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

[Tom Butterworth]

> 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