Message-ID: <20170316173656.GI31595@intel.com>
Date:   Thu, 16 Mar 2017 19:36:56 +0200
From:   Ville Syrjälä <ville.syrjala@...ux.intel.com>
To:     "Sharma, Shashank" <shashank.sharma@...el.com>
Cc:     Brian Starkey <brian.starkey@....com>,
        Local user for Liviu Dudau <liviu.dudau@....com>,
        dri-devel@...ts.freedesktop.org, linux-media@...r.kernel.org,
        linux-kernel@...r.kernel.org, mihail.atanassov@....com,
        "Cyr, Aric" <Aric.Cyr@....com>,
        "Wentland, Harry" <Harry.Wentland@....com>,
        Alex Deucher <alexdeucher@...il.com>
Subject: Re: DRM Atomic property for color-space conversion

On Thu, Mar 16, 2017 at 07:05:12PM +0200, Sharma, Shashank wrote:
> Regards
> 
> Shashank
> 
> 
> On 3/16/2017 5:55 PM, Brian Starkey wrote:
> > Hi,
> >
> > On Thu, Mar 16, 2017 at 05:14:07PM +0200, Sharma Shashank wrote:
> >> Regards
> >>
> >> Shashank
> >>
> >>
> >> On 3/16/2017 4:37 PM, Local user for Liviu Dudau wrote:
> >>> On Thu, Mar 16, 2017 at 04:30:59PM +0200, Ville Syrjälä wrote:
> >>>> On Thu, Mar 16, 2017 at 04:20:29PM +0200, Sharma, Shashank wrote:
> >>>>> Regards
> >>>>>
> >>>>> Shashank
> >>>>>
> >>>>>
> >>>>> On 3/16/2017 4:07 PM, Ville Syrjälä wrote:
> >>>>>> On Tue, Jan 31, 2017 at 03:55:41PM +0000, Brian Starkey wrote:
> >>>>>>> On Tue, Jan 31, 2017 at 05:15:46PM +0200, Ville Syrjälä wrote:
> >>>>>>>> On Tue, Jan 31, 2017 at 12:33:29PM +0000, Brian Starkey wrote:
> >>>>>>>>> Hi,
> >>>>>>>>>
> >>>>>>>>> On Mon, Jan 30, 2017 at 03:35:13PM +0200, Ville Syrjälä wrote:
> >>>>>>>>>> On Fri, Jan 27, 2017 at 05:23:24PM +0000, Brian Starkey wrote:
> >>>>>>>>>>> Hi,
> >>>>>>>>>>>
> >>>>>>>>>>> We're looking to enable the per-plane color management 
> >>>>>>>>>>> hardware in
> >>>>>>>>>>> Mali-DP with atomic properties, which has sparked some 
> >>>>>>>>>>> conversation
> >>>>>>>>>>> around how to handle YCbCr formats.
> >>>>>>>>>>>
> >>>>>>>>>>> As it stands today, it's assumed that a driver will 
> >>>>>>>>>>> implicitly "do the
> >>>>>>>>>>> right thing" to display a YCbCr buffer.
> >>>>>>>>>>>
> >>>>>>>>>>> YCbCr data often uses different gamma curves and signal 
> >>>>>>>>>>> ranges (e.g.
> >>>>>>>>>>> BT.609, BT.701, BT.2020, studio range, full-range), so its 
> >>>>>>>>>>> desirable
> >>>>>>>>>>> to be able to explicitly control the YCbCr to RGB conversion 
> >>>>>>>>>>> process
> >>>>>>>>>>> from userspace.
> >>>>>>>>>>>
> >>>>>>>>>>> We're proposing adding a "CSC" (color-space conversion) 
> >>>>>>>>>>> property to
> >>>>>>>>>>> control this - primarily per-plane for framebuffer->pipeline 
> >>>>>>>>>>> CSC, but
> >>>>>>>>>>> perhaps one per CRTC too for devices which have an RGB 
> >>>>>>>>>>> pipeline and
> >>>>>>>>>>> want to output in YUV to the display:
> >>>>>>>>>>>
> >>>>>>>>>>> Name: "CSC"
> >>>>>>>>>>> Type: ENUM | ATOMIC;
> >>>>>>>>>>> Enum values (representative):
> >>>>>>>>>>> "default":
> >>>>>>>>>>>     Same behaviour as now. "Some kind" of YCbCr->RGB conversion
> >>>>>>>>>>>     for YCbCr buffers, bypass for RGB buffers
> >>>>>>>>>>> "disable":
> >>>>>>>>>>>     Explicitly disable all colorspace conversion (i.e. use an
> >>>>>>>>>>>     identity matrix).
> >>>>>>>>>>> "YCbCr to RGB: BT.709":
> >>>>>>>>>>>     Only valid for YCbCr formats. CSC in accordance with BT.709
> >>>>>>>>>>>     using [16..235] for (8-bit) luma values, and [16..240] for
> >>>>>>>>>>>     8-bit chroma values. For 10-bit formats, the range 
> >>>>>>>>>>> limits are
> >>>>>>>>>>>     multiplied by 4.
> >>>>>>>>>>> "YCbCr to RGB: BT.709 full-swing":
