[FFmpeg-devel] [libav-devel] [RFC] Exporting the alpha mode from decoders

[Vittorio Giovara]

On Fri, Feb 6, 2015 at 11:32 AM, wm4 <nfxjfg at googlemail.com> wrote:
> This is a proposal for an API extension.
>
> Currently, some pixel formats support alpha, but whether the alpha
> component contains something useful or just garbage is not part of the
> pixel format definition. This applies at least to packed RGB formats,
> where the 4th component is either alpha or garbage depending on the
> context.
>
> This means that if a decoder outputs such a packed RGB format, an API
> user has no idea whether it has alpha or not. E.g. take PNG and
> Camtasia (tscc.c). PNG with alpha outputs AV_PIX_FMT_RGBA, and Camtasia
> can output AV_PIX_FMT_RGB32 (which is ARGB or BGRA depending on
> endian). Camtasia supports no alpha, and the alpha component literally
> contains garbage (AFAIK and judging by the single sample I've seen). An
> application decoding both of these can't know whether the output frame
> has alpha or not, unless every codec is special-cased.
>
> One possible solution is duplicating all these pixel formats, so you'd
> have e.g. AV_PIX_FMT_RGBA and AV_PIX_FMT_RGBX. But I think we all agree
> that we don't want more pixel formats.
>
> The other solution, which I want to advocate here, is adding a field to
> AVFrame that indicates the alpha mode. Something like:
>
> typedef enum AVAlphaMode {
>     // if an alpha component is present, its function is unknown, and
>     // it may be garbage
>     AV_ALPHA_UNKNOWN,
>     // the alpha component contains premultiplied alpha
>     // (not sure if any file format actually uses this)
>     AV_ALPHA_PREMULTIPLIED,
>     // the alpha component contains straight alpha
>     // (e.g. PNG)
>     AV_ALPHA_STRAIGHT,
> } AVAlphaMode;
>
> typedef struct AVFrame {
>     ...
>     AVAlphaMode alpha_mode;
> } AVFrame;
>
> Thoughts?

The problem looks interesting. I am not sure samples with
premultiplied alpha exist (or what swscale does in that case).
Another approach could be to expand avframe->flag, in order to signal
when alpha channel contains garbage, rather than introducing a new
field.
-- 
Vittorio