Ffmpeg C++: Seeking to Frame 0 - c++

Question

I have been successfully using:
avformat_seek_file(avFormatContext_, streamIndex_, 0, frame, frame, AVSEEK_FLAG_FRAME)
This along with the example code included at the bottom has allowed me to seek to specific iframes in my videos and read frames from there until I reach the frame I want.
The problem is the video files I am using have forward and backward interpolation so the first keyframe is not at frame 0, but something like frame 8.
What I am looking for is a way to seek to the frames that exist before the first B-Frame in my video files. Any help would be appreciated.
seekFrame:
bool QVideoDecoder::seekFrame(int64_t frame)
{
if(!ok)
return false;
//printf("**** seekFrame to %d. LLT: %d. LT: %d. LLF: %d. LF: %d. LastFrameOk: %d\n",(int)frame,LastLastFrameTime,LastFrameTime,LastLastFrameNumber,LastFrameNumber,(int)LastFrameOk);
// Seek if:
// - we don't know where we are (Ok=false)
// - we know where we are but:
// - the desired frame is after the last decoded frame (this could be optimized: if the distance is small, calling decodeSeekFrame may be faster than seeking from the last key frame)
// - the desired frame is smaller or equal than the previous to the last decoded frame. Equal because if frame==LastLastFrameNumber we don't want the LastFrame, but the one before->we need to seek there
if( (LastFrameOk==false) || ((LastFrameOk==true) && (frame<=LastLastFrameNumber || frame>LastFrameNumber) ) )
{
//printf("\t avformat_seek_file\n");
if(ffmpeg::avformat_seek_file(pFormatCtx,videoStream,0,frame,frame,AVSEEK_FLAG_FRAME)<0)
return false;
avcodec_flush_buffers(pCodecCtx);
DesiredFrameNumber = frame;
LastFrameOk=false;
}
//printf("\t decodeSeekFrame\n");
return decodeSeekFrame(frame);
return true;
}
decodeSeekFrame:
bool QVideoDecoder::decodeSeekFrame(int after)
{
if(!ok)
return false;
//printf("decodeSeekFrame. after: %d. LLT: %d. LT: %d. LLF: %d. LF: %d. LastFrameOk: %d.\n",after,LastLastFrameTime,LastFrameTime,LastLastFrameNumber,LastFrameNumber,(int)LastFrameOk);
// If the last decoded frame satisfies the time condition we return it
//if( after!=-1 && ( LastDataInvalid==false && after>=LastLastFrameTime && after <= LastFrameTime))
if( after!=-1 && ( LastFrameOk==true && after>=LastLastFrameNumber && after <= LastFrameNumber))
{
// This is the frame we want to return
// Compute desired frame time
ffmpeg::AVRational millisecondbase = {1, 1000};
DesiredFrameTime = ffmpeg::av_rescale_q(after,pFormatCtx->streams[videoStream]->time_base,millisecondbase);
//printf("Returning already available frame %d # %d. DesiredFrameTime: %d\n",LastFrameNumber,LastFrameTime,DesiredFrameTime);
return true;
}
// The last decoded frame wasn't ok; either we need any new frame (after=-1), or a specific new frame with time>after
bool done=false;
while(!done)
{
// Read a frame
if(av_read_frame(pFormatCtx, &packet)<0)
return false; // Frame read failed (e.g. end of stream)
//printf("Packet of stream %d, size %d\n",packet.stream_index,packet.size);
if(packet.stream_index==videoStream)
{
// Is this a packet from the video stream -> decode video frame
int frameFinished;
avcodec_decode_video2(pCodecCtx,pFrame,&frameFinished,&packet);
//printf("used %d out of %d bytes\n",len,packet.size);
//printf("Frame type: ");
//if(pFrame->pict_type == FF_B_TYPE)
// printf("B\n");
//else if (pFrame->pict_type == FF_I_TYPE)
// printf("I\n");
//else
// printf("P\n");
/*printf("codecctx time base: num: %d den: %d\n",pCodecCtx->time_base.num,pCodecCtx->time_base.den);
printf("formatctx time base: num: %d den: %d\n",pFormatCtx->streams[videoStream]->time_base.num,pFormatCtx->streams[videoStream]->time_base.den);
printf("pts: %ld\n",pts);
printf("dts: %ld\n",dts);*/
// Did we get a video frame?
if(frameFinished)
{
ffmpeg::AVRational millisecondbase = {1, 1000};
int f = packet.dts;
int t = ffmpeg::av_rescale_q(packet.dts,pFormatCtx->streams[videoStream]->time_base,millisecondbase);
if(LastFrameOk==false)
{
LastFrameOk=true;
LastLastFrameTime=LastFrameTime=t;
LastLastFrameNumber=LastFrameNumber=f;
}
else
{
// If we decoded 2 frames in a row, the last times are okay
LastLastFrameTime = LastFrameTime;
LastLastFrameNumber = LastFrameNumber;
LastFrameTime=t;
LastFrameNumber=f;
}
//printf("Frame %d # %d. LastLastT: %d. LastLastF: %d. LastFrameOk: %d\n",LastFrameNumber,LastFrameTime,LastLastFrameTime,LastLastFrameNumber,(int)LastFrameOk);
// Is this frame the desired frame?
if(after==-1 || LastFrameNumber>=after)
{
// It's the desired frame
// Convert the image format (init the context the first time)
int w = pCodecCtx->width;
int h = pCodecCtx->height;
img_convert_ctx = ffmpeg::sws_getCachedContext(img_convert_ctx,w, h, pCodecCtx->pix_fmt, w, h, ffmpeg::PIX_FMT_RGB24, SWS_BICUBIC, NULL, NULL, NULL);
if(img_convert_ctx == NULL)
{
printf("Cannot initialize the conversion context!\n");
return false;
}
ffmpeg::sws_scale(img_convert_ctx, pFrame->data, pFrame->linesize, 0, pCodecCtx->height, pFrameRGB->data, pFrameRGB->linesize);
// Convert the frame to QImage
LastFrame=QImage(w,h,QImage::Format_RGB888);
for(int y=0;y<h;y++)
memcpy(LastFrame.scanLine(y),pFrameRGB->data[0]+y*pFrameRGB->linesize[0],w*3);
// Set the time
DesiredFrameTime = ffmpeg::av_rescale_q(after,pFormatCtx->streams[videoStream]->time_base,millisecondbase);
LastFrameOk=true;
done = true;
} // frame of interest
} // frameFinished
} // stream_index==videoStream
av_free_packet(&packet); // Free the packet that was allocated by av_read_frame
}
//printf("Returning new frame %d # %d. LastLastT: %d. LastLastF: %d. LastFrameOk: %d\n",LastFrameNumber,LastFrameTime,LastLastFrameTime,LastLastFrameNumber,(int)LastFrameOk);
//printf("\n");
return done; // done indicates whether or not we found a frame
}