> >>>>>>>>>>>     Only valid for YCbCr formats. CSC in accordance with 
> >>>>>>>>>>> BT.709,
> >>>>>>>>>>>     but using the full range of each channel.
> >>>>>>>>>>> "YCbCr to RGB: Use CTM":*
> >>>>>>>>>>>     Only valid for YCbCr formats. Use the matrix applied via 
> >>>>>>>>>>> the
> >>>>>>>>>>>     plane's CTM property
> >>>>>>>>>>> "RGB to RGB: Use CTM":*
> >>>>>>>>>>>     Only valid for RGB formats. Use the matrix applied via the
> >>>>>>>>>>>     plane's CTM property
> >>>>>>>>>>> "Use CTM":*
> >>>>>>>>>>>     Valid for any format. Use the matrix applied via the 
> >>>>>>>>>>> plane's
> >>>>>>>>>>>     CTM property
> >>>>>>>>>>> ... any other values for BT.601, BT.2020, RGB to YCbCr etc. 
> >>>>>>>>>>> etc. as
> >>>>>>>>>>> they are required.
> >>>>>>>>>> Having some RGB2RGB and YCBCR2RGB things in the same property 
> >>>>>>>>>> seems
> >>>>>>>>>> weird. I would just go with something very simple like:
> >>>>>>>>>>
> >>>>>>>>>> YCBCR_TO_RGB_CSC:
> >>>>>>>>>> * BT.601
> >>>>>>>>>> * BT.709
> >>>>>>>>>> * custom matrix
> >>>>>>>>>>
> >>>>>>>>> I think we've agreed in #dri-devel that this CSC property
> >>>>>>>>> can't/shouldn't be mapped on-to the existing (hardware 
> >>>>>>>>> implementing
> >>>>>>>>> the) CTM property - even in the case of per-plane color 
> >>>>>>>>> management -
> >>>>>>>>> because CSC needs to be done before DEGAMMA.
> >>>>>>>>>
> >>>>>>>>> So, I'm in favour of going with what you suggested in the 
> >>>>>>>>> first place:
> >>>>>>>>>
> >>>>>>>>> A new YCBCR_TO_RGB_CSC property, enum type, with a list of fixed
> >>>>>>>>> conversions. I'd drop the custom matrix for now, as we'd need 
> >>>>>>>>> to add
> >>>>>>>>> another property to attach the custom matrix blob too.
> >>>>>>>>>
> >>>>>>>>> I still think we need a way to specify whether the source data 
> >>>>>>>>> range
> >>>>>>>>> is broadcast/full-range, so perhaps the enum list should be 
> >>>>>>>>> expanded
> >>>>>>>>> to all combinations of BT.601/BT.709 + broadcast/full-range.
> >>>>>>>> Sounds reasonable. Not that much full range YCbCr stuff out there
> >>>>>>>> perhaps. Well, apart from jpegs I suppose. But no harm in being 
> >>>>>>>> able
> >>>>>>>> to deal with it.
> >>>>>>>>
> >>>>>>>>> (I'm not sure what the canonical naming for 
> >>>>>>>>> broadcast/full-range is,
> >>>>>>>>> we call them narrow and wide)
> >>>>>>>> We tend to call them full vs. limited range. That's how our
> >>>>>>>> "Broadcast RGB" property is defined as well.
> >>>>>>>>
> >>>>>>> OK, using the same ones sounds sensible.
> >>>>>>>
> >>>>>>>>>> And trying to use the same thing for the crtc stuff is 
> >>>>>>>>>> probably not
> >>>>>>>>>> going to end well. Like Daniel said we already have the
> >>>>>>>>>> 'Broadcast RGB' property muddying the waters there, and that 
> >>>>>>>>>> stuff
> >>>>>>>>>> also ties in with what colorspace we signal to the sink via
> >>>>>>>>>> infoframes/whatever the DP thing was called. So my gut 
> >>>>>>>>>> feeling is
> >>>>>>>>>> that trying to use the same property everywhere will just end up
> >>>>>>>>>> messy.
> >>>>>>>>> Yeah, agreed. If/when someone wants to add CSC on the output 
> >>>>>>>>> of a CRTC
> >>>>>>>>> (after GAMMA), we can add a new property.
> >>>>>>>>>
> >>>>>>>>> That makes me wonder about calling this one 
> >>>>>>>>> SOURCE_YCBCR_TO_RGB_CSC to
> >>>>>>>>> be explicit that it describes the source data. Then we can 
> >>>>>>>>> later add
> >>>>>>>>> SINK_RGB_TO_YCBCR_CSC, and it will be reasonably obvious that its
> >>>>>>>>> value describes the output data rather than the input data.
