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POVideo encoding and keyframes
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copied!<p>I am transcoding a video frame by frame and using x264+ffmpeg to encode. The original video plays fine, but the first few frames of my transcoded vide show grey artefacts. I understand this is because of time compression and these artefacts disappear after a few frames. </p> <p>See these two pictures which are the first and second frames. The third frame is normal (i.e. no grey artefact and not blurry like the second one) <img src="https://i.stack.imgur.com/YmhPz.png" alt="First frame"> <img src="https://i.stack.imgur.com/MQTGJ.png" alt="Second frame"></p> <p>How can I force the first frame to be a key frame (ie fully encoded in my output video) so that these artefacts do not show?</p> <p><strong>Edit - more details</strong></p> <p>Here is what I am doing more in details. I used bit form differents tutorials to read a video frame by frame and reencode each frame to a new video. My encoding parameters are the following:</p> <pre><code>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; </code></pre> <p>Then I process each frame, probably doing something wrong there. The decoding bit:</p> <pre><code>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); } } } </code></pre> <p>The actual writing:</p> <pre><code>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; } </code></pre>

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