linux-kernel - Re: [PATCH 2/3] docs: media: Document CLL and Mastering display

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Date:   Tue, 10 Nov 2020 12:28:07 +0200
From:   Stanimir Varbanov <stanimir.varbanov@...aro.org>
To:     Hans Verkuil <hverkuil-cisco@...all.nl>,
        linux-media@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-arm-msm@...r.kernel.org
Cc:     Nicolas Dufresne <nicolas.dufresne@...labora.com>,
        Ezequiel Garcia <ezequiel@...labora.com>
Subject: Re: [PATCH 2/3] docs: media: Document CLL and Mastering display



On 11/10/20 11:50 AM, Hans Verkuil wrote:
> On 09/11/2020 18:31, Stanimir Varbanov wrote:
>> Document Content light level and Mastering display colour volume.
>>
>> Signed-off-by: Stanimir Varbanov <stanimir.varbanov@...aro.org>
>> ---
>>  .../media/v4l/ext-ctrls-codec.rst             | 61 +++++++++++++++++++
>>  1 file changed, 61 insertions(+)
>>
>> diff --git a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> index ce728c757eaf..39d0aab5ca3d 100644
>> --- a/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> +++ b/Documentation/userspace-api/media/v4l/ext-ctrls-codec.rst
>> @@ -4382,3 +4382,64 @@ enum v4l2_mpeg_video_hevc_size_of_length_field -
>>        - Selecting this value specifies that HEVC slices are expected
>>          to be prefixed by Annex B start codes. According to :ref:`hevc`
>>          valid start codes can be 3-bytes 0x000001 or 4-bytes 0x00000001.
>> +
>> +``V4L2_CID_MPEG_VIDEO_HEVC_CLL_INFO (struct)``
>> +    The Content Light Level defines upper bounds for the nominal target
>> +    brightness light level of the pictures.
>> +
>> +.. c:type:: v4l2_ctrl_hevc_cll_info
>> +
>> +.. cssclass:: longtable
>> +
>> +.. flat-table:: struct v4l2_ctrl_hevc_cll_info
>> +    :header-rows:  0
>> +    :stub-columns: 0
>> +    :widths:       1 1 2
>> +
>> +    * - __u16
>> +      - ``max_content_light_level``
>> +      - An upper bound on the maximum light level among all individual
>> +        samples for the pictures of coded video sequence, cd/m2.
>> +    * - __u16
>> +      - ``max_pic_average_light_level``
>> +      - An upper bound on the maximum average light level among the
>> +        samples for any idividual picture of coded video sequence, cd/m2.
> 
> idividual -> individual
> 
> In the CTA-861-G spec value 0 is used to indicate that this information is
> not present. How is that handled here? Can it be 0 as well in an HEVC stream?

ITU-T Rec. H265 says: When equal to 0, no such upper bound is indicated
by max_content_light_level.

So, the meaning is the same as in CTA-861-G.

> 
> Same for the next control.
> 
>> +
>> +``V4L2_CID_MPEG_VIDEO_HEVC_MASTERING_DISPLAY (struct)``
>> +    The mastering display defines the colour volume (the colour primaries,
>> +    white point and luminance range) of a display considered to be the
>> +    mastering display for current video content.
>> +
>> +.. c:type:: v4l2_ctrl_hevc_mastering_display
>> +
>> +.. cssclass:: longtable
>> +
>> +.. flat-table:: struct v4l2_ctrl_hevc_mastering_display
>> +    :header-rows:  0
>> +    :stub-columns: 0
>> +    :widths:       1 1 2
>> +
>> +    * - __u16
>> +      - ``display_primaries_x[3]``
>> +      - Specifies the normalized x chromaticity coordinate of the colour
>> +        primary component of the mastering display.
> 
> CTA-861-G defines this as: "coded as unsigned 16-bit values in units
> of 0.00002, where 0x0000 represents zero and 0xC350 represents 1.0000."
> 
> Is that true here as well? If so, then this should be documented because
> "normalized x chromaticity coordinate" doesn't say anything meaningful.

Yes, it is the same. Will document that in next version.

> 
>> +    * - __u16
>> +      - ``display_primaries_y[3]``
>> +      - Specifies the normalized y chromaticity coordinate of the colour
>> +        primary component of the mastering display.
>> +    * - __u16
>> +      - ``white_point_x``
>> +      - Specifies the normalized x chromaticity coordinate of the white
>> +        point of the mastering display.
>> +    * - __u16
>> +      - ``white_point_y``
>> +      - Specifies the normalized y chromaticity coordinate of the white
>> +        point of the mastering display.
>> +    * - __u32
>> +      - ``max_luminance``
>> +      - Specifies the nominal maximum display luminance of the mastering
>> +        display.
> 
> In CTA-861-G this is in 1 cd/m^2 units.

In Rec. H265 max_luminance is in the range of 50 000 to 100 000 000 and
units of 0.0001 cd/m2.

> 
>> +    * - __u32
>> +      - ``min_luminance``
>> +      - specifies the nominal minimum display luminance of the mastering
>> +        display.
> 
> And this in units of 0.0001 cd/m^2.

min_luminance - range of 1 to 50 000 and units of 0.0001 cd/m2.

I will update all these in next patchset version.

> 
> Regards,
> 
> 	Hans
> 

-- 
regards,
Stan