Re: [PATCH v4 03/11] drm/fourcc: Add DRM_FORMAT_Y8

Tomi Valkeinen
2025-03-27T14:21:23Z

Hi,

On 27/03/2025 11:20, Pekka Paalanen wrote:
> On Wed, 26 Mar 2025 15:55:18 +0200
> Tomi Valkeinen <tomi.valkeinen@ideasonboard.com> wrote:
>
>> Hi,
>>
>> On 26/03/2025 15:52, Geert Uytterhoeven wrote:
>>> Hi Tomi,
>>>
>>> On Wed, 26 Mar 2025 at 14:23, Tomi Valkeinen
>>> <tomi.valkeinen@ideasonboard.com> wrote:
>>>> Add greyscale Y8 format.
>>>>
>>>> Acked-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
>>>> Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>
>>>
>>> Thanks for your patch!
>>>
>>>> --- a/include/uapi/drm/drm_fourcc.h
>>>> +++ b/include/uapi/drm/drm_fourcc.h
>>>> @@ -405,6 +405,9 @@ extern "C" {
>>>> #define DRM_FORMAT_YUV444 fourcc_code('Y', 'U', '2', '4') /* non-subsampled Cb (1) and Cr (2) planes */
>>>> #define DRM_FORMAT_YVU444 fourcc_code('Y', 'V', '2', '4') /* non-subsampled Cr (1) and Cb (2) planes */
>>>>
>>>> +/* Greyscale formats */
>>>> +
>>>> +#define DRM_FORMAT_Y8 fourcc_code('G', 'R', 'E', 'Y') /* 8-bit Y-only */
>>>
>>> This format differs from e.g. DRM_FORMAT_R8, which encodes
>>> the number of bits in the FOURCC format. What do you envision
>>> for e.g. DRM_FORMAT_Y16? fourcc_code('G', 'R', '1', '6')?
>>
>> I wanted to use the same fourcc as on V4L2 side. Strictly speaking it's
>> not required, but different fourccs for the same formats do confuse.
>>
>> So, generally speaking, I'd pick an existing fourcc from v4l2 side if
>> possible, and if not, invent a new one.
>
> Hi Tomi,
>
> what's the actual difference between DRM_FORMAT_R8 and DRM_FORMAT_Y8?
>
> Is the difference that when R8 gets expanded to RGB, it becomes (R, 0,
> 0), but Y8 gets expanded to (c1 * Y, c2 * Y, c3 * Y) where c1..c3 are
> defined by MatrixCoefficients (H.273 terminology)?
>
> That would be my intuitive assumption following how YCbCr is handled.
> Is it obvious enough, or should there be a comment to that effect?

You raise an interesting point. Is it defined how a display driver, that
supports R8 as a format, shows R8 on screen? I came into this in the
context of grayscale formats, so I thought R8 would be handled as (R, R,
R) in RGB. But you say (R, 0, 0), which... also makes sense.

I think that's a new argument in favor of Y8: Y8 means Y-only, so the
meaning is more explicit.

How I see that the display controller would deal with Y8 (depending on
the HW):

- Take the Y value as a greyscale value, if the HW supports greyscale
format directly.
- Use the Y as YCbCr (Y, Cb-neutral, Cr-neutral), and use that if the HW
supports YCbCr directly.
- Use the Y as YCbCr as above, and convert to RGB in the usual way.

And as it's an YUV format, the limited/full range applies, which I
believe is not usually applied to RGB formats.

Does this make sense?

Tomi