Steam Deck integrated display: saturation boosting or reduction?

[Pekka Paalanen]

This is a continuation of
https://gitlab.freedesktop.org/wayland/wayland-protocols/-/merge_requests/14#note_2152254
because this is off-topic in that thread.

> No, we did widening. The Deck's internal display has a modest gamut
> that is < 71% sRGB.

If games do wide (well, full sRGB or wider) gamut, then why would you
need to make that gamut even wider to fit nicely into a significantly
smaller gamut display?

Here's what I think happened.

You have a game that produces saturation up to P3, let's say. When you
did the colorimetrically correct matrix conversion (CTM) from BT.2020
to the "modest gamut", you found out that it is horribly clipping
colors, right?

If you then removed that CTM, it means that you are
re-interpreting BT.2020 RGB encoding *as if* it was "modest gamut" RGB
encoding. This happens if you simply apply the input image EOTF and
then apply the display inverse-EOTF and do nothing to the color gamut
in between. Adjusting dynamic range does not count here. This is an
extreme case of saturation reduction.

(Note: Doing nothing to numbers equals to applying a major semantic
operation. Like telling someone something in cm and they take that
number in mm instead. Or metric vs. imperial units. Color space
primaries and white point define the units for RGB values, and if you
have other RGB values, they are not comparable without the proper CTM
conversion.)

That does not look good either, so after that re-interpretation you
added saturation boosting that nicely makes use of the capabilities of
the integrated display's "modest gamut" so that the image looks more
"vibrant" and less de-saturated. However, the total effect is still
saturation reduction, because the re-interpretation of the game content
RGB values is such a massive saturation reduction that your boosting
does not overcome it.

I could make up an analogue: Someone says they are making all sticks
50% longer than what you ask. You ask them to make a stick 100 long.
They give you a stick that you measure to be 15 long, and they still
claim it is 50% longer than what you asked. How is this possible? The
length spaces are different: you were thinking and measuring in cm,
they did mm. They did give you a stick of 150 mm, which is 50% longer
than 100 mm. But from your perspective, the stick is 85% smaller than
you asked. If one had started by converting to a mutual length space
first (referring to the correct CTM), there would be an initial
agreement of how long is 1.


Thanks,
pq
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