Steam Deck integrated display: saturation boosting or reduction?

[Pekka Paalanen]

On Sat, 4 Nov 2023 13:11:02 +0000
Joshua Ashton <joshua at froggi.es> wrote:

> Hello,

Hi Joshua,

thanks for replying. The reason I started this discussion is that I
would like us to come to the same page on terminology and what the math
actually means. I cannot and do not want to claim anything wrong in
your math or implementation, but I would like to describe it from a
certain point of view.

> The existing behaviour before any of our colour work was that the native 
> display's primaries were being used for SDR content. (Ie. just scanning 
> out game's buffer directly)

Ok. That means it was using an implicit conversion from whatever the
games were emitting into the display's native primaries. Using an
(implicit) identity CTM unavoidably changes colorimetry when the source
and destination color spaces differ.

If games were graded to produce BT.2020 encoding, this would amount to
extreme saturation reduction. If games were graded to produce sRGB
(perhaps implicitly, like almost all SDR apps do), this would result in
some saturation reduction.

When I say "graded", I mean something that can also happen implicitly
by a developer thinking: "it looks good on my monitor, and I'd like
everyone to see it like I do". The developer may not even think about
it. We have no knowledge of what the content was graded for, but it
still happened.

> Games are not submitting us any primaries for the buffers they are sending.
> I mean they are saying they are sRGB so "technically 709", but 
> colorimetry for SDR content (outside of mastering) is very wishy-washy.

Right.

> Deck Display Info:
> static constexpr displaycolorimetry_t displaycolorimetry_steamdeck_spec
> {
> 	.primaries = { { 0.602f, 0.355f }, { 0.340f, 0.574f }, { 0.164f, 0.121f 
> } },
> 	.white = { 0.3070f, 0.3220f },  // not D65
> };
> 
> static constexpr displaycolorimetry_t displaycolorimetry_steamdeck_measured
> {
> 	.primaries = { { 0.603f, 0.349f }, { 0.335f, 0.571f }, { 0.163f, 0.115f 
> } },
> 	.white = { 0.296f, 0.307f }, // not D65
> };
> 
> https://github.com/ValveSoftware/gamescope/blob/master/src/color_helpers.h#L451
> 
> For the rest of this, consider displaycolorimetry_steamdeck_measured to 
> be what we use for the internal display.
> 
> To improve the rendering of content on the Deck's internal display with 
> the modest gamut, we go from the display's native primaries (sub 709) to 
> somewhere between the native primaries (0.0) and a hypothetical wider 
> gamut display (1.0) that we made up.

What do you mean by "we go from ... to ..."? What does "go" stand for
here?

Do I understand right, that you choose the target primaries from
somewhere between the display's native primaries and the below
hypothetical display's wider gamut primaries, based on end user
vibrance setting?

It's really curious if it indeed is *target* primaries for your color
transformation, given that you are still driving the internal display.

> The hypothetical display's primaries were decided based by making 
> content look appealing:
> static constexpr displaycolorimetry_t displaycolorimetry_widegamutgeneric
> {
> 	.primaries = { { 0.6825f, 0.3165f }, { 0.241f, 0.719f }, { 0.138f, 
> 0.050f } },
> 	.white = { 0.3127f, 0.3290f },  // D65
> };
> 
> We have a single knob for this in the UI, in code it's "SDR Gamut 
> Wideness", but known in the UI as "Color Vibrance". It's the knob that 
> picks the target color gamut that gets mapped to the native display.
> 
> This is how that single value interacts to pick the target primaries:
> 
> https://github.com/ValveSoftware/gamescope/blob/master/src/color_helpers.cpp#L798
> 
> We then use the result there to do a simple saturation fit based on the 
> kob and some additional parameters that control how we interpolate.
> (blendEnableMinSat, blendEnableMaxSat, blendAmountMin, blendAmountMax)
> 
> Those parameters also change with the SDR Gamut Wideness value, based on 
> things that "look nice". :P
> 
> https://github.com/ValveSoftware/gamescope/blob/master/src/color_helpers.cpp#L769

Alright, that's another part of the color transformation you do, the
one with the most "perceptual intent" in it, I'd say.

> We also do some other things like Bradford chromatic adaptation to fix 
> the slightly-off whitepoint too.
> 
> We use all this to generate a 3D LUT with that saturation fit, chromatic 
> adaptation and use Shaper + 3D LUT at scanout time to apply it.
> (We also have a shader based fallback path)
> 
> The goal of all of this work is less 'color accuracy' and more 'making 
> the display more inline with consumer expectations'.

Yes, that is called "perceptual rendering intent" and it is 'color
accurate' in that context, assuming your goal is to give the user an as
close experience as possible compared to a highly capable display or to
the game developer's (mastering) display. You might not know the facts
of what it should look like, but you do your best anyway.

> We wanted to try and make the display appear much more 'vivid' and 
> colourful without introducing horrible clipping.
> 
> We also use this same logic for wider gamut displays (where 0.0 = sRGB 
> and 1.0 = native) and for SDR content on HDR.
> 
> Hope this helps!

Very much, but I still have a question: what primaries do you
associate with game content pixels?

How is the matrix dest_from_source in
https://github.com/ValveSoftware/gamescope/blob/master/src/color_helpers.cpp#L701
formed, what are its inputs?

This whole email started from you claiming to do saturation boosting,
while you are obviously doing saturation reduction in total. Turns out
the saturation boosting (yes, you do that) is not the whole color
transformation you do. So we're simply talking about different portions
of the color transformation and both claims are right, I believe.


Thanks,
pq

> On 11/3/23 13:00, Pekka Paalanen wrote:
> > 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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