New uAPI for color management proposal and feedback request

Thu May 20 07:58:26 UTC 2021

On Wed, 19 May 2021 16:49:35 +0300
Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:

> On Wed, May 19, 2021 at 12:34:05PM +0300, Pekka Paalanen wrote:
> > On Wed, 12 May 2021 16:04:16 +0300
> > Ville Syrjälä <ville.syrjala at linux.intel.com> wrote:
> >   
> > > On Wed, May 12, 2021 at 02:06:56PM +0200, Werner Sembach wrote:  
> > > > Hello,
> > > > 
> > > > In addition to the existing "max bpc", and "Broadcast RGB/output_csc" drm properties I propose 4 new properties:
> > > > "preferred pixel encoding", "active color depth", "active color range", and "active pixel encoding"
> > > > 
> > > > 
> > > > Motivation:
> > > > 
> > > > Current monitors have a variety pixel encodings available: RGB, YCbCr 4:4:4, YCbCr 4:2:2, YCbCr 4:2:0.
> > > > 
> > > > In addition they might be full or limited RGB range and the monitors accept different bit depths.
> > > > 
> > > > Currently the kernel driver for AMD and Intel GPUs automatically configure the color settings automatically with little
> > > > to no influence of the user. However there are several real world scenarios where the user might disagree with the
> > > > default chosen by the drivers and wants to set his or her own preference.
> > > > 
> > > > Some examples:
> > > > 
> > > > 1. While RGB and YCbCr 4:4:4 in theory carry the same amount of color information, some screens might look better on one
> > > > than the other because of bad internal conversion. The driver currently however has a fixed default that is chosen if
> > > > available (RGB for Intel and YCbCr 4:4:4 for AMD). The only way to change this currently is by editing and overloading
> > > > the edid reported by the monitor to the kernel.
> > > > 
> > > > 2. RGB and YCbCr 4:4:4 need a higher port clock then YCbCr 4:2:0. Some hardware might report that it supports the higher
> > > > port clock, but because of bad shielding on the PC, the cable, or the monitor the screen cuts out every few seconds when
> > > > RGB or YCbCr 4:4:4 encoding is used, while YCbCr 4:2:0 might just work fine without changing hardware. The drivers
> > > > currently however always default to the "best available" option even if it might be broken.
> > > > 
> > > > 3. Some screens natively only supporting 8-bit color, simulate 10-Bit color by rapidly switching between 2 adjacent
> > > > colors. They advertise themselves to the kernel as 10-bit monitors but the user might not like the "fake" 10-bit effect
> > > > and prefer running at the native 8-bit per color.
> > > > 
> > > > 4. Some screens are falsely classified as full RGB range wile they actually use limited RGB range. This results in
> > > > washed out colors in dark and bright scenes. A user override can be helpful to manually fix this issue when it occurs.
> > > > 
> > > > There already exist several requests, discussion, and patches regarding the thematic:
> > > > 
> > > > - https://gitlab.freedesktop.org/drm/amd/-/issues/476
> > > > 
> > > > - https://gitlab.freedesktop.org/drm/amd/-/issues/1548
> > > > 
> > > > - https://lkml.org/lkml/2021/5/7/695
> > > > 
> > > > - https://lkml.org/lkml/2021/5/11/416
> > > >   
> > 
> > ...
> >   
> > > > Adoption:
> > > > 
> > > > A KDE dev wants to implement the settings in the KDE settings GUI:
> > > > https://gitlab.freedesktop.org/drm/amd/-/issues/476#note_912370
> > > > 
> > > > Tuxedo Computers (my employer) wants to implement the settings desktop environment agnostic in Tuxedo Control Center. I
> > > > will start work on this in parallel to implementing the new kernel code.    
> > > 
> > > I suspect everyone would be happier to accept new uapi if we had
> > > multiple compositors signed up to implement it.  
> > 
> > I think having Weston support for these would be good, but for now it
> > won't be much of an UI: just weston.ini to set, and the log to see what
> > happened.
> > 
> > However, knowing what happened is going to be important for color
> > calibration auditing:
> > https://gitlab.freedesktop.org/wayland/weston/-/issues/467
> > 
> > Yes, please, very much for read-only properties for the feedback part.
> > Properties that both userspace and kernel will write are hard to deal
> > with in general.
> > 
> > Btw. "max bpc" I can kind of guess that conversion from framebuffer
> > format to the wire bpc happens automatically and only as the final
> > step,  
> 
> Well, there could be dithering and whatnot also involved. So it's
> not super well specified atm either.

I tend to forget that dithering is a thing. I guess it could be
temporal and/or spatial depending on hardware?

> > but "Broadcast RGB" is more complicated: is the output from the
> > abstract pixel pipeline sent as-is and "Broadcast RGB" is just another
> > inforframe bit to the monitor, or does "Broadcast RGB" setting actually
> > change what happens in the pixel pipeline *and* set infoframe bits?  
> 
> It does indeed compress the actual pixel data. There was once a patch
> porposed to introduce a new enum value that only sets the infoframe and
> thus would allow userspace to pass through already limited range data.
> Shouldn't be hard to resurrect that if needed.

Right, thanks for confirming. I mentioned this mostly for Werner to
point out that existing properties might do surprising things.
Especially if one has looked at HDR related properties which only set
infoframe bits and nothing more.

> > My vague recollection is that framebuffer was always assumed to be in
> > full range, and then if "Broadcast RGB" was set to limited range, the
> > driver would mangle the pixel pipeline to convert from full to limited
> > range. This means that it would be impossible to have limited range
> > data in a framebuffer, or there might be a double-conversion by
> > userspace programming a LUT for limited->full and then the driver
> > adding full->limited. I'm also confused how full/limited works when
> > framebuffer is in RGB/YCbCr and the monitor wire format is in RGB/YCbCr
> > and there may be RGB->YCbCR or YCbCR->RGB conversions going on - or
> > maybe even FB YCbCR -> RGB -> DEGAMMA -> CTM -> GAMMA -> YCbCR.
> > 
> > I wish someone drew a picture of the KMS abstract pixel pipeline with
> > all the existing KMS properties in it. :-)  
> 
> Here's an ugly one for i915:
> 
>     (input RGB vs. YCbCr?)
> [FB] -> [YCbCr?] -> [YCbCr->RGB conversion     ] -> [plane blending] -> ...
>       |             [YCbCr color range/encoding] |
>       \ [RGB?] ----------------------------------/
> 
>                                            (output RGB limited vs. RGB full vs. YCbCr?)
> ... -> [DEGAMMA_LUT] -> [CTM] -> [GAMMA_LUT] -> [YCbCr?] -> [RGB->YCbCr conversion      ] -> [to port]
>                                               |             [always BT.709/limited range]
>                                               \ [RGB?] -> ...
> 
> ... -> [RGB passthrough             ] -> [to port]
>      | [Broadcast RGB=full or       ]
>      | [Broadcast RGB=auto + IT mode]
>      |
>      \ [RGB full->limited conversion] -> [to port]
>        [Broadcast RGB=limited or    ]
>        [Broadcast RGB=auto + CE mode]
> 
> I guess having something like that in the docs would be nice. Not sure
> if there's a way to make something that looks decent for html/etc.

It would be super useful indeed.

Thanks,
pq
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