[PATCH v3 07/32] drm/amd/display: document AMDGPU pre-defined transfer functions

[Pekka Paalanen]

On Thu, 28 Sep 2023 16:16:57 -0400
Harry Wentland <harry.wentland at amd.com> wrote:

> On 2023-09-25 15:49, Melissa Wen wrote:
> > Brief documentation about pre-defined transfer function usage on AMD
> > display driver and standardized EOTFs and inverse EOTFs.
> > 
> > v3:
> > - Document BT709 OETF (Pekka)
> > - Fix description of sRGB and pure power funcs (Pekka)
> > 
> > Co-developed-by: Harry Wentland <harry.wentland at amd.com>
> > Signed-off-by: Harry Wentland <harry.wentland at amd.com>
> > Signed-off-by: Melissa Wen <mwen at igalia.com>
> > ---
> >  .../amd/display/amdgpu_dm/amdgpu_dm_color.c   | 39 +++++++++++++++++++
> >  1 file changed, 39 insertions(+)
> > 
> > diff --git a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
> > index d03bdb010e8b..14f9c02539c6 100644
> > --- a/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
> > +++ b/drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm_color.c
> > @@ -85,6 +85,45 @@ void amdgpu_dm_init_color_mod(void)
> >  }
> >  
> >  #ifdef AMD_PRIVATE_COLOR
> > +/* Pre-defined Transfer Functions (TF)
> > + *
> > + * AMD driver supports pre-defined mathematical functions for transferring
> > + * between encoded values and optical/linear space. Depending on HW color caps,
> > + * ROMs and curves built by the AMD color module support these transforms.
> > + *
> > + * The driver-specific color implementation exposes properties for pre-blending
> > + * degamma TF, shaper TF (before 3D LUT), and blend(dpp.ogam) TF and
> > + * post-blending regamma (mpc.ogam) TF. However, only pre-blending degamma
> > + * supports ROM curves. AMD color module uses pre-defined coefficients to build
> > + * curves for the other blocks. What can be done by each color block is
> > + * described by struct dpp_color_capsand struct mpc_color_caps.
> > + *
> > + * AMD driver-specific color API exposes the following pre-defined transfer
> > + * functions:
> > + *
> > + * - Linear/Unity: linear/identity relationship between pixel value and
> > + *   luminance value;
> > + * - Gamma 2.2, Gamma 2.4, Gamma 2.6: pure power functions;
> > + * - sRGB: 2.4: The piece-wise transfer function from IEC 61966-2-1:1999;
> > + * - BT.709: has a linear segment in the bottom part and then a power function
> > + *   with a 0.45 (~1/2.22) gamma for the rest of the range; standardized by
> > + *   ITU-R BT.709-6;
> > + * - PQ (Perceptual Quantizer): used for HDR display, allows luminance range
> > + *   capability of 0 to 10,000 nits; standardized by SMPTE ST 2084.
> > + *  
> 
> I think it's important to highlight that the AMD color model is
> designed with an assumption that SDR (sRGB, BT.709, G2.2, etc.)
> peak white maps (normalized to 1.0 FP) to 80 nits in the PQ system.
> This has the implication that PQ EOTF (NL-to-L) maps to [0.0..125.0].
> 125.0 = 10,000 nits / 80 nits
> 
> I think we'll want table or some other way describing this:
> 
> (Using L to mean linear and NL to mean non-linear.)
> 
> == sRGB, BT709, Gamma 2.x ==
> NL form is either UNORM or [0.0, 1.0]
> L form is [0.0, 1.0]
> 
> Note that HDR multiplier can wide range beyond [0.0, 1.0].
> In practice this means that PQ TF is needed for any subsequent
> L-to-NL transforms.
> 
> == PQ ==
> NL form is either UNORM or FP16 CCCS (Windows canonical composition color space, see [1])
> L form is [0.0, 125.0]

Hi,

what is [1]?


Thanks,
pq

> == Unity, Default ==
> NL form is either UNORM or FP16 CCCS
> L form is either [0.0, 1.0] (mapping from UNORM) or CCCS (mapping from CCCS FP16)
> 
> Harry
> 
> > + * In the driver-specific API, color block names attached to TF properties
> > + * suggest the intention regarding non-linear encoding pixel's luminance
> > + * values. As some newer encodings don't use gamma curve, we make encoding and
> > + * decoding explicit by defining an enum list of transfer functions supported
> > + * in terms of EOTF and inverse EOTF, where:
> > + *
> > + * - EOTF (electro-optical transfer function): is the transfer function to go
> > + *   from the encoded value to an optical (linear) value. De-gamma functions
> > + *   traditionally do this.
> > + * - Inverse EOTF (simply the inverse of the EOTF): is usually intended to go
> > + *   from an optical/linear space (which might have been used for blending)
> > + *   back to the encoded values. Gamma functions traditionally do this.
> > + */
> >  static const char * const
> >  amdgpu_transfer_function_names[] = {
> >  	[AMDGPU_TRANSFER_FUNCTION_DEFAULT]		= "Default",  
> 
> 

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