		0.1302	0.0456
//				White:		0.3127	0.329
  • Do a wrong interpretation of the original sRGB using those primaries and convert to AP1.
  • Lerp the correct sRGB->AP1 with the CustomWide->AP1. In the [0, 0, 15] examples, the lerp factor is aroundish 99% CustomWide.

There are certainly better gamut expansion methods out there :slight_smile: