[FFmpeg-devel] [RFC][PATCH] avcodec: add a get_enc_buffer() callback to AVCodecContext

[Lynne]

Feb 20, 2021, 21:53 by jamrial at gmail.com:

> This callback is functionally the same as get_buffer2() is for decoders, and
> implements for the new encode API the functionality of the old encode API had
> where the user could provide their own buffers.
>
> Signed-off-by: James Almer <jamrial at gmail.com>
> ---
> As suggested by Anton, here's get_buffer() for encoders. This way, users of the
> new encode API can still provide their own buffers, like they could with the
> old API, now that it's being removed.
>
> The initial implementation of the default callback uses the existing method of
> simply calling av_new_packet() for it. In the future, a buffer pool could be
> used instead.
> Is AV_GET_ENC_BUFFER_FLAG_REF useful here? Is there an encoder that keeps a
> reference to a previous packet around?
>

Could be useful, and keeps it symmetrical with decoding, so I'd keep it,
just in case.


> Alternative names for the callback field and public default callback function
> are welcome, hence it being RFC.
>

get_enc_buffer() -> get_encoder_buffer()
avcodec_default_get_enc_buffer-> avcodec_default_get_encoder_buffer()
That's all I'd suggest. It's just 4 more characters.
 

> +    /**
> +     * This callback is called at the beginning of each packet to get a data
> +     * buffer for it.
> +     *
> +     * The following field will be set in the packet before this callback is
> +     * called:
> +     * - size (may by zero)
> +     * This callback must use the above value to calculate the required buffer size,
> +     * which must padded by at least AV_INPUT_BUFFER_PADDING_SIZE bytes.
> +     *
> +     * This callback must fill the following fields in the packet:
> +     * - data
> +     * - buf must contain a pointer to an AVBufferRef structure. The packet's
> +     *   data pointer must be contained in it.
> +     *   See: av_buffer_create(), av_buffer_alloc(), and av_buffer_ref().
>

Does a size of zero mean you can have a NULL packet data and buffer
or a AV_INPUT_BUFFER_PADDING_SIZE-sized buffer?


> +     *
> +     * If AV_CODEC_CAP_DR1 is not set then get_enc_buffer() must call
> +     * avcodec_default_get_enc_buffer() instead of providing a buffer allocated by
> +     * some other means.
> +     *
> +     * If AV_GET_ENC_BUFFER_FLAG_REF is set in flags then the packet may be reused
> +     * (read and/or written to if it is writable) later by libavcodec.
> +     *
> +     * @see avcodec_default_get_enc_buffer()
> +     *
> +     * - encoding: Set by libavcodec, user can override.
> +     * - decoding: unused
> +     */
> +    int (*get_enc_buffer)(struct AVCodecContext *s, AVPacket *pkt, int flags);
>

Maybe mention thread safety? Since in a frame-threaded encoder
this may be called from different threads.

Rest looks good.