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POUnderstanding FFMPEG Video Encoding
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copied!<p>Got this from the encoding example in ffmpeg. I can somewhat follow the authors example for audio encoding, but I find myself befuddled looking at the C code (I commented in block numbers to help me reference what I'm talking about)...</p> <pre><code>static void video_encode_example(const char *filename) { AVCodec *codec; AVCodecContext *c= NULL; int i, out_size, size, x, y, outbuf_size; FILE *f; AVFrame *picture; uint8_t *outbuf, *picture_buf; //BLOCK ONE printf("Video encoding\n"); /* find the mpeg1 video encoder */ codec = avcodec_find_encoder(CODEC_ID_MPEG1VIDEO); if (!codec) { fprintf(stderr, "codec not found\n"); exit(1); //BLOCK TWO } c= avcodec_alloc_context(); picture= avcodec_alloc_frame(); /* put sample parameters */ c->bit_rate = 400000; /* resolution must be a multiple of two */ c->width = 352; c->height = 288; /* frames per second */ c->time_base= (AVRational){1,25}; c->gop_size = 10; /* emit one intra frame every ten frames */ c->max_b_frames=1; c->pix_fmt = PIX_FMT_YUV420P; //BLOCK THREE /* open it */ if (avcodec_open(c, codec) < 0) { fprintf(stderr, "could not open codec\n"); exit(1); } f = fopen(filename, "wb"); if (!f) { fprintf(stderr, "could not open %s\n", filename); exit(1); } //BLOCK FOUR /* alloc image and output buffer */ outbuf_size = 100000; outbuf = malloc(outbuf_size); size = c->width * c->height; picture_buf = malloc((size * 3) / 2); /* size for YUV 420 */ picture->data[0] = picture_buf; picture->data[1] = picture->data[0] + size; picture->data[2] = picture->data[1] + size / 4; picture->linesize[0] = c->width; picture->linesize[1] = c->width / 2; picture->linesize[2] = c->width / 2; //BLOCK FIVE /* encode 1 second of video */ for(i=0;i<25;i++) { fflush(stdout); /* prepare a dummy image */ /* Y */ for(y=0;y<c->height;y++) { for(x=0;x<c->width;x++) { picture->data[0][y * picture->linesize[0] + x] = x + y + i * 3; } } //BLOCK SIX /* Cb and Cr */ for(y=0;y<c->height/2;y++) { for(x=0;x<c->width/2;x++) { picture->data[1][y * picture->linesize[1] + x] = 128 + y + i * 2; picture->data[2][y * picture->linesize[2] + x] = 64 + x + i * 5; } } //BLOCK SEVEN /* encode the image */ out_size = avcodec_encode_video(c, outbuf, outbuf_size, picture); printf("encoding frame %3d (size=%5d)\n", i, out_size); fwrite(outbuf, 1, out_size, f); } //BLOCK EIGHT /* get the delayed frames */ for(; out_size; i++) { fflush(stdout); out_size = avcodec_encode_video(c, outbuf, outbuf_size, NULL); printf("write frame %3d (size=%5d)\n", i, out_size); fwrite(outbuf, 1, out_size, f); } //BLOCK NINE /* add sequence end code to have a real mpeg file */ outbuf[0] = 0x00; outbuf[1] = 0x00; outbuf[2] = 0x01; outbuf[3] = 0xb7; fwrite(outbuf, 1, 4, f); fclose(f); free(picture_buf); free(outbuf); avcodec_close(c); av_free(c); av_free(picture); } //BLOCK TEN </code></pre> <p>Here's what I can get from the authors code block by block...</p> <p>BLOCK ONE: Initializing Variables and pointers. I couldn't find the AVFrame struct yet in the ffmpeg source code so I don't know what its referencing</p> <p>BLOCK TWO: Uses a codec from the file, if not found close. </p> <p>BLOCK THREE: Sets sample video parameters. Only thing I don't really get is gop size. I read about intra frames and I still don't get what they are.</p> <p>BLOCK FOUR: Open the file for writing...</p> <p>BLOCK FIVE: Here's where they really start losing me. Part is probably because I don't know exactly what AVFrame is, but why do they only use 3/2 of the image size?</p> <p>BLOCK SIX & SEVEN: I don't understand what they are trying to accomplish with this math.</p> <p>BLOCK EIGHT: It looks like the avcodec function does all the work here, not concerned with that for the time being..</p> <p>BLOCK NINE: Since it's outside the 25 frame for loop I assume it gets the leftover frames?</p> <p>BLOCK TEN: Close, free mem, etc...</p> <p>I know this is a large block of code to be confused with, any input would be helpful. I got put in over my head at work. Thanks in advance SO.</p>

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