Re: Proposal for HTMLCanvasElement HDR compositing modes

On Mon, Mar 15, 2021 at 8:26 PM Lars Borg <borg@adobe.com> wrote:

> Chris C,
>
>
>
> One concern I have is the term scene light.
>
> If this means scene-referred, then that’s a problem as sRGB is defined
> only for display light, not for scene light.
>
> So then additional conversions would be needed to convert sRGB to canvas.
>
> And if scene light doesn’t mean scene-referred then what does it mean? It
> causes confusion.
>
> Please find a way to stay in display-referred color space.
>

I'm quite happy to change the terminology -- I made a point of staking out
definitions of the terms I would use as I would use them to ensure that
these concerns would jump out immediately. Success!

(This is in a break with the custom of not defining terms in specifications
and hoping that all readers and authors have the same definition in mind).

I see the point here. Indeed, "scene light" is a bad term for this.

We cannot work in true display light, because the true display is unknown
and unknowable. The term "reference display light" would  have been nice,
but it's slightly different (or at least more general) than what I'm aiming
at here.

Maybe we can call it "canvas reference display light" or
"device-independent display light".

The key properties I want this space to have are:

   - PQ signal is mapped to the space by applying the PQ inverse-OETF (and
   maybe a linear scale)
   - This is true for any linear light space, so it's not an onerous demand
   - HLG signal is mapped to the space by applying the HLG inverse-OETF
   (and maybe a linear scale)
   - If we define our reference display to have a maximum luminance of 334
      nits, then this is the case
      - Applying and then un-applying an HLG OOTF in various parts of the
      pipeline feels gross (maybe it shouldn't?)
      - sRGB signal is mapped into the space by applying the ordinary
   piecewise-with-gamma-2.4 transfer function (and maybe a linear scale)

The first and second points (especially for HLG, not wanting any OOTF
applied) made this feel like "scene light" to me. With respect to sRGB and
scene light, BT2100 itself is already sloppy with the definition of scene
light, so extending that definition to sRGB doesn't feel like doing
anything more untoward to the concept.

But, I see that "scene light" has a specific meaning (particularly with
respect to being "before artistic changes have been made"), which makes it
inappropriate as a thing to recover from a signal.