Re: DRM Atomic property for color-space conversion

[Brian Starkey <brian.starkey@arm.com>]

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.

-Brian

>
>-- 
>Ville Syrjälä
>Intel OTC
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