Difference between Frames and Packets in FFMPEG

Question

I am trying to decode a mpeg video file using LibAV. There are two terms which I am not able to grok properly, Frames and Packets.

As per my present understanding, Frames are uncompressed video frames and packets are the compressed frames.

Questions :

Packet has multiple frames, right?
Can a frame be only part of one Packet? I refer to the case where a half of the frame information is in packet1 and another half in packet2? Is it possible?
How will we know how many frames are in a packet in LibAV?

Rémi Desmartin Rémi Desmartin · Accepted Answer · 2020-05-30T16:17:17

To answer your first and third questions:

according to the doc for the AVPacket class: "For video, it should typically contain one compressed frame. For audio it may contain several compressed frames.
the decode video example gives this code that reads all frames within a packet; you can also use it to count the frames:

static void decode(AVCodecContext *dec_ctx, AVFrame *frame, AVPacket *pkt,
                   const char *filename)
{
    char buf[1024];
    int ret;
    ret = avcodec_send_packet(dec_ctx, pkt);
    if (ret < 0) {
        fprintf(stderr, "Error sending a packet for decoding\n");
        exit(1);
    }
    while (ret >= 0) {
        ret = avcodec_receive_frame(dec_ctx, frame);
        if (ret == AVERROR(EAGAIN) || ret == AVERROR_EOF)
            return;
        else if (ret < 0) {
            fprintf(stderr, "Error during decoding\n");
            exit(1);
        }
        printf("saving frame %3d\n", dec_ctx->frame_number);
        fflush(stdout);
        /* the picture is allocated by the decoder. no need to
           free it */
        snprintf(buf, sizeof(buf), filename, dec_ctx->frame_number);
        pgm_save(frame->data[0], frame->linesize[0],
                 frame->width, frame->height, buf);
    }
}

Difference between Frames and Packets in FFMPEG

2 Answers