Re: [RFC PATCH 0/3] A drm_plane API to support HDR planes

[Harry Wentland <harry.wentland@amd.com>]

On 2021-05-18 3:56 a.m., Pekka Paalanen wrote:
> On Mon, 17 May 2021 15:39:03 -0400
> Vitaly Prosyak <vitaly.prosyak@amd.com> wrote:
> 
>> On 2021-05-17 12:48 p.m., Sebastian Wick wrote:
>>> On 2021-05-17 10:57, Pekka Paalanen wrote:  
>>>> On Fri, 14 May 2021 17:05:11 -0400
>>>> Harry Wentland <harry.wentland@amd.com> wrote:
>>>>  
>>>>> On 2021-04-27 10:50 a.m., Pekka Paalanen wrote:  
>>>>>> On Mon, 26 Apr 2021 13:38:49 -0400
>>>>>> Harry Wentland <harry.wentland@amd.com> wrote:  
>>>>
>>>> ...
>>>>  
>>>>>>> ## Mastering Luminances
>>>>>>>
>>>>>>> Now we are able to use the PQ 2084 EOTF to define the luminance of
>>>>>>> pixels in absolute terms. Unfortunately we're again presented with
>>>>>>> physical limitations of the display technologies on the market   
>>>>> today.  
>>>>>>> Here are a few examples of luminance ranges of displays.
>>>>>>>
>>>>>>> | Display                  | Luminance range in nits |
>>>>>>> | ------------------------ | ----------------------- |
>>>>>>> | Typical PC display       | 0.3 - 200 |
>>>>>>> | Excellent LCD HDTV       | 0.3 - 400 |
>>>>>>> | HDR LCD w/ local dimming | 0.05 - 1,500 |
>>>>>>>
>>>>>>> Since no display can currently show the full 0.0005 to 10,000 nits
>>>>>>> luminance range the display will need to tonemap the HDR content,   
>>>>> i.e  
>>>>>>> to fit the content within a display's capabilities. To assist with
>>>>>>> tonemapping HDR content is usually accompanied with a metadata that
>>>>>>> describes (among other things) the minimum and maximum mastering
>>>>>>> luminance, i.e. the maximum and minimum luminance of the display   
>>>>> that  
>>>>>>> was used to master the HDR content.
>>>>>>>
>>>>>>> The HDR metadata is currently defined on the drm_connector via the
>>>>>>> hdr_output_metadata blob property.
>>>>>>>
>>>>>>> It might be useful to define per-plane hdr metadata, as different
>>>>>>> planes might have been mastered differently.  
>>>>>>
>>>>>> I don't think this would directly help with the dynamic range   
>>>>> blending  
>>>>>> problem. You still need to establish the mapping between the optical
>>>>>> values from two different EOTFs and dynamic ranges. Or can you know
>>>>>> which optical values match the mastering display maximum and minimum
>>>>>> luminances for not-PQ?
>>>>>>  
>>>>>
>>>>> My understanding of this is probably best illustrated by this example:
>>>>>
>>>>> Assume HDR was mastered on a display with a maximum white level of 500
>>>>> nits and played back on a display that supports a max white level of 
>>>>> 400
>>>>> nits. If you know the mastering white level of 500 you know that 
>>>>> this is
>>>>> the maximum value you need to compress down to 400 nits, allowing 
>>>>> you to
>>>>> use the full extent of the 400 nits panel.  
>>>>
>>>> Right, but in the kernel, where do you get these nits values from?
>>>>
>>>> hdr_output_metadata blob is infoframe data to the monitor. I think this
>>>> should be independent of the metadata used for color transformations in
>>>> the display pipeline before the monitor.
>>>>
>>>> EDID may tell us the monitor HDR metadata, but again what is used in
>>>> the color transformations should be independent, because EDIDs lie,
>>>> lighting environments change, and users have different preferences.
>>>>
>>>> What about black levels?
>>>>
>>>> Do you want to do black level adjustment?
>>>>
>>>> How exactly should the compression work?
>>>>
>>>> Where do you map the mid-tones?
>>>>
>>>> What if the end user wants something different?  
>>>
>>> I suspect that this is not about tone mapping at all. The use cases
>>> listed always have the display in PQ mode and just assume that no
>>> content exceeds the PQ limitations. Then you can simply bring all
>>> content to the color space with a matrix multiplication and then map the
>>> linear light content somewhere into the PQ range. Tone mapping is
>>> performed in the display only.
> 
> The use cases do use the word "desktop" though. Harry, could you expand
> on this, are you seeking a design that is good for generic desktop
> compositors too, or one that is more tailored to "embedded" video
> player systems taking the most advantage of (potentially
> fixed-function) hardware?
> 

The goal is to enable this on a generic desktop, such as generic Wayland
implementations or ChromeOS. We're not looking for a custom solution for
some embedded systems, though the solution we end up with should obviously
not prevent an implementation on embedded video players.

> What matrix would one choose? Which render intent would it
> correspond to?
> 
> If you need to adapt different dynamic ranges into the blending dynamic
> range, would a simple linear transformation really be enough?
> 
>>> From a generic wayland compositor point of view this is uninteresting.
>>>  
>> It a compositor's decision to provide or not the metadata property to 
>> the kernel. The metadata can be available from one or multiple clients 
>> or most likely not available at all.
>>
>> Compositors may put a display in HDR10 ( when PQ 2084 INV EOTF and TM 
>> occurs in display ) or NATIVE mode and do not attach any metadata to the 
>> connector and do TM in compositor.
>>
>> It is all about user preference or compositor design, or a combination 
>> of both options.
> 
> Indeed. The thing here is that you cannot just add KMS UAPI, you also
> need to have the FOSS userspace to go with it. So you need to have your
> audience defined, userspace patches written and reviewed and agreed
> to be a good idea. I'm afraid this particular UAPI design would be
> difficult to justify with Weston. Maybe Kodi is a better audience?
> 

I'm not sure designing a UAPI for Kodi that's not going to work for
Wayland-compositors is the right thing. From a KMS driver maintainer
standpoint I don't want an API for each userspace.

The idea here is to do design and discussion in public so we can eventually
arrive at a UAPI for HDR and CM that works for Wayland and by extension
for every other userspace.

> But then again, one also needs to consider whether it is enough for a
> new UAPI to satisfy only part of the possible audience and then need
> yet another new UAPI to satisfy the rest. Adding new UAPI requires
> defining the interactions with all existing UAPI as well.
> 
> Maybe we do need several different UAPIs for the "same" things if the
> hardware designs are too different to cater with just one.
> 

I feel we should have a section in the RFC that sketches how different HW
deals with this currently. It would be good if we can arrive at a UAPI that
captures at least the common functionality of various HW.

Harry

> 
> Thanks,
> pq
>