Reading samples via AVAssetReader

Question

How do you read samples via AVAssetReader? I've found examples of duplicating or mixing using AVAssetReader, but those loops are always controlled by the AVAssetWriter loop. Is it possible just to create an AVAssetReader and read through it, getting each sample?

Thanks.

Answer 1

It's unclear from your question whether you are talking about video samples or audio samples. To read video samples, you need to do the following:

1. Construct an AVAssetReader:

   asset_reader = [[AVAssetReader alloc] initWithAsset:asset error:&error]; (error checking goes here)

2. Get the video track(s) from your asset:

   NSArray* video_tracks = [asset tracksWithMediaType:AVMediaTypeVideo]; AVAssetTrack* video_track = [video_tracks objectAtIndex:0];

3. Set the desired video frame format:

   NSMutableDictionary* dictionary = [[NSMutableDictionary alloc] init];
   [dictionary setObject:[NSNumber numberWithInt:<format code from CVPixelBuffer.h>] forKey:(NSString*)kCVPixelBufferPixelFormatTypeKey];

   Note that certain video formats just will not work, and if you're doing something real-time, certain video formats perform better than others (BGRA is faster than ARGB, for instance).

4. Construct the actual track output and add it to the asset reader:

   AVAssetReaderTrackOutput* asset_reader_output = [[AVAssetReaderTrackOutput alloc] initWithTrack:video_track outputSettings:dictionary];
   [asset_reader addOutput:asset_reader_output];

5. Kick off the asset reader:

   [asset_reader startReading];

6. Read off the samples:

   CMSampleBufferRef buffer;
   while ( [asset_reader status]==AVAssetReaderStatusReading )
         buffer = [asset_reader_output copyNextSampleBuffer];
   

Answer 2

damian's answer worked for me with video, and one minor modification: In step 3, I think the dictionary needs to be mutable:

NSMutableDictionary* dictionary = [[NSMutableDictionary alloc] init];