per-plane LUTs and CSCs?

[Laurent Pinchart]

Hi Pekka,

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).

-- 
Regards,

Laurent Pinchart