2017-10-10 16:08 GMT+02:00 Tom Butterworth <bangno...@gmail.com>: > > > > On 9 Oct 2017, at 22:36, Carl Eugen Hoyos <ceffm...@gmail.com> wrote: > > > > 2017-10-09 21:34 GMT+02:00 Martin Vignali <martin.vign...@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 _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
