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• Documentation for ffmpeg progress output

  20 juillet 2016, par Mahoni

  With -progress continuous data about the transcoding can be posted or written to a file. Where can I find documentation what which key means exactly ? Specifically, q (Quality) and speed but also the other ones

  frame=161
  
  fps=39.6
  
  stream_0_0_q=16.0
  
  stream_1_0_q=13.0
  
  stream_2_0_q=13.0
  
  stream_3_0_q=11.0
  
  stream_4_0_q=10.0
  
  bitrate=N/A
  
  total_size=N/A
  
  out_time_ms=5456211
  
  out_time=00:00:05.456211
  
  dup_frames=10
  
  drop_frames=0
  
  speed=1.34x
  
  progress=continue

  You get similar output with the verbose logging. I haven’t found it in the source code either where I thought I could find more information about it

• About image opacity

  23 octobre 2013, par Mikko Koppanen — Imagick

  There is a common misconception that Imagick::setImageOpacity() would work to reduce the opacity of the image. However, as the name says the method actually sets the opacity throughout the image and thus affects also transparent areas.

  To demonstrate let’s first look at this image of a red circle on a transparent background :

  

  Now, let’s apply setImageOpacity on the image :

  php

  < view plain text >

  1. < ?php
  2. $im = new Imagick (’red-circle.png’) ;
  3. $im->setImageOpacity (0.5) ;
  4. $im->writeImage (’red-circle-setopacity.png’) ;
  5.  ?>

  

  As we can see from the resulting image the transparent background is affected as well.

  In order to actually reduce the opacity of the opaque parts Imagick::evaluateImage can be used instead :

  php

  < view plain text >

  1. < ?php
  2. $im = new Imagick (’red-circle.png’) ;
  3.  
  4. /* Divide the alpha channel value by 2 */
  5. $im->evaluateImage(Imagick: :EVALUATE_DIVIDE, 2, Imagick: :CHANNEL_ALPHA) ;
  6. $im->writeImage (’red-circle-divide.png’) ;
  7.  ?>

  And here are the results :

  

  As the background is already fully transparent so the divide operation causes no changes to it.

  Similar example is available in the PHP manual http://php.net/imagick.evaluateimage and I added a note to setImageOpacity page as well (at the time of writing it has not synced to documentation mirrors yet).

• FFmpeg, reduce initial delay while reading remote video file

  5 juillet 2016, par Tim Autin

  I’m using ffmpeg to read a video mp4 file stored on a web server. I use ffmpeg to convert this file into still images, with this command :

  ./ffmpeg -y -r 25 -i http://webserver.com/file.mp4 -q:v 5 /destination/image_%03d.jpg

  It works well, but I’d like to reduce the initial delay before the first image is generated (around 3 sec in WiFi, a lot more in 3G). I tried adding these options :

  • -probesize 32
  • -analyzeduration 32

  But it didn’t change anything. Is there anything else I can do ? When we are streaming, the x264 encoder has a "-tune zerolatency" option, can I use something similar when reading a file ?