Video coding and keyframes

Question

I encode the video frame by frame and use x264 + ffmpeg for encoding. The original video plays beautifully, but the first few frames of my transcoded video show gray artifacts. I understand that this is due to time compression, and these artifacts disappear after a few frames.

See these two images, which are the first and second frames. The third frame is normal (i.e. there is no gray artifact and does not blur like the second)

How can I make the first frame to be a key frame (i.e. fully encoded in my output video) so that these artifacts are not displayed?

Edit - more details

Here is what I do in more detail. I used bit-difference tutorials to read video frame by frame and transcoded each frame to a new video. My encoding options are as follows:

avcodec_get_context_defaults3(c, *codec); c->codec_id = codec_id; c->bit_rate = output_bitrate; /* Resolution must be a multiple of two. */ c->width = output_width; c->height = output_height; /* timebase: This is the fundamental unit of time (in seconds) in terms * of which frame timestamps are represented. For fixed-fps content, * timebase should be 1/framerate and timestamp increments should be * identical to 1. */ st->r_frame_rate.num = output_framerate_num; st->r_frame_rate.den = output_framerate_den; c->time_base.den = output_timebase_den; c->time_base.num = output_timebase_num; c->gop_size = 3; /* emit one intra frame every twelve frames at most */ c->pix_fmt = STREAM_PIX_FMT; if (c->codec_id == AV_CODEC_ID_MPEG2VIDEO) { /* just for testing, we also add B frames */ c->max_b_frames = 2; } if (c->codec_id == AV_CODEC_ID_MPEG1VIDEO) { /* Needed to avoid using macroblocks in which some coeffs overflow. * This does not happen with normal video, it just happens here as * the motion of the chroma plane does not match the luma plane. */ c->mb_decision = 2; } c->max_b_frames = 2; c->scenechange_threshold = 0; c->rc_buffer_size = 0; c->me_method = ME_ZERO; 

Then I process each frame, maybe something is wrong there. Decoding Bit:

 while(av_read_frame(gFormatCtx, &packet)>=0) { // Is this a packet from the video stream? if(packet.stream_index==gVideoStreamIndex) { // Decode video frame avcodec_decode_video2(gVideoCodecCtx, pCurrentFrame, &frameFinished, &packet); // Did we get a video frame? if(frameFinished) { [...] if(firstPts == -999) /*Initial value*/ firstPts = packet.pts; deltaPts = packet.pts - firstPts; double seconds = deltaPts*av_q2d(gFormatCtx->streams[gVideoStreamIndex]->time_base); [...] muxing_writeVideoFrame(pCurrentFrame, packet.pts); } } } 

Actual letter:

 int muxing_writeVideoFrame(AVFrame *frame, int64_t pts) { frameCount = frameCount +1; if(frameCount > 0) { if (video_st) video_pts = (double)video_st->pts.val * video_st->time_base.num / video_st->time_base.den; else video_pts = 0.0; if (video_st && !(video_st && audio_st && audio_pts < video_pts)) { frame->pts = pts;//av_rescale_q(frame_count, video_st->codec->time_base, video_st->time_base); write_video_frame(oc, video_st, frame); } } return 0; } static int write_video_frame(AVFormatContext *oc, AVStream *st, AVFrame *frame) { int ret; static struct SwsContext *sws_ctx; //LOGI(10, frame_count); AVCodecContext *c = st->codec; /* encode the image */ AVPacket pkt; int got_output; av_init_packet(&pkt); pkt.data = NULL; // packet data will be allocated by the encoder pkt.size = 0; ret = avcodec_encode_video2(c, &pkt, frame, &got_output); if (ret < 0) { fprintf(stderr, "Error encoding video frame: %s\n", av_err2str(ret)); exit(1); } /* If size is zero, it means the image was buffered. */ if (got_output) { if (c->coded_frame->key_frame) pkt.flags |= AV_PKT_FLAG_KEY; pkt.stream_index = st->index; /* Write the compressed frame to the media file. */ ret = av_interleaved_write_frame(oc, &pkt); } else { ret = 0; } if (ret != 0) { LOGI(10, av_err2str(ret)); exit(1); } frame_count++; return got_output; } 

Answer 1

This is because your video does not start with a keyframe . To fix this, I use AviDemux . Since version 2.6

• MP4 Muxer
• MP4v2 Muxer

and possibly more support for videos starting without a keyframe.

Answer 2

Your problem, most likely, in the decoding part is not in the encoding (x264 cannot create such artifacts). And, as mentioned above, it looks like you are starting to decode not from the key frame (or maybe your stream is open-hop, and you are not dropping the first B-frames).