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• Issue with MJPEG stream playback speed when encoding with FFmpeg

  6 janvier, par Arciiix

  I'm working with a MJPEG stream source

  


  Stream #0:0: Video: mjpeg (Baseline), yuvj420p(pc, bt470bg/unknown/unknown), 1280x720 [SAR 1:1 DAR 16:9], 25 tbr, 25 tbn

  


  I want to capture this stream and save it to an MP4 file using FFmpeg, but I am running into a weird issue.

  


  The only way it works fine is when I just copy the stream without reencoding and using wallclock as timestamps (cmd 1) :

  


  ffmpeg -use_wallclock_as_timestamps 1 -i "http://IP/video/1280x720" -c:v copy output_good.mp4


  


  However, when I try encoding the stream with libx264, the playback speed of the resulting video becomes slower than 100%, causing the video to gradually fall out of sync with real-time. This happens even when I use any encoding, also when I explicitly set the frame rate or vsync. For example, these commands fail :

  


  

  • CMD 2 : ffmpeg -i "http://IP/video/1280x720" -c:v libx264 -r 30 -preset fast output_bad1.mp4

  


  • CMD 3 : ffmpeg -use_wallclock_as_timestamps 1 -i "http://IP/video/1280x720" -c:v libx264 output.mp4

  


  


  https://www.youtube.com/watch?v=PF0fuCu7Xao

  


  As you can see in the comparison of the resulting videos, the playback gradually slows down with CMD 2 and CMD 3, while it's alright with CMD 1.

  


  What I've noticed is that on the FFmpeg stdout, when using encoding, the speed= and FPS= go up and up, e.g. to 31 FPS even though my source is technically at 25 FPS.

  


  What I've tried :

  


  

  • different encoding codecs (e.g. libvpx)

  


  • -re

  


  • preset ultrafast on encoding

  


  • vsync (fps_mode), both ctr and vfr

  


  • fps=30 filter

  


  


  Has anyone encountered a similar issue or know how to force FFmpeg to preserve the correct playback speed while encoding or enforcing a frame rate ? Thanks !

  


• How to do Slow Motion video in IOS

  17 janvier 2017, par 2vision2

  I have to do "slow motion" in a video file along with audio, in-between some frames and need to store the ramped video as a new video.

  Ref : http://www.youtube.com/watch?v=BJ3_xMGzauk (watch from 0 to 10s)

  From my analysis, I’ve found that AVFoundation framework can be helpful.

  Ref :
  http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/00_Introduction.html

  Copy and pasted from the above link :

  "
  Editing
  AV Foundation uses compositions to create new assets from existing pieces of media (typically, one or more video and audio tracks). You use a mutable composition to add and remove tracks, and adjust their temporal orderings. You can also set the relative volumes and ramping of audio tracks ; and set the opacity, and opacity ramps, of video tracks. A composition is an assemblage of pieces of media held in memory. When you export a composition using an export session, it’s collapsed to a file.
  On iOS 4.1 and later, you can also create an asset from media such as sample buffers or still images using an asset writer.

  "

  Questions :
  Can I do " slow motion " the video/audio file using the AVFoundation framework ? Or Is there any other package available ? If i want to handle audio and video separately, please guide me how to do ?

  Update : : Code For AV Export Session :

   NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
  
      NSString *outputURL = paths[0];
  
      NSFileManager *manager = [NSFileManager defaultManager];
  
      [manager createDirectoryAtPath:outputURL withIntermediateDirectories:YES attributes:nil error:nil];
  
      outputURL = [outputURL stringByAppendingPathComponent:@"output.mp4"];
  
      // Remove Existing File
  
      [manager removeItemAtPath:outputURL error:nil];
  
      AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:self.inputAsset presetName:AVAssetExportPresetLowQuality];
  
      exportSession.outputURL = [NSURL fileURLWithPath:outputURL]; // output path;
  
      exportSession.outputFileType = AVFileTypeQuickTimeMovie;
  
      [exportSession exportAsynchronouslyWithCompletionHandler:^(void) {
  
          if (exportSession.status == AVAssetExportSessionStatusCompleted) {
  
              [self writeVideoToPhotoLibrary:[NSURL fileURLWithPath:outputURL]];
  
              ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
  
              [library writeVideoAtPathToSavedPhotosAlbum:[NSURL fileURLWithPath:outputURL] completionBlock:^(NSURL *assetURL, NSError *error){
  
                  if (error) {
  
                      NSLog(@"Video could not be saved");
  
                  }
  
              }];
  
          } else {
  
              NSLog(@"error: %@", [exportSession error]);
  
          }
  
      }];

• How to do Slow Motion video in IOS

  4 mai 2022, par 2vision2

  I have to do "slow motion" in a video file along with audio, in-between some frames and need to store the ramped video as a new video.

  



  Ref : http://www.youtube.com/watch?v=BJ3_xMGzauk (watch from 0 to 10s)

  



  From my analysis, I've found that AVFoundation framework can be helpful.

  



  Ref :
http://developer.apple.com/library/ios/#documentation/AudioVideo/Conceptual/AVFoundationPG/Articles/00_Introduction.html

  



  Copy and pasted from the above link :

  



  "
Editing
AV Foundation uses compositions to create new assets from existing pieces of media (typically, one or more video and audio tracks). You use a mutable composition to add and remove tracks, and adjust their temporal orderings. You can also set the relative volumes and ramping of audio tracks ; and set the opacity, and opacity ramps, of video tracks. A composition is an assemblage of pieces of media held in memory. When you export a composition using an export session, it's collapsed to a file.
On iOS 4.1 and later, you can also create an asset from media such as sample buffers or still images using an asset writer.

  



  "

  



  Questions :
Can I do " slow motion " the video/audio file using the AVFoundation framework ? Or Is there any other package available ? If i want to handle audio and video separately, please guide me how to do ?

  



  Update : : Code For AV Export Session :

  



   NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
    NSString *outputURL = paths[0];
    NSFileManager *manager = [NSFileManager defaultManager];
    [manager createDirectoryAtPath:outputURL withIntermediateDirectories:YES attributes:nil error:nil];
    outputURL = [outputURL stringByAppendingPathComponent:@"output.mp4"];
    // Remove Existing File
    [manager removeItemAtPath:outputURL error:nil];
    AVAssetExportSession *exportSession = [[AVAssetExportSession alloc] initWithAsset:self.inputAsset presetName:AVAssetExportPresetLowQuality];
    exportSession.outputURL = [NSURL fileURLWithPath:outputURL]; // output path;
    exportSession.outputFileType = AVFileTypeQuickTimeMovie;
    [exportSession exportAsynchronouslyWithCompletionHandler:^(void) {
        if (exportSession.status == AVAssetExportSessionStatusCompleted) {
            [self writeVideoToPhotoLibrary:[NSURL fileURLWithPath:outputURL]];
            ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
            [library writeVideoAtPathToSavedPhotosAlbum:[NSURL fileURLWithPath:outputURL] completionBlock:^(NSURL *assetURL, NSError *error){
                if (error) {
                    NSLog(@"Video could not be saved");
                }
            }];
        } else {
            NSLog(@"error: %@", [exportSession error]);
        }
    }];