[Libav-user] Encoding FLV gives green screen output

[Brad O'Hearne]

Sam, 

Early on in my video encoding work I had encountered something similar. I don't know if this can help you or not -- while I'm capturing from QTKit, I am converting pixel formats and encoding to FLV. There's enough similarity there that I thought it might be worth taking a look-see at my code example. If it helps, great, if not, sorry about that, and I wish you the best of luck in figuring it out. Here's the code:

https://github.com/BigHillSoftware/QTFFmpeg

Hopefully there's something in the source that will help. 

Cheers, 

Brad

On May 5, 2013, at 4:59 AM, yy-zed <samhalligan15 at gmail.com> wrote:

> I've written a piece of C++ code that can capture webcam video frame, decode
> them, convert them to YUV420P, encode them and then write them to a file. If
> I use the mpeg2 codec and write to a .mpg file, everything works perfectly.
> But, if I use flv, then the output produced is just a green screen. I'm not
> sure if there are different encoder settings I need to set for encoding flv
> video?Or maybe I'm doing something wrong in sws_scale? Here's my code(the
> relevant parts):
> 
> Encoder settings:
> 
> c->codec_id = codec_id;
> c->bit_rate = 400000;
> // Resolution must be a multiple of two.
> c->width    = 352;
> c->height   = 288;
> c->time_base.den = STREAM_FRAME_RATE;
> c->time_base.num = 1;
> //emit one intra frame every twelve frames at most
> c->gop_size      = 12;
> c->pix_fmt       = STREAM_PIX_FMT;
> 
> Encoding a frame:
> 
> int ret;
> uint8_t *buffer = NULL;
> static struct SwsContext *sws_ctx;
> 
> //Setup the codec context, and set its width and height to be equal to the
> input video width and height
> AVCodecContext *c = st->codec;
> c->width = pCodecCtx->width;
> c->height = pCodecCtx->height;
> 
> av_init_packet(&packet);
> frameYUV = avcodec_alloc_frame();
> 
> //Determine how big the buffer will need to be to store the captured frame
> numBytes =
> avpicture_get_size(STREAM_PIX_FMT,pCodecCtx->width,pCodecCtx->height);
> 
> //Allocate the needed buffer size
> buffer = new uint8_t[numBytes];
> sws_ctx =
> sws_getContext(pCodecCtx->width,pCodecCtx->height,pCodecCtx->pix_fmt,
>                        pCodecCtx->width,pCodecCtx->height,
>                        STREAM_PIX_FMT,SWS_BICUBIC,NULL,NULL,NULL);
> 
> //Fill the output frame
> avpicture_fill((AVPicture
> *)frameYUV,buffer,STREAM_PIX_FMT,pCodecCtx->width,pCodecCtx->height);
> 
> //Read a video frame in
> av_read_frame(pFormatCtx,&packet);
> 
> //Decode the contents of packet into pFrame
> avcodec_decode_video2(pCodecCtx,pFrame,&frameFinished,&packet);
> 
> //Scale pFrame into frameYUV, and convert to PIXFMTYUV420P
> sws_scale
> (
>    sws_ctx,
>    (uint8_t const * const *)pFrame->data,
>    pFrame->linesize,
>    0,
>    pCodecCtx->height,
>    frameYUV->data,
>    frameYUV->linesize
> );
> av_init_packet(&samsPacket);
> //Encode frameYUV
> avcodec_encode_video2(c, &samsPacket, frameYUV, &gotSamsPacket);
> 
> AVPacket pkt = { 0 };
> int got_packet;
> av_init_packet(&pkt);
> // encode the image
> ret = avcodec_encode_video2(c, &pkt, frame, &got_packet);
> if (ret < 0){
>    debugLogStreamSave->debug("Error encoding video frame");
>    exit(1);
> }
> if (!ret && got_packet && pkt.size){
>    pkt.stream_index = st->index;
> 
>    // Write the compressed frame to our output
>    ret = av_interleaved_write_frame(oc, &pkt);
> 
> Really scratching my head over this one, any help would be really
> appreciated!
> 
> 
> 
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