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• what random function the ffmpeg filter expression called ?

  8 octobre 2020, par Wang

  for example this random(-1) in geq :

  


  ffmpeg -f lavfi -i nullsrc=s=10x10:r=25:d=4 -vf geq=lum='255':cb='255*random(-1)':cr='255*random(-1)',scale=200:200:flags=neighbor -bf 0 test.mp4


  


  I always get exactly the same color of first 2 pixels in the first frame. And the first frame always has very similar colors. Star from 3rd frame its start to getting better. Moreover, the random() seems only introduce random inside single frame. The sequence of first pixel of each frame is exactly the same :

  


  ffmpeg -f lavfi -i nullsrc=s=1x1:r=25:d=4 -vf geq=lum=255:cb='255*random(-1)':cr='255*random(-1)',scale=10:10:flags=neighbor -vframes 10 f%05d.png


  


  

  


  

  


  I looked into source code, but I cannot see what random function actually get called :

  


          case e_random:{
            int idx= av_clip(eval_expr(p, e->param[0]), 0, VARS-1);
            uint64_t r= isnan(p->var[idx]) ? 0 : p->var[idx];
            r= r*1664525+1013904223;
            p->var[idx]= r;
            return e->value * (r * (1.0/UINT64_MAX));
        }



  


  How do I suppose to get proper randomness in the geq ?

  


• Fix motion blur in still frames from mpeg2video

  14 mars 2017, par Bird

  I’m extracting still frames from a video using the basic code :

  ffmpeg -i video.MXF -vf fps=1 output_%04d.png  

  In some videos, this yields images that, when the camera was moving, look much blurrier than when watching the video (see example below). The still frames from when the camera was not moving look sharper (closer to how it looks in video playback).

  The video specs are : mpeg2video, yuv422p, 1280x720 (according to FFprobe).

  Is this inherent within the video coding or structure ? The video looks so nice when in motion, but even when I pause in VLC the frame goes from sharp to blurred.

  Are there any additions to my FFmpeg code that could result in sharper images ? I tried adding a yadif filter, but it made no difference (the video isn’t interlaced anyways).

  Unfortunately I can’t post a video sample online, but below is an example of a sharper image and a blurry image ; both look in focus during video playback and are about a second apart in the video (that’s the same orange sea star on the left side).

  
  

• Cannot play amr rtp stream using ffmpeg

  1er juin 2020, par Vikram Dattu

  I want to receive and play amr packets streamed using RTP.

  



  Packets are sent to my macbook on port 9999

  



  I can receive packets using simple C program for UDP receiver.
I use bind to 9999 port and use recvfrom to receive packets.

  



  However when I use ffmpeg command to do the same, it is not able to receive single packet.

  



  Command used is : ffplay -protocol_whitelist file,udp,rtp -i test.sdp

  



  Contents of test.sdp :

  



  v=0
o=Andrew 2890844526 2890844526 IN IP4 192.168.1.106
s= SDP Blog
c=IN IP4 192.168.1.106
t=0 0
m=audio 9999 RTP/AVP 100
a=rtpmap:100 AMR/8000/1
a=fmtp:100 octet-align=1


  



  192.168.1.106 is sender's address.

  



  I have tried changing this address to 127.0.0.1. Doesn't help.

  



  I am stuck at the point and not cannot find anything helpful.

  



  By the way,
sample sine wav generator example to stream on same device (loopback on 127.0.0.1) works.