Re: [RFC v2 02/22] drm: Add Enhanced Gamma and color lut range attributes

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

On Wed, Nov 10, 2021 at 10:49:24AM +0200, Pekka Paalanen wrote:
> On Wed, 10 Nov 2021 00:02:16 +0200
> Ville Syrjälä <ville.syrjala@linux.intel.com> wrote:
> 
> > On Tue, Nov 09, 2021 at 03:47:58PM -0500, Harry Wentland wrote:
> > > On 2021-11-08 04:54, Pekka Paalanen wrote:  
> > > > On Thu, 4 Nov 2021 12:27:56 -0400
> > > > Harry Wentland <harry.wentland@amd.com> wrote:
> > > >   
> > > >> On 2021-11-04 04:38, Pekka Paalanen wrote:  
> > > >>> On Wed, 3 Nov 2021 11:08:13 -0400
> > > >>> Harry Wentland <harry.wentland@amd.com> wrote:
> > > >>>     
> > > >>>> On 2021-09-06 17:38, Uma Shankar wrote:    
> > > >>>>> Existing LUT precision structure is having only 16 bit
> > > >>>>> precision. This is not enough for upcoming enhanced hardwares
> > > >>>>> and advance usecases like HDR processing. Hence added a new
> > > >>>>> structure with 32 bit precision values.
> > > >>>>>
> > > >>>>> This also defines a new structure to define color lut ranges,
> > > >>>>> along with related macro definitions and enums. This will help
> > > >>>>> describe multi segmented lut ranges in the hardware.
> > > >>>>>
> > > >>>>> Signed-off-by: Uma Shankar <uma.shankar@intel.com>
> > > >>>>> ---
> > > >>>>>  include/uapi/drm/drm_mode.h | 58 +++++++++++++++++++++++++++++++++++++
> > > >>>>>  1 file changed, 58 insertions(+)
> 
> ...
> 
> > > >> If the framebuffer is not in FP16 the question then becomes how
> > > >> the integer pixel values relate to LUT addressing.  
> > > > 
> > > > Traditionally, and in any API I've seen (GL, Vulkan), a usual mapping
> > > > is to match minimum unsigned integer value to 0.0, and unsigned maximum
> > > > integer value to 1.0. This is how things work on the cable too, right?
> > > > (Also taking full vs. limited range video signal into account. And
> > > > conversion to cable-YUV if that happens.)
> > > > 
> > > > If you want integer format FB values to map to something else, then you
> > > > have to tag the FB with that range information, somehow. New UAPI.
> > > >   
> > > 
> > > On the cable we send integer values, not floating point. AMD HW uses
> > > floating point internally, though, and the PWL API defines floating
> > > point entries, so on some level we need to be clear what the floating
> > > point entries mean. Either we document that to be [0.0, 1.0] or we
> > > have some UAPI to define it. I'm leaning toward the latter but have
> > > to think about it some more.  
> > 
> > As for Intel hw if you have an integer pixel value of 0xff... (with
> > however many bits you have with a specific pixel format) it will get
> > extended to 0.fff... (to whatever precision the pipe has internally).
> > So if we go by that a fixed point 1.0 value in the proposed
> > drm_color_lut_range would be considered just outside the gamut. And
> > pretty sure fp16 input of 1.0 should also result in a 0.fff... internal
> > value as well [1]. I think that definition pretty much matches how GL
> > UNORM<->float conversion works as well.
> 
> Does it work that way in GL though?
> 
> I've always thought that with GL_UNSIGNED_BYTE, 0xff maps to 1.0, not
> 255.0/256.0.
> 
> Taking a random spec: OpenGL ES 2.0.25
> 
> Section 2.1.2 Data Conversions says:
> 
> 	Normalized unsigned integers represent numbers in the range
> 	[0, 1]. The conversion from a normalized unsigned integer c to
> 	the corresponding floating-point f is defined as
> 	f = c / (2^b - 1)
> 
> Note how the divisor has -1.

That seems to match what I said, or at least tried to say (~0 <-> 1.0 in
float). drm_color_lut_range being fixed point would follow the ~0 side of
that. Or at least that interpretation would very easily map to our hw.

-- 
Ville Syrjälä
Intel