HTML Canvas - Transforms for HDR and WCG

Hi all,

My colleagues and I have spent some time thinking about the HTML canvas proposals and the input and output scaling required for each for HDR and wider colour gamut functionality.  I've added some plots showing some ideas on how input and output formats could be transformed for the new canvases.
New extended sRGB canvas format.
[cid:image001.jpg@01D724C1.B3CCA3E0]
In this canvas (1,1,1) is equal to SDR White and (0,0,0) is equal to Black.  Values are not however clipped outside this range and brighter parts of the image can be created using values over 1 and wider colour gamuts can be created using negative values.  For each input signal, we can simply convert to linear rec2020 and then scale such that the SDR reference white signal has the value (1,1,1).  Note, linearly scaling PQ display light signals without an OOTF adjustment, does not maintain their subjective appearance.
For SDR outputs this would cause clipping of the HDR signals above (1,1,1).  For HLG, should the canvas be able to determine the output format required, we could instead use an sRGB EOTF on the input and use the compatible nature of the HLG signal to create a tone-mapped sRGB image.

[cid:image002.jpg@01D724C1.B3CCA3E0]
HLG Canvas
[Diagram  Description automatically generated]
In this canvas (1,1,1) is equal to Peak White and (0,0,0) is equal to Black..  The display-independent working space is an BT.2100 HLG signal.
Input signals that are not HLG are converted to normalised linear display light.  These are converted to the HLG display-independent space using an HLG inverse EOTF.   The EOTF LW parameter is selected such that the HLG OOTF becomes unity (i.e. a pass-through) simplifying the conversion process and avoiding a requirement for a 3D-LUT.

  *   SDR display-light signals are scaled so that their nominal peak equals HDR Reference White (0.265) and matrixed to the correct colourspace prior to conversion to HLG.
  *   PQ signals are scaled so that 203 cd/m2 equals HDR Reference White (0..265) prior to conversion to HLG.
The HLG conversion maps HDR reference white in normalized linear display light (0.265,0.265,0.265) to (0.75,0.75,0.75) in the HLG non-linear signal.  Formats which are already linear, e.g. "sRGB-linear", "rec2020-linear" are treated similarly but do not require an initial EOTF being applied to convert to linear.
Note, BT.1886 can be used with either BT.709 or BT.2020 signals.  The correct matrices should be chosen match the input signal and the output display.
PQ Canvas
[A picture containing graphical user interface  Description automatically generated]

In this canvas (1,1,1) is equal to 10000 cd/m2 and (0,0,0) is equal to 0 cd/m2.  Input signals that are not PQ are converted to linear and scaled so that HDR reference white is set to 0.0203.  They are then matrixed to the correct colourspace and converted to PQ.  The conversion maps (0.0203,0.0203,0.0203) to (0.58,0.58,0.58).  Formats which are already linear, e.g. "sRGB-linear", "rec2020-linear" are treated similarly but do not require an initial EOTF being applied to convert to linear.

Best Regards

Simon

Simon Thompson
Senior R&D Engineer