per-plane LUTs and CSCs?

[Laurent Pinchart]

Hi Vitaly,

On Thu, Sep 10, 2020 at 01:09:03PM -0400, Vitaly Prosyak wrote:
> On 2020-09-10 6:56 a.m., Laurent Pinchart wrote:
> > On Thu, Sep 10, 2020 at 01:28:03PM +0300, Pekka Paalanen wrote:
> >> On Thu, 10 Sep 2020 12:30:09 +0300 Laurentiu Palcu wrote:
> >>> On Thu, Sep 10, 2020 at 11:50:26AM +0300, Pekka Paalanen wrote:
> >>>> On Thu, 10 Sep 2020 09:52:26 +0200 Daniel Vetter wrote:
> >>>>> On Thu, Sep 10, 2020 at 10:25:43AM +0300, Pekka Paalanen wrote:
> >>>>>> On Wed, 9 Sep 2020 13:57:28 +0300 Laurentiu Palcu wrote:
> >>>>>>      
> >>>>>>> Hi all,
> >>>>>>>
> >>>>>>> I was wondering whether you could give me an advise on how to proceed further
> >>>>>>> with the following issue as I'm preparing to upstream the next set of patches
> >>>>>>> for the iMX8MQ display controller(DCSS). The display controller has 3 planes,
> >>>>>>> each with 2 CSCs and one degamma LUT. The CSCs are in front and after the LUT
> >>>>>>> respectively. Then the output from those 3 pipes is blended together and then
> >>>>>>> gamma correction is applied using a linear-to-nonlinear LUT and another CSC, if
> >>>>>>> needed.
> >>>>
> >>>> Hi,
> >>>>
> >>>> hmm, so FB -> CSC -> LUT -> CSC -> blending?
> >>>>
> >>>> Is it then
> >>>> 	blending -> LUT -> CSC -> encoder
> >>>> or
> >>>> 	blending -> CSC -> LUT -> encoder?
> >>>
> >>> The DCSS pipeline topology is this:
> >>>
> >>> FB1->CSC_A->LUT->CSC_B-\
> >>> FB2->CSC_A->LUT->CSC_B-|-blender->LUT->CSC_O->encoder
> >>> FB3->CSC_A->LUT->CSC_B-/
> >>>
> >>> Basically, CSC_A is used to convert to a common colorspace if needed
> >>> (YUV->RGB) as well as to perform pixel range conversions: limited->full.
> >>> CSC_B is for gamut conversions(like 709->2020). The CSC_O is used to
> >>> convert to the colorspace used by the output (like RGB->YUV).
> >>
> >> I didn't realize that it would be correct to do RGB-YUV conversion in
> >> non-linear space, but yeah, that's what most software do too I guess.
> >>
> >>>> Are all these LUTs per-channel 1D LUTs or something else?
> >>>
> >>> LUTs are 3D, per pixel component.
> >>
> >> Sorry, which one?
> >>
> >> An example of a 3D LUT is 32x32x32 entries with each entry being a
> >> triplet, while a 1D LUT could be 1024 entries with each entry being a
> >> scalar. 1D LUTs are used per-channel so you need three of them, 3D LUTs
> >> you need just one for the color value triplet mapping.
> >>
> >> A 3D LUT can express much more than a 4x4 CTM. A 1D LUT cannot do the
> >> channel mixing that a CTM can.
> >>
> >> So if you truly have 3D LUTs everywhere, I wonder why the CSC matrix
> >> blocks exist...
> >
> > Possibly because the 3D LUT uses interpolation (it's a 17x17x17 LUT in
> > R-Car), having a separate CSC can give more precision (as well as
> > allowing the two problems to be decoupled, at a relatively low hardware
> > cost).
> 
> If you put nonlinear signal to 3DLUT then your
> precision would be improved.
> How many bits each color value has in 3DLUT ?

The whole display pipeline uses 8 bits per component.

-- 
Regards,

Laurent Pinchart