How to convert YUV420P image to JPEG using ffmpeg's libraries?

Question

I'm trying to convert a YUV420P image (AV_PIX_FMT_YUV420P) to a JPEG using ffmpeg's libavformat and libavcodec. This is my code so far:

AVFormatContext* pFormatCtx;
AVOutputFormat* fmt;
AVStream* video_st;
AVCodecContext* pCodecCtx;
AVCodec* pCodec;

uint8_t* picture_buf;
AVFrame* picture;
AVPacket pkt;
int y_size;
int got_picture=0;
int size;

int ret=0;

FILE *in_file = NULL;                            //YUV source
int in_w = 720, in_h = 576;                      //YUV's width and height
const char* out_file = "encoded_pic.jpg";    //Output file

in_file = fopen(argv[1], "rb");
av_register_all();

pFormatCtx = avformat_alloc_context();

fmt = NULL;
fmt = av_guess_format("mjpeg", NULL, NULL);
pFormatCtx->oformat = fmt;

//Output URL
if (avio_open(&pFormatCtx->pb,out_file, AVIO_FLAG_READ_WRITE) < 0)
{
    printf("Couldn't open output file.");
    return -1;
}

video_st = avformat_new_stream(pFormatCtx, 0);

if (video_st==NULL)
    return -1;

pCodecCtx = video_st->codec;
pCodecCtx->codec_id = fmt->video_codec;
pCodecCtx->codec_type = AVMEDIA_TYPE_VIDEO;
pCodecCtx->pix_fmt = AV_PIX_FMT_YUV420P;

pCodecCtx->width = in_w;
pCodecCtx->height = in_h;

pCodecCtx->time_base.num = 1;
pCodecCtx->time_base.den = 25;

//Output some information
av_dump_format(pFormatCtx, 0, out_file, 1);

pCodec = avcodec_find_encoder(pCodecCtx->codec_id);

if (!pCodec)
{
    printf("Codec not found.");
    return -1;
}

if (avcodec_open2(pCodecCtx, pCodec,NULL) < 0){
    printf("Could not open codec.\n");
    return -1;
}

picture = avcodec_alloc_frame();
size = avpicture_get_size(pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);

picture_buf = (uint8_t *)av_malloc(size);

if (!picture_buf)
    return -1;

avpicture_fill((AVPicture *)picture, picture_buf, pCodecCtx->pix_fmt, pCodecCtx->width, pCodecCtx->height);

//Write Header
avformat_write_header(pFormatCtx,NULL);

y_size = pCodecCtx->width * pCodecCtx->height;
av_new_packet(&pkt,y_size*3);

//Read YUV
if (fread(picture_buf, 1, y_size*3/2, in_file) <=0)
{
    printf("Could not read input file.");
    return -1;
}

picture->data[0] = picture_buf;  // Y
picture->data[1] = picture_buf+ y_size;  // U
picture->data[2] = picture_buf+ y_size*5/4; // V

//Encode
ret = avcodec_encode_video2(pCodecCtx, &pkt,picture, &got_picture);
if(ret < 0)
{
    printf("Encode Error.\n");
    return -1;
}

if (got_picture==1)
{
    pkt.stream_index = video_st->index;
    ret = av_write_frame(pFormatCtx, &pkt);
}

av_free_packet(&pkt);
//Write Trailer
av_write_trailer(pFormatCtx);

printf("Encode Successful.\n");

if (video_st)
{
    avcodec_close(video_st->codec);
    av_free(picture);
    av_free(picture_buf);
}

avio_close(pFormatCtx->pb);
avformat_free_context(pFormatCtx);

fclose(in_file);

I'm getting this error:

Output #0, mjpeg, to 'encoded_pic.jpg':
Stream #0.0: Video: mjpeg, yuv420p, 720x576, q=2-31, 200 kb/s, 90k tbn, 25 tbc
[mjpeg @ 0x78c3a0] Specified pix_fmt is not supported
Could not open codec.

If there any mention of this conversion not being allowed in ffmpeg's documentation? If there is, how can we circumvent it? How does ffmpeg internally convert it when we use the command-line version of ffmpeg?

Answer 1

The following code is your issue:

.pix_fmts       = (const enum AVPixelFormat[]){
    AV_PIX_FMT_YUVJ420P, AV_PIX_FMT_YUVJ422P, AV_PIX_FMT_YUVJ444P, AV_PIX_FMT_NONE
},

As you can see, it wants AV_PIX_FMT_YUVJ420P, not YUV420P. This is an old leftover and is largely considered deprecated, but it has not yet been removed or marked for removal, and is still used in some places. The literal meaning of YUVJ420P is "YUV420P with JPEG color-range" (i.e. pixels use 0-255 range instead of 16-235 range, where 0 instead of 16 is black, and 255 instead of 235 is white). You can alternatively also do that using AV_PIX_FMT_YUV420P and then setting e.g. avctx->color_range to AVCOL_RANGE_JPEG.

You probably don't care about that, you want to know, how can I convert AV_PIX_FMT_YUV420P to AV_PIX_FMT_YUVJ420P? Well, in this case, your input is JPEG so it probably uses JPEG range data (check that by checking avctx->color_range or avframe->color_range). Then you can simply change avframe->pix_fmt from AV_PIX_FMT_YUV420P to AV_PIX_FMT_YUVJ420P. If for whatever reason color_range is AVCOL_RANGE_MPEG, you can probably use libswscale to convert between the AV_PIX_FMT_YUV420P with MPEG-range and AV_PIX_FMT_YUVJ420P, see e.g. this example.