[libcamera-devel] [PATCH 7/8] libcamera: v4l2_videodevice: Map V4L2_PIX_FMT_GREY to DRM FourCC

[Laurent Pinchart]

Hi Nicolas,

On Sat, Mar 14, 2020 at 07:14:12AM -0400, Nicolas Dufresne wrote:
> Le samedi 14 mars 2020 à 01:38 +0200, Laurent Pinchart a écrit :
> > DRM has a format for 8-bit greyscale data, DRM_FORMAT_R8. Despite the
> > 'R' name, which comes from GL/Vulkan to mean single-channel data, the
> > format maps to greyscale for display. We can thus map it to
> > V4L2_PIX_FMT_GREY.
> 
> The mapping looks fine to me, but may lead to surprising result in
> absence of colorimetry support. Most YCbCr 4:0:0 will be limited range,
> while on the GFX side, most of R8 usage will be full range. Just be
> aware.

Thanks for the comment. In the specific case I'm trying to support, data
comes from a sensor that doesn't have a Bayer filter, so it's really
full range greyscale (it's actually 10 bits scaled down to 8 bits). In
the general case, we need to convey colorimetry information, I agree
with you, and that's something that can't be conveyed just through a
4CC, we will need a dedicated API.

> > Signed-off-by: Laurent Pinchart <laurent.pinchart at ideasonboard.com>
> > ---
> >  src/libcamera/v4l2_videodevice.cpp | 9 ++++++++-
> >  1 file changed, 8 insertions(+), 1 deletion(-)
> > 
> > diff --git a/src/libcamera/v4l2_videodevice.cpp b/src/libcamera/v4l2_videodevice.cpp
> > index f68b67b33293..f89bf2ff781e 100644
> > --- a/src/libcamera/v4l2_videodevice.cpp
> > +++ b/src/libcamera/v4l2_videodevice.cpp
> > @@ -1473,12 +1473,15 @@ PixelFormat V4L2VideoDevice::toPixelFormat(uint32_t v4l2Fourcc)
> >  	case V4L2_PIX_FMT_NV21M:
> >  		return DRM_FORMAT_NV21;
> >  
> > +	/* Greyscale formats. */
> > +	case V4L2_PIX_FMT_GREY:
> > +		return DRM_FORMAT_R8;
> > +
> >  	/* Compressed formats. */
> >  	case V4L2_PIX_FMT_MJPEG:
> >  		return DRM_FORMAT_MJPEG;
> >  
> >  	/* V4L2 formats not yet supported by DRM. */
> > -	case V4L2_PIX_FMT_GREY:
> >  	default:
> >  		/*
> >  		 * \todo We can't use LOG() in a static method of a Loggable
> > @@ -1557,6 +1560,10 @@ uint32_t V4L2VideoDevice::toV4L2Fourcc(PixelFormat pixelFormat, bool multiplanar
> >  	case DRM_FORMAT_NV21:
> >  		return V4L2_PIX_FMT_NV21;
> >  
> > +	/* Greyscale formats. */
> > +	case DRM_FORMAT_R8:
> > +		return V4L2_PIX_FMT_GREY;
> > +
> >  	/* Compressed formats. */
> >  	case DRM_FORMAT_MJPEG:
> >  		return V4L2_PIX_FMT_MJPEG;

-- 
Regards,

Laurent Pinchart