> >>>>>>>> Source and sink have a slight connotation in my mind wrt. the 
> >>>>>>>> box that
> >>>>>>>> produces the display signal and the box that eats the signal. 
> >>>>>>>> So trying
> >>>>>>>> to use the same terms to describe the internals of the pipeline 
> >>>>>>>> inside
> >>>>>>>> the "source box" migth lead to some confusion. But we do 
> >>>>>>>> probably need
> >>>>>>>> some decent names for these to make the layout of the pipeline 
> >>>>>>>> clear.
> >>>>>>>> Input/output are the other names that popped to my mind but 
> >>>>>>>> those aren't
> >>>>>>>> necessarily any better. But in the end I think I could live 
> >>>>>>>> with whatever
> >>>>>>>> names we happen to pick, as long as we document the pipeline 
> >>>>>>>> clearly.
> >>>>>>>>
> >>>>>>>> Long ago I did wonder if we should just start indexing these 
> >>>>>>>> things
> >>>>>>>> somehow, and then just looking at the index should tell you the 
> >>>>>>>> order
> >>>>>>>> of the operations. But we already have the ctm/gamma w/o any 
> >>>>>>>> indexes so
> >>>>>>>> that idea probably isn't so great anymore.
> >>>>>>>>
> >>>>>>>>> I want to avoid confusion caused by ending up with two
> >>>>>>>>> {CS}_TO_{CS}_CSC properties, where one is describing the data 
> >>>>>>>>> to the
> >>>>>>>>> left of it, and the other describing the data to the right of 
> >>>>>>>>> it, with
> >>>>>>>>> no real way of telling which way around it is.
> >>>>>>>> Not really sure what you mean. It should always be
> >>>>>>>> <left>_to_<right>_csc.
> >>>>>>> Agreed, left-to-right. But for instance on a CSC property 
> >>>>>>> representing
> >>>>>>> a CRTC output CSC (just before hitting the connector), which 
> >>>>>>> happens
> >>>>>>> to be converting RGB to YCbCr:
> >>>>>>>
> >>>>>>> CRTC -> GAMMA -> RGB_TO_YCBCR_CSC
> >>>>>>>
> >>>>>>> ...the enum value "BT.601 Limited" means that the data on the 
> >>>>>>> *right*
> >>>>>>> of RGB_TO_YCBCR_CSC is "BT.601 Limited"
> >>>>>>>
> >>>>>>> On the other hand for a CSC on the input of a plane, which 
> >>>>>>> happens to
> >>>>>>> be converting YCbCr to RGB:
> >>>>>>>
> >>>>>>> RAM -> YCBCR_TO_RGB_CSC -> DEGAMMA
> >>>>>>>
> >>>>>>> ...the enum value "BT.601 Limited" means that the data on the 
> >>>>>>> *left*
> >>>>>>> of YCBCR_TO_RGB_CSC is "BT.601 Limited".
> >>>>>>>
> >>>>>>> Indicating in the property name whether its value is describing the
> >>>>>>> data on the left or the right is needed (and I don't think 
> >>>>>>> inferring
> >>>>>>> that "it's always the YCBCR one" is the correct approach).
> >>>>>>>
> >>>>>>> In my example above, "SOURCE_xxx" would mean the enum value is
> >>>>>>> describing the "source" data (i.e. the data on the left) and
> >>>>>>> "SINK_xxx" would mean the enum value is describing the "sink" data
> >>>>>>> (i.e. the data on the right). This doesn't necessarily need to 
> >>>>>>> infer a
> >>>>>>> particular point in the pipeline.
> >>>>>> Right, so I guess you want the values to be named "<a> to <b>" as 
> >>>>>> well?
> >>>>>> Yes, I think we'll be wanting that as well.
> >>>>>>
> >>>>>> So what we might need is something like:
> >>>>>> enum YCBCR_TO_RGB_CSC
> >>>>>>   * YCbCr BT.601 limited to RGB BT.709 full
> >>>>>>   * YCbCr BT.709 limited to RGB BT.709 full <this would be the 
> >>>>>> likely default value IMO>
> >>>>>>   * YCbCr BT.601 limited to RGB BT.2020 full
> >>>>>>   * YCbCr BT.709 limited to RGB BT.2020 full
> >>>>>>   * YCbCr BT.2020 limited to RGB BT.2020 full
> >>>>>>
> >>>>>> And thanks to BT.2020 we'll need a RGB->RGB CSC property as well. 
