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 10:17:59 -0500
Harry Wentland <harry.wentland@amd.com> wrote:

> On 2021-11-10 06:55, Ville Syrjälä wrote:
> > 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.

...

> > 
> > 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.
> >   
> 
> If I understand you right Intel HW represents 0xff (assuming 8 bpc) as
> the largest (representable) float that is less than 1.0. That float would
> be bigger than 255.0/256.0 but smaller than 256.0/256.0.

I was just really confused and re-reading what Ville wrote originally
now makes sense to me.

So, not what Harry wrote. Let me attempt to reiterate and mark fixed
point hex values with a h to discriminate from float values.

With 8-bit:
0x00 -> 0.000000..0h = 0.0 float
0xff -> 0.ffffff..fh = 1.0 float

Then 1.000..0h is the first value out of range.


Thanks,
pq

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