Re: DRM Atomic property for color-space conversion

From: Brian Starkey
Date: Tue Jan 31 2017 - 10:56:46 EST

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:

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:
>* 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

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

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:


...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:


...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.


Ville Syrjälä
Intel OTC
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