> >>>>>> Eg:
> >>>>>> enum RGB_TO_RGB_CSC
> >>>>>>   * bypass (or separate 709->709, 2020->2020?) <this would be the 
> >>>>>> default>
> >>>>>>   * RGB BT.709 full to RGB BT.2020 full
> >
> > I like this approach, from a point of view of being explicit and
> > discoverable by userspace. It also happens to map quite nicely to our
> > hardware... we have a matrix before degamma, so we could do a
> > CSC + Gamut conversion there in one go, which is apparently not 100%
> > mathematically correct, but in general is good enough.
> >
> > ... however having talked this over a bit with someone who understands
> > the detail a lot better than me, it sounds like the "correct" thing to
> > do as per the spec is:
> >
> > CSC -> DEGAMMA -> GAMUT
> >
> > e.g.
> >
> > YCbCr bt.601 limited to RGB bt.601 full -> degamma ->
> >     RGB bt.601 full to RGB bt.709 full
> >
> > So that sounds like what we need to support in the API, and also
> > sounds more like the "separate properties" approach.
> I agree.
> >
> >>>>>>
> >>>>>> Alternatives would involve two properties to define the input and 
> >>>>>> output
> >>>>>> from the CSC separately, but then you lose the capability to see 
> >>>>>> which
> >>>>>> combinations are actually supoorted.
> >>>>> I was thinking about this too, or would it make more sense to 
> >>>>> create two
> >>>>> properties:
> >>>>> - one for gamut mapping (cases like RGB709->RGB2020)
> >>>>> - other one for Color space conversion (cases lile YUV 709 -> RGB 
> >>>>> 709)
> >>>>>
> >>>>> Gamut mapping can represent any of the fix function mapping, 
> >>>>> wereas CSC
> >>>>> can bring up any programmable matrix
> >>>>>
> >>>>> Internally these properties can use the same HW unit or even same 
> >>>>> function.
> >>>>> Does it sound any good ?
> >
> > It seems to me that actually the two approaches can be combined into
> > the same thing:
> >  * We definitely need a YCbCr-to-RGB conversion before degamma
> >    (for converting YUV data to RGB, in some flavour)
> >  * We definitely need an RGB-to-RGB conversion after gamma to handle
> >    709 layers blended with Rec.2020.
> > The exact conversion each of those properties represents (CSC + gamut,
> > CSC only, gamut only) can be implicit in the enum name.
> >
> > For hardware which has a fixed-function CSC before DEGAMMA with a
> > matrix after DEGAMMA, I'd expect to see something like below. None of
> > the YCBCR_TO_RGB_CSC values include a gamut conversion, because that
> > is instead exposed with the RGB_TO_RGB_CSC property (which represents
> > the hardware matrix)
> >
> > YCBCR_TO_RGB_CSC (before DEGAMMA):
> >     YCbCr BT.601 limited to RGB BT.601 full
> >     YCbCr BT.709 limited to RGB BT.709 full
> >     YCbCr BT.2020 limited to RGB BT.2020 full
> >
> > RGB_TO_RGB_CSC (after DEGAMMA):
> >     RGB BT.601 full to RGB BT.709 full
> >     RGB BT.709 full to RGB BT.2020 full
> >
> >
> > On the other hand, on hardware which does a CSC + Gamut conversion in
> > one go, before DEGAMMA (like ours), you might have:
> >
> > YCBCR_TO_RGB_CSC (before DEGAMMA):
> >     YCbCr BT.601 limited to RGB BT.601 full
> >     YCbCr BT.601 limited to RGB BT.709 full
> >     YCbCr BT.709 limited to RGB BT.709 full
> >     YCbCr BT.2020 limited to RGB BT.2020 full
> >
> > RGB_TO_RGB_CSC (after DEGAMMA):
> >     Not supported
> >
> > Userspace can parse the two properties to figure out its options to
> > get from desired input -> desired output. It is perhaps a little
> > verbose, but it's descriptive and flexible.
> >
> Seems to be a good idea, Ville ?

