per-plane LUTs and CSCs?

[Vitaly Prosyak]

On 2020-09-10 1:51 p.m., Laurent Pinchart wrote:
> 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.

It is pretty low , please, use always nonlinear signal in the pipe and it allows you to compress and 'win' about 2 bits.

-- 
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