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

[Harry Wentland]

On 2023-09-29 03:35, Pekka Paalanen wrote:
> 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]?
> 

Oops.

[1] https://learn.microsoft.com/en-us/windows/win32/direct3darticles/high-dynamic-range

Harry

> 
> 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",  
>>
>>
>