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

[Pekka Paalanen <ppaalanen@gmail.com>]

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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.


Thanks,
pq

> [1] though IIRC some our hw did get that a bit wrong and it
>     actually generates a 1.0 fixed point value for the pipe


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