FFMPEG: While decoding video, is possible to generate result to user's provided buffer?

Question

In ffmpeg decoding video scenario, H264 for example, typically we allocate an AVFrame and decode the compressed data, then we get the result fr

Answer 1

I found the way out.

There is a public member of AVCodecContext, get_buffer2, which is a callback function. While calling avcodec_decode_video2, this callback function will be invoked, and this callback function is responsible to delegate buffers and some informations to AVFrame, then avcodec_decode_video2 generate the result to the buffers of AVFrame.

The callback function, get_buffer2, is set avcodec_default_get_buffer2 as default. However, we can override this as our privided function. For example:

void our_buffer_default_free(void *opaque, uint8_t *data)
{
    // empty
}
int our_get_buffer(struct AVCodecContext *c, AVFrame *pic, int flags)
{
    assert(c->codec_type == AVMEDIA_TYPE_VIDEO);
    pic->data[0] = lrY.pBits;
    pic->data[1] = lrU.pBits;
    pic->data[2] = lrV.pBits;
    picture->linesize[0] = lrY.Pitch;
    picture->linesize[1] = lrU.Pitch;
    picture->linesize[2] = lrV.Pitch;
    pic->buf[0] = av_buffer_create(pic->data[0], pic->linesize[0] * pic->height, our_buffer_default_free, NULL, 0);
    pic->buf[1] = av_buffer_create(pic->data[1], pic->linesize[1] * pic->height / 2, our_buffer_default_free, NULL, 0);
    pic->buf[2] = av_buffer_create(pic->data[2], pic->linesize[2] * pic->height / 2, our_buffer_default_free, NULL, 0);
    return 0;
}

Before decoding, we override the callback function:

context->get_buffer2 = our_get_buffer;

Then avcodec_decode_video2 will generate the result to our provided buffers.

By the way, for C++ programs which often implementing these processes in classes, we can record this pointer first:

context->opaque = this;

And define the overridden callback function as static member:

static int myclass::my_get_buffer(struct AVCodecContext *c, AVFrame *pic, int flags)
{
    auto this_pointer = static_cast<decode_context*>(c->opaque);
    return this_pointer->my_get_buffer_real(c, pic, flags);
}
int myclass::my_get_buffer_real(struct AVCodecContext *c, AVFrame *pic, int flags)
{
    // ditto with above our_get_buffer.
    // ...
}