[FFmpeg-devel] libavcodec/hap : add HapAlphaOnly decoding/encoding

[Martin Vignali]

2017-10-10 16:08 GMT+02:00 Tom Butterworth <bangnoise at gmail.com>:

>
>
> > On 9 Oct 2017, at 22:36, Carl Eugen Hoyos <ceffmpeg at gmail.com> wrote:
> >
> > 2017-10-09 21:34 GMT+02:00 Martin Vignali <martin.vignali at gmail.com>:
> >
> >>>> IMHO, from a compositing point of view, alpha only and gray is
> >>>> the same thing.
> >>>> It's common (in cgi compositing for example), to use alpha (or
> >>>> matte) as separate gray only file.
> >>>>
> >>> If this is what you believe, I don't think an RGBA representation
> >>> should be committed.
> >>>
> >> This is not about what i believe ! :-)
> >
> > I think it is.
> > I asked you to explain why it is a good idea to map alpha only
> > to rgba and if this is what users expect. Your answer seems
> > to indicate that you share my feeling that it would be unexpected.
> >
> > Carl Eugen
>
> Perhaps I am being slow (which is very possible) - I don’t understand how
> by using a gray pixel format, transcoding between an RGBA codec and Hap
> Alpha-Only would work as expected without users having to add
> counterintuitive filtering to swizzle channels?
>

Before answering, i'm fine with both pix_fmt option ! even if i found one
really more easy to understand (the gray)


I think if a user use a separate file to store alpha, is not so strange to
use gray (lot of software works like that (in compositing, edit, color
grading)).
If a user is not familiar with channel manipulation, he will probably just
use an rgba picture.
But we can also add a part in the ffmpeg doc, to explain how to encode from
RGBA, and what is expected as default.


> I would not expect there is much of an audience for this codec, but such
> audience there is is likely to be transcoding to Hap Alpha from something
> like ProRes 4444 and to expect the alpha channel to be selected - eg the
> following must work without extra options:
>
> ffmpeg -i MyProRes4444.mov -vcodec hap -format hap_alpha_only out.mov
>

Depend of the default way to use this codec.
If it use to replace for example an HAP video for animated alpha for
example if we want to have a still picture with an animated alpha : we
export one still frame, and one HAPAlpha Only video).

If we consider this is a "valid" use of this codec (which i think it is, or
i miss something),

the user will output a gray picture in his media creation software (as rgb
or gray), and encode it
ffmpeg -i myGrayMatte.mov -vcodec hap -format hap_alpha_only.mov

And if the user really want to extract the alpha plane of an rgba input,
it's probably only something like that (untested)
ffmpeg -i myRgbaInput -vf extractplanes=a -vcodec hap -format
hap_alpha_only.mov


>
> Furthermore, how is an API user to know that the frames they acquire or
> emit are in fact alpha content when they are tagged as gray?
>

We can maybe consider, that the data format value in the mov  is enough ?


>
> Patches 0001, 0002, 0006 and 0007 LGTM and are uncontentious. As they are
> required for Hap Q Alpha support I will commit these shortly, assuming
> nobody objects.
>
>
Ok for that, so we can discuss in parallel about HAPQAlpha support (with
RGBA pix_fmt output this time ! :-)

Best

Martin