Looks pretty sane to me.

Though how we'll do the degamma/gamma is rather unclear still.

I think we might be looking at two variants of hardware:
- A fully programmable one with separate stages:
  csc -> degamma -> gamut -> gamma
- A totally fixed one with just a few different variants
  of the pipeline baked into the hardware 

If we want to expose the gamma/degamma to the user, how exactly are we
going to do that with the latter form or hardware. I guess we could
specify that if the degamma property is not exposed, there will be an
implicit degamma stage between the two csc and gamut. And if it is
exposed the output from the first csc is non-linear and thus needs
the degamma programmed to make it so before the gamut mapping.

And perhaps a similar rule could work for the gamma; If it's present
the output from the gamut mapping is expected to be linear, and
otherwise non-linear. Not quite sure about this. In fact I don't yet
know what our hardware would output from the end of the fixed pipeline.

> 
> - Shashank
> >>>> It's certainly possible. One problem is that we can't inform userspace
> >>>> upfront which combinations are supported. Whether that's a real 
> >>>> problem
> >>>> I'm not sure. With atomic userspace can of course check upfront if
> >>>> something can be done or not, but the main problem is then coming up
> >>>> with a fallback strategy that doesn't suck too badly.
> >>>>
> >
> > The approach above helps limit the set exposed to userspace to be only
> > those which are supported - because devices which don't have separate
> > hardware for the two stages won't expose values for both.
> >
> >>>> Anyways, I don't think I have any strong favorites here. Would be nice
> >>>> to hear what everyone else thinks.
> >>> I confess to a lack of experience in the subject here, but what is 
> >>> the more common
> >>> request coming from userspace: converting YUV <-> RGB but keeping 
> >>> the gammut mapping
> >>> separate, or YUV (gammut x) <-> RGB (gammut y) ? In other words: I 
> >>> can see the usefulness
> >>> of having an explicit way of decomposing the color mapping process 
> >>> and control the
> >>> parameters, but how often do apps or compositors go through the 
> >>> whole chain?
> >> Right now, more or less the interest is on the RGB->YUV conversion 
> >> side, coz till now BT 2020 gamut was not in
> >> picture. REC 601 and 709 have very close gamuts, so it was ok to 
> >> blend frames mostly without bothering about
> >> gamut, but going fwd, ones REC 2020 comes into picture, we need to 
> >> bother about mapping gamuts too, else
> >> blending Rec709 buffers and Rec2020 buffers together would cause very 
> >> visible gamut mismatch.
> >>
> >> So considering futuristic developments, it might be ok to consider 
> >> both. Still, as Ville mentioned, it would be good
> >> to hear from other too.
> >>
> >
> > Yeah I agree that we definitely need to consider both for anything we
> > come up with now.
> >
> > Cheers,
> > Brian
> >
> >> - Shashank
> >>>
> >>> Best regards,
> >>> Liviu
> >>>
> >>>> -- 
> >>>> Ville Syrjälä
> >>>> Intel OTC
> >>

-- 
Ville Syrjälä
Intel OTC

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