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• iOS Libavcodec - Trim and convert a video

  14 juillet 2015, par AnujAroshA

  In my iOS project, I am trying to take an .mp4 video file, cut out a small clip in the middle of the file, and convert the output to .mov file. I am using libavcodec to do this.

  I built the FFmpeg libraries for iOS using this script and added to my project.

  The code I am using to trim and convert the video file is this.

  The issues with final output of the file are :

  1. When I copy the output from the iPhone to my mac, the video meta data for duration and video dimensions are blank. If I do the same for the original, I get the video length and dimensions correctly. I must not be creating the video meta data on the new video correctly.

  2. The frame rate on the outputted video appears to be wrong. I expect to see e.g. 250 frames over 10 seconds (25fps), but instead I see 250 frames over 4 seconds, then only the last frame for the remaining 6 seconds.

  3. The code should seek to 100s in the video before starting trimming. Instead, the code appears to crop the video starting at the beginning.

• Getting accurate time from FFMPeg with Objective C (Audio Queue Services)

  2 avril 2012, par Winston

  My iPhone app plays an audio file using FFMPeg.

  I'm getting the elapsed time (to show to user) from the playing audio (in minutes and seconds after converting from microseconds, given by FFMPeg) like so :

  AudioTimeStamp currentTimeStamp;
  
  AudioQueueGetCurrentTime (audioQueue, NULL, &currentTimeStamp, NULL);
  
  
  
  getFFMPEGtime = currentTimeStamp.mSampleTime/self.basicAudioDescription.mSampleRate;
  
  
  
  self.currentAudioTime = [NSString stringWithFormat: @"%02d:%02d",
  
                              (int) getFFMPEGtime / (int)60000000,
  
                              (int) ((getFFMPEGtime % 60000000)/1000000)];

  Everything works fine, but when I scrub back or forward to play another portion of the song, the elapsed time will go back to zero, no matter the current position. The timer will always zero out.

  I know I'm suposed to do some math to keep track of the old time and the new time, maybe constructing another clock or so, perhaps implementing another callback function, etc... I'm not sure what way I should go.

  My questions are :

  1) What's the best approach to keep track of the elapsed time when going back/forward in a song, avoiding the clock to always going back to zero ?

  2) Should I look deeply into FFMPeg functions or should I stick with Objective-C and Cocoa Touch for solving this problem ?

  Please, I need some advices/ideas from experienced programmers. I'm stuck. Thanks beforehand !

• How can I fix choppy ffmpeg RTP streaming over wifi ?

  19 décembre 2015, par awidgery

  I have a Raspberry Pi, with a USB mic and a WiFi dongle dongle connected.

  I’m trying to stream audio only from the Pi, with the intention of receiving the stream over wifi to a custom iOS mobile app using VLCKit. I’m using ffmpeg on the Pi as I need a reasonably low (<2s) latency for this project, and using Icecast/Darkice gave around 15s latency.

  The code executed on the Pi is :

  ffmpeg -f alsa -i plughw:1,0 -acodec libmp3lame -ab 128k -ac 1 -ar 44100  -f rtp rtp://234.5.5.5:1234

  On the Pi end I have a device playing (Christmas !) music constantly into the USB mic for testing purposes. The Pi is only connected by WiFi - not ethernet.

  For testing receiving the stream, I’m using VLC (on a Macbook/iPhone).

  When the Mac is connected through Ethernet, the stream works fine, as you can see here :

  https://goo.gl/photos/HZgNh7z4HgaqHBaP7

  However, when the Mac is connected via WiFi, the stream is choppy, as you can see here :

  https://goo.gl/photos/qjAVH6djqS9Jbvmh6

  You can also see a ping trace from the Mac to the Pi, and the VLC stats. As you can see there doesn’t seem to be a correlation between either of these and the choppiness.

  I’ve tried the VLC iOS app and the choppiness is the same as the Mac on WiFi.

  How can I decrease/remove this chop, even if doing so increases latency a bit ?