Re: [PATCH] drm/i915/icl: Handle YCbCr to RGB conversion for BT2020 case

["Ville Syrjälä" <ville.syrjala@linux.intel.com>]

On Tue, May 07, 2019 at 02:35:15PM +0000, Shankar, Uma wrote:
> 
> 
> >-----Original Message-----
> >From: Intel-gfx [mailto:intel-gfx-bounces@lists.freedesktop.org] On Behalf Of Ville
> >Syrjälä
> >Sent: Tuesday, May 7, 2019 7:37 PM
> >To: Sharma, Shashank <shashank.sharma@intel.com>
> >Cc: intel-gfx@lists.freedesktop.org
> >Subject: Re: [Intel-gfx] [PATCH] drm/i915/icl: Handle YCbCr to RGB conversion for
> >BT2020 case
> >
> >On Tue, May 07, 2019 at 06:32:57PM +0530, Shashank Sharma wrote:
> >> From: Uma Shankar <uma.shankar@intel.com>
> >>
> >> Currently input csc for YCbCR to RGB conversion handles only
> >> BT601 and Bt709. Extending it to support BT2020 as well.
> >>
> >> Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> >> Signed-off-by: Shashank Sharma <shashank.sharma@intel.com>
> >> ---
> >>  drivers/gpu/drm/i915/intel_sprite.c | 24 ++++++++++++++++++++++++
> >>  1 file changed, 24 insertions(+)
> >>
> >> diff --git a/drivers/gpu/drm/i915/intel_sprite.c
> >> b/drivers/gpu/drm/i915/intel_sprite.c
> >> index 44aaeac1b2ed..2536e757bec2 100644
> >> --- a/drivers/gpu/drm/i915/intel_sprite.c
> >> +++ b/drivers/gpu/drm/i915/intel_sprite.c
> >> @@ -433,6 +433,18 @@ icl_program_input_csc(struct intel_plane *plane,
> >>  			0x9EF8, 0x7800, 0xABF8,
> >>  			0x0, 0x7800,  0x7ED8,
> >>  		},
> >> +		/*
> >> +		 * BT.2020 full range YCbCr -> full range RGB
> >> +		 * The matrix required is :
> >> +		 * [1.000, 0.000, 1.474,
> >> +		 *  1.000, -0.1645, -0.5713,
> >> +		 *  1.000, 1.8814, 0.0000]
> >> +		 */
> >> +		[DRM_COLOR_YCBCR_BT2020] = {
> >> +			0x7BC8, 0x7800, 0x0,
> >> +			0x8928, 0x7800, 0xAA88,
> >> +			0x0, 0x7800, 0x7F10,
> >> +		},
> >>  	};
> >>
> >>  	/* Matrix for Limited Range to Full Range Conversion */ @@ -461,6
> >> +473,18 @@ icl_program_input_csc(struct intel_plane *plane,
> >>  			0x8888, 0x7918, 0xADA8,
> >>  			0x0, 0x7918,  0x6870,
> >>  		},
> >> +		/*
> >> +		 * BT.2020 Limited range YCbCr -> full range RGB
> >> +		 * The matrix required is :
> >> +		 * [1.164, 0.000, 1.717,
> >> +		 *  1.138, -0.1873, -0.6504,
> >> +		 *  1.1380, 2.1417, 0.0000]
> >
> >Where are those 1.138 coming from?
> 
> Hi Ville,
> This is the original YCBCR to RGB BT2020 matrix:
> {
>         1.00000000000,  0.00000000000,  1.47460000000,
>         1.00000000000,  -0.16455312684, -0.57135312684,
>         1.00000000000,  1.88140000000,  0.00000000000
> };
> 
> We have to convert Limited Range YCbCr to Full Range RGB. Hence we need to apply a scale factor:
>  yscalefactor = 219.0 * normalizingfactor;
>  cbcrscalefactor = 224.0 * normalizingfactor;
> 
>  /* Scale factors are inverted for LR to FR conversion */
>  yscalefactor = 1.0 / yscalefactor;
>  cbcrscalefactor = 1.0 / cbcrscalefactor;
> 
> This yields the above results. 

Those are the coefficients for Y, so they should still
be the same for all three output channels.

igt_color_encoding gives me:
|1.1644, 0.0000, 1.6787,|
|1.1644,-0.1873,-0.6504,|
|1.1644, 2.1418, 0.0000,|

Looks like we're also misprogramming the Y pre-offset
for the full range YCbCr case.

-- 
Ville Syrjälä
Intel
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