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• Is there a way to eliminate seek time when decoding part of a video using ffmpeg ?

  17 décembre 2019, par Babis

  I’ve got some MKV videos encoded with FFV1. For each of the frames, I want to run some complex and time-intensive python or matlab code, so I’m using multithreading, where each thread works on an individual image.

  I’ve tried extracting a single frame from the video using -ss, but it’s terribly inefficient.

  The most efficient way is to decompress everything into images in one go, but then I’m writing to disk, and then I’ll be reading from disk, therefore it’s not ideal either.

  I’ve tried using a ram disk to export images to, and reading them from python/matlab, but it’s not great performance-wise either. Also, I have to split the export into several batches, as the video file is 20GB and all of the exported images will not fit into memory

  Is there a way to rapidly extract individual frames from ffmpeg directly into RAM (or ram disk), so that they can be used by another program ? For example using something like a lookup-table.

  For reference, each video is about 20GB, comprised of 50000 frames, and they are all keyframes (it’s for archival purposes)

• ffmpeg use complex_filter with alphamerge only for a part of the video

  7 mars 2019, par Andy P

  I am trying to apply a filter to only the first few seconds of a video clip - and leave the rest of the video unchanged.

  why ?
  I got some video clips that I wanted to put on a website - unfortunatelly those clips are starting with a black background, which does not fit the website’s design. Therefor I was changing the background to transparent.

  I got that filter working from many of the great answers here (thanks to Gyan) and those videos are playing fine in common browsers :

  ffmpeg -i ${1} -filter_complex "[0]split[m][a];
  
  [a]geq='if(lt(lum(X,Y),16),0,255)',hue=s=0[al];
  
  [m][al]alphamerge,format=yuva420p" -c:v libvpx-vp9 -b:v 0 -crf 18 -an -auto-alt-ref 0 ${1}.webm

  the problem now : of course this replaces all black pixels during the video, which leads to many artefacts later on. Therefor I am searching for a way to apply that filter only to the first 5-ish seconds.

  I think I need a second split and a crop or a trim and a concat filter with a timestamp - but I can’t make it work :(

  ffmpeg -i ${1} -filter_complex "[0]split[f][s];
  
  [f]trim=start=0,duration=5[ft];
  
  [s]trim=start=6[st];
  
  [st]split[m][a];
  
  [a]geq='if(lt(lum(X,Y),16),0,255)',hue=s=0[al];
  
  [m][al]alphamerge,format=yuva420p[mal];
  
  [ft][mal]concat" -c:v libvpx-vp9 -b:v 0 -crf 18 -an -auto-alt-ref 0 ${1}.webm

  /edit : I am changing the subject slighty, to reflect the actual problem.

• How to set the reference frame number correctly in ffmpeg x265

  8 mai 2020, par luodonghui1234

  I have a problem when using x265 in ffmpeg. The following code is that I use x265 params to set the reference frame number of P frame. I have checked the x265 document and set the — ref parameter to achieve the purpose I want, but it is invalid. The resolution of the encoded P frame header to num_negative_pics is always 2. I need to set this number to 1, that is to say, any P frame only needs to refer to the previous frame.

  



  m_cVideoCodeCtxt->flags |= AV_CODEC_FLAG_CLOSED_GOP;
m_cVideoCodeCtxt->rc_max_rate    = m_iRate;
m_cVideoCodeCtxt->rc_min_rate    = (int)m_iRate / 2;
m_cVideoCodeCtxt->rc_buffer_size = m_iRate;
m_cVideoCodeCtxt->bit_rate       = (int)m_iRate;

m_cVideoCodeCtxt->width          = 1920;    
m_cVideoCodeCtxt->height         = 1080;    
m_cVideoCodeCtxt->time_base      = { 1, 30 };
m_cVideoCodeCtxt->framerate      = { 30 ,1};
m_cVideoCodeCtxt->frame_number   = 1;  
m_cVideoCodeCtxt->gop_size       = 30; 
m_cVideoCodeCtxt->max_b_frames   = 0; 
m_cVideoCodeCtxt->thread_count   = 4;
m_cVideoCodeCtxt->pix_fmt        = pix_fmt;
m_cVideoCodeCtxt->codec_type     = AVMEDIA_TYPE_VIDEO;
m_cVideoCodeCtxt->codec_id       = m_fVideoFmCtxt->video_codec_id;
int ret = av_opt_set(m_cVideoCodeCtxt->priv_data, "preset", "ultrafast", 0);
ret = av_opt_set(m_cVideoCodeCtxt->priv_data, "tune", "zerolatency", 0);

CString cstrX265Parms = L"";
cstrX265Parms.Format(L"keyint=%d:ref=1:no-open-gop=1:weightp=1:b_pyramid=1:weightb=0:cutree=0:rc-lookahead=0:bframes=0:scenecut=0:b-adapt=0:repeat-headers=1:max-merge=1", m_iFrameRate);
USES_CONVERSION;
char* cX265Parms = W2A(cstrX265Parms);
ret = av_opt_set(m_cVideoCodeCtxt->priv_data, "x265-params", cX265Parms, 0);


  



  Is there a problem with my code ? Or need to cooperate with other parameter settings