Add HDR metadata to rec2100-pq canvases

[palemieux]

Given the objective:

To display PQ encoded HDR images and video in a canvas such that PQ images drawn in the canvas will appear exactly the same as they would if displayed via an or

It should be possible for the application to (optionally) associate the following information with a rec2100-pq canvas:

Mastering Display Color Volume Metadata as specified in SMPTE ST 2086

• Mastering Display Primaries: list of 3 (x, y) chromaticity coordinates. Each chromaticity coordinate consists of two uint16 elements, in order, the normalized x and y chromaticity coordinates of the color primary in units of 0.00002

EXAMPLE: Color primaries of the P3D65 system specified in SMPTE ST 2113: { 34000, 16000 } { 13250, 34500 } { 7500, 3000 }

• Mastering Display White Point Chromaticity: (x, y) chromaticity coordinate. Each chromaticity coordinate consists of two uint16 elements, in order, the normalized x and y chromaticity coordinates of the color primary in units of 0.00002

EXAMPLE: Illuminant D65 specified in SMPTE RP 177: { 15635, 16450 }

• Mastering Display Maximum Luminance: uint32 value in units of 0.0001 cd/m²

EXAMPLE: Reference white in Recommendation ITU-R BT.2035: 1000000

• Mastering Display Maximum Luminance: uint32 value in units of 0.0001 cd/m²

EXAMPLE: Reference black in Recommendation ITU-R BT.2035: 100

MaxCLL and MaxFALL as specified in Annex P.1 and P.2 of CTA 861-G

• Maximum Content Light Level: uint16 in units of 1 cd/m²

• Maximum Frame-average Light Level: uint16 in units of 1 cd/m²

[ccameron-chromium]

An API to solve this issue could be

  interface ColorVolumeMetadata {
    // Color volume in xy coordinates
    float? redPrimary[];
    float? greenPrimary[];
    float? bluePrimary[];

    // Light level, units less clear here.
    float? maxContentLightLevel;
    float? maxFrameAverageLightLevel;
  };

And then, to CanvasHighDynamicRangeOptions add

  partial interface CanvasHighDynamicRangeOptions {
    ColorVolumeMetadata? color_volume_metadata;
  }

Using these, an application would be able to call

  myCanvas.configureHighDyanmicRange({
      color_volume_metadata:{maxContentLightLevel:1000}})

The units for maxContentLightLevel should be nits for PQ content. For extended-srgb-linear content, nits is not a good fit. I would say that for such a space, 1.0 should refer to SDR brightness.

[ccameron-chromium]

We will also want to add a configureHighDyanmicRange method to OffscreenCanvas