[FFmpeg-devel] libavcodec/hapdec : add support for HapAphaOnly decoding

[Martin Vignali]

2017-09-27 13:25 GMT+02:00 Tom Butterworth <bangnoise at gmail.com>:

>
>
> > On 27 Sep 2017, at 11:59, Martin Vignali <martin.vignali at gmail.com>
> wrote:
> >
> >>
> >>> +        ctx->tex_fun = ctx->dxtc.rgtc1u_block;
> >>> +        avctx->pix_fmt = AV_PIX_FMT_RGB0;
> >>
> >> The rgtc1u_block function places the single channel value in every
> channel
> >> except the alpha channel
> >> The pixel format you have chosen is one which disregards the alpha
> channel
> >>
> >> The output of this alpha-only decoder should be in the alpha channel of
> a
> >> pixel format which carries alpha. Probably other channels should have
> zero
> >> values.
> >>
> >>
> > Hi Tom,
> >
> > After Carl answer, and thinking about that,
> > The goal of HapAlphaOnly, is to store alpha, but can also store gray only
> > (much better than HAP)
> > In fact, it can be call HAP Gray !
>
> But it isn’t
>
> > For example from my part, i would like to use this codec, for matte, but
> > also for gray only layer (use with color blending)
>
> This is fine, but let’s concentrate on supporting Hap Alpha Only, not your
> non-standard use of it. If you would like this flexibility in Hap, that
> should be argued for on the Hap project, not within FFmpeg.
>
> Hap Alpha Only is defined as an *alpha* channel stored in RGTC1U, and the
> implementation in FFmpeg must follow the definition (see
> https://github.com/Vidvox/hap/blob/master/documentation/HapVideoDRAFT.md <
> https://github.com/Vidvox/hap/blob/master/documentation/HapVideoDRAFT.md>
> ).
>
> The Alpha Only encoder and decoder must ingest and emit a pixel format
> which carries alpha. It may be that adding a new alpha-only pixel format is
> not seen as desirable, in which case an existing format with alpha should
> be selected. This allows users to, eg, transcode from an RGBA format to Hap
> Alpha Only and have the channel they expect be preserved from the input
> video. Any other behaviour (eg having RGB converted to grey and then stored
> as alpha) is not acceptable.
>
> > If a user want to compress a gray picture, it can be done using RGB to
> Gray
> > conversion
> > And if user want to compress only alpha or a specific channel, i think
> > ffmpeg have a filter for that (extractplanes (untested)),
>
> Your non-standard usage can still be achieved by channel switching in a
> filter, the default behaviour must be according to the standard, which is
> to encode and decode the alpha channel.
>
> Cheers - Tom
> <http://ffmpeg.org/mailman/listinfo/ffmpeg-devel>
>


Ok, in that case, the HAP Alpha Encoding current patch is fine (it take
alpha from RGBA input)
I will change the 24 to 32 as requested and send a new patch

Before sending a new patch for the decoding :
did you think, it's better to have [255,255,255, alpha] or [0,0,0, alpha]
for RGTC1 alpha decoding in RGBA ?

Martin