Re: [PATCH v2 1/1] virtio-video: Add virtio video device specification

[Tomasz Figa <tfiga@chromium.org>]

On Sat, Dec 21, 2019 at 12:46 AM Dmitry Sepp
<dmitry.sepp@opensynergy.com> wrote:
> On Freitag, 20. Dezember 2019 16:26:50 CET Keiichi Watanabe wrote:
> > On Thu, Dec 19, 2019 at 10:28 PM Dmitry Sepp
> > <dmitry.sepp@opensynergy.com> wrote:
> > > On Mittwoch, 18. Dezember 2019 14:02:14 CET Keiichi Watanabe wrote:
[snip]
> > > > +enum virtio_video_format {
> > > > + VIRTIO_VIDEO_FORMAT_UNDEFINED = 0,
> > > > + /* Raw formats */
> > > > + VIRTIO_VIDEO_FORMAT_NV12 = 1,
> > > > + VIRTIO_VIDEO_FORMAT_YUV420,
> > > > + VIRTIO_VIDEO_FORMAT_YVU420,
> > >
> > > Let's add some variants of RGB, like RGBA, ARGB. We need it for the
> > > encoder in particular .
> >
> > Sounds good.
> > BTW, which "ARGB8888" or "A8R8G8B8" is preferred? While the first one
> > comes from DRM's FourCC, the second one comes from virtio_gpu_formats.
> > I personally prefer the first one, then we can have a naming convention
> > like: VIRTIO_VIDEO_FORMAT_<name from drm_fourcc.h>
> >
>
> I'd go with ARGB8888 and BGRA8888 (might be with X variants).

Just to make sure we're talking about the same formats. DRM naming
convention is based on the little endian convention, which for 32-bit
formats means that you interpret the whole pixel as a packed 32-bit
word on a little endian system. For ARGB8888 that would mean (bit 31)
ARGB (bit 0) in the 32-bit word and (byte 0) B, G, R, A (byte 3) when
looking at separate bytes in memory. Does that correspond to your
expected format?

We also have to be specific about the A and X formats, as the A format
should be supported only if the hardware (host) doesn't ignore the
alpha channel. I haven't seen any hardware capable of encoding alpha
channel yet, but apparently for WebM the standard is to just encode
the alpha channel into another stream as Y, together with dummy U and
V values. [1] That sounds like something that would be handled by two
separate encoding streams and not just one that accepts RGBA on the
input.

[1] http://wiki.webmproject.org/alpha-channel

Best regards,
Tomasz