linux-kernel - Re: [PATCH] media: doc: pixfmt-yuv: Fix 4:4:4 subsampling info

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Date:   Mon, 25 Jan 2021 11:02:01 -0300
From:   Helen Koike <helen.koike@...labora.com>
To:     Laurent Pinchart <laurent.pinchart@...asonboard.com>
Cc:     linux-media@...r.kernel.org, hverkuil@...all.nl,
        hans.verkuil@...co.com, kernel@...labora.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] media: doc: pixfmt-yuv: Fix 4:4:4 subsampling info



On 1/25/21 10:57 AM, Helen Koike wrote:
> 
> 
> On 1/23/21 6:56 AM, Laurent Pinchart wrote:
>> Hi Helen,
>>
>> Thank you for the patch.
>>
>> On Fri, Jan 22, 2021 at 03:27:23PM -0300, Helen Koike wrote:
>>> YUV 4:4:4 is not subsampled, fix this in the docs.
>>>
>>> Fixes: da785536e007 ("media: doc: pixfmt-yuv: Move all semi-planar YUV formats to common file")
>>> Signed-off-by: Helen Koike <helen.koike@...labora.com>
>>> ---
>>>  Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst | 4 ++--
>>>  1 file changed, 2 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst b/Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst
>>> index 7d4d39201a3f..bcb4ef24c334 100644
>>> --- a/Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst
>>> +++ b/Documentation/userspace-api/media/v4l/pixfmt-yuv-planar.rst
>>> @@ -396,8 +396,8 @@ number of lines as the luma plane.
>>>  NV24 and NV42
>>>  -------------
>>>  
>>> -Semi-planar YUV 4:4:4 formats. The chroma plane is subsampled by 2 in the
>>> -horizontal direction. Chroma lines contain half the number of pixels and the
>>> +Semi-planar YUV 4:4:4 formats. No sub-sampling.
>>
>> "The chroma plane is not subsampled." ?
> 
> Ack.
> 
>>
>>> +Chroma lines contain the same number of pixels and the
>>>  same number of bytes as luma lines, and the chroma plane contains the same
>>>  number of lines as the luma plane.
>>
>> That's not quite right, the chroma lines contain twice the number of
>> pixels and bytes, as there's one Cb and one Cr value in the chroma line
>> for each Y value in the luma line.

Actually, it is the same number o pixels, but twice the number o bytes.
Since a trio (YCbCr) compose a pixel.

At least this is how I understand comparing the logic of the text description
of NV16 YUV4:2:2.

Regards,
Helen

>>
>>
>> Maybe the text could be reflowed ?
>>
> 
> Ack.
> 
> I'll submit v2 updating the text.
> 
> Thanks,
> Helen
>