On Thu, 23 Jan 2025 15:16:29 -0500 Harry Wentland <harry.wentland@xxxxxxx> wrote: > On 2025-01-17 04:06, Pekka Paalanen wrote: > > On Thu, 16 Jan 2025 10:56:22 +0200 > > Pekka Paalanen <pekka.paalanen@xxxxxxxxxxxxx> wrote: > > > >> On Thu, 19 Dec 2024 21:33:37 -0700 > >> Alex Hung <alex.hung@xxxxxxx> wrote: > >> > >>> From: Harry Wentland <harry.wentland@xxxxxxx> > >>> > >>> The BT.709 and BT.2020 OETFs are the same, the only difference > >>> being that the BT.2020 variant is defined with more precision > >>> for 10 and 12-bit per color encodings. > >>> > >>> Both are used as encoding functions for video content, and are > >>> therefore defined as OETF (opto-electronic transfer function) > >>> instead of as EOTF (electro-optical transfer function). > >>> > >>> Signed-off-by: Alex Hung <alex.hung@xxxxxxx> > >>> Signed-off-by: Harry Wentland <harry.wentland@xxxxxxx> > >> > >> Hi, > >> > >> why would a display system ever use BT.2020 or BT.709 OETF or its > >> inverse? > > > > Sorry, this is more for my own curiosity, not an argument against the > > patch. Since hardware designers decided to incorporate these curves > > explicitly, what use was in mind? It's likely something I have > > overlooked. > > > > I'm not entirely sure myself, but gamescope does use it for displaying > game streaming content. > > https://github.com/ValveSoftware/gamescope/blob/2f88849ac9dc7da5c678d5d7d3e9b58f38add1bf/src/Backends/DRMBackend.cpp#L2509 I see, apparently they are being used as a pair to make a roundtrip to some optical space for applying a color transformation matrix. IOW, the CTM is being applied in the assumed "camera" side of broadcasting or grading. ITU-R BT.2408 seems to refer to this as "scene referred mapping", whose aim is to match cameras rather than displays (and has nothing to do with the scene). I cannot claim to understand this case, but I suspect it should be used only before color grading (manual or automatic artistic adjustments, or IOW picture formation(?)) in a pipeline. I would assume that game streaming content has been prepared for a display at its source, which implies that grading has been done. That means that inverse-OETF leads to an unknown color space. Applying a matrix in an unknown color space has unintended effects. That may not be a big problem, even less if the matrix is visually adjusted, but it's theoretically not quite right AFAIU. Thanks, pq
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