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• Why does ffmpeg have bigger latency on dark images ?

  19 novembre 2017, par doodoroma

  I have a c# application to stream real camera images using ffmpeg. The input images are in raw, 8-bit gray-scale format. I created an ffmpeg stream using the standard input to feed the images and send the output packages to websocket clients.

  I start an external ffmpeg process using this config :

  -f rawvideo -pixel_format gray -video_size " + camera.Width.ToString() + "x" + camera.Height.ToString() + " -framerate 25 -i - -f mpeg1video -b:v 512k -s 320x240 -

  Typical image size is 1040*1392 pixels

  I display the stream on the browser, using jsmpeg library

  This works with a reasonable latency ( 500ms on localhost), but when the camera image is really dark (black image), the latency is extremely big ( 2-3 seconds on localhost). When there is something bright on the image again after a black period, it takes 2-3 seconds to "synchronize".

  I was thinking that black images are really easy to compress and will generate really small packages, jsmpeg has almost no information to display and wait until a complete data package arrives, but I couldn’t prove my theory.

  I played with ffmpeg parameters like bitrate and fps but nothing has changed.

  Is there any settings which I could try ?

• How to capture a layered window with transparency background properly ? (using BitBlt)

  25 octobre 2016, par Mitra M

  I want to capture a WPF window (WPF layered window) with transparency background.

  To do that I tried FFmpeg, But :

  1 - If I set AllowTransparency (this is a property of WPF window) to false ,I can capture the window by gdigrab (this is an ffmpeg device), but output has black background.(I don’t want black background)

  2 - If I set AllowTransparency to true then gdigrab won’t work. (get black frame only)

  I have read David’s nice article, he has said :

  if you use BitBlt to do this, you could “or in” the CAPTUREBLT flag if
  you wanted to capture windows that are layered

  The gdigrab uses BitBlt, this is gdigrab.c code snippet :

  /* Blit screen grab */
  
      if (!BitBlt(dest_hdc, 0, 0,
  
                  clip_rect.right - clip_rect.left,
  
                  clip_rect.bottom - clip_rect.top,
  
                  source_hdc,
  
                  clip_rect.left, clip_rect.top, SRCCOPY | CAPTUREBLT)) {
  
          WIN32_API_ERROR("Failed to capture image");
  
          return AVERROR(EIO);
  
      }

  You can see the flags . (SRCCOPY | CAPTUREBLT).

  Please tell me :

  1- Why the gdigrab can not capture a WPF window properly ?

  2 - What changes in this code should be done to do this ?

  (Sorry for my English, I used translate.google)

  Thanks

• Using ffmpeg to print onto video clip actual time and duration from original clip

  24 mai 2023, par Kes

  I am using arch linux and bash and ffmpeg, all are up to date and the latest versions.

  


  I am clipping a video that is 30 seconds long and wish to clip from 5 secs to 10 seconds to a new file, from the original.

  


  In the bottom right hand corner of the clip I wish to show timestamps from the original video as follows

  


  

  • in the 5th second "00:00:05/ 00:00:30"

  


  • in the 6th second "00:00:06/ 00:00:30"
    
etc

  


  • in the 10th second "00:00:10/ 00:00:30"

  


  


  This is an apparentley simple question(?) but the syntax of the command is not at all obvious and I am hoping an expert may shed some light on this.

  


  All I have so far for the drawtext part, which does not do what I want as it only counts the elapsed time from t=0 of the clip, whereas I want it to show the timestamp and total duration of the original clip

  


  drawtext I started with

  


  "drawtext=text='%{pts\:gmtime\:0\:%M\\\\\:%S}':fontsize=24:fontcolor=black:x=(w-text_w-10):y=(h-text_h-10)"


  


  ffmpeg line with drawtext I have started with

  


  ffmpeg -ss 00:00:05 -i  "$in_file" -filter_complex "drawtext=fontfile=font.ttf:text='sample text':x=10:y=10:fontsize=12:fontcolor=white:box=1:boxcolor=black@0.5:boxborderw=5,drawtext=text='%{duration\:hms}':fontsize=12:fontcolor=black:x=(w-text_w-10):y=(h-text_h-10)" -t 5 -c:a copy -c:v libx264 out_file.mp4