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

[Pekka Paalanen <pekka.paalanen@haloniitty.fi>]

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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?


Thanks,
pq

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