Intentionally skip frames while processing video using AVFoundation - iphone

Question

I'm trying to process a local video file and simply do some analysis on the pixel data. Nothing is being output.
My current code iterates through each frame of the video but I'd actually like to skip ~15 frames at a time to speed things up. Is there a way to skip over frames without decoding them?
In Ffmpeg, I could simply call av_read_frame without calling avcodec_decode_video2.
Thanks! Here's my current code:
- (void) readMovie:(NSURL *)url
{
[self performSelectorOnMainThread:#selector(updateInfo:) withObject:#"scanning" waitUntilDone:YES];
startTime = [NSDate date];
AVURLAsset * asset = [AVURLAsset URLAssetWithURL:url options:nil];
[asset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:#"tracks"] completionHandler:
^{
dispatch_async(dispatch_get_main_queue(),
^{
AVAssetTrack * videoTrack = nil;
NSArray * tracks = [asset tracksWithMediaType:AVMediaTypeVideo];
if ([tracks count] == 1)
{
videoTrack = [tracks objectAtIndex:0];
videoDuration = CMTimeGetSeconds([videoTrack timeRange].duration);
NSError * error = nil;
// _movieReader is a member variable
_movieReader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
if (error)
NSLog(#"%#", error.localizedDescription);
NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey;
NSNumber* value = [NSNumber numberWithUnsignedInt: kCVPixelFormatType_32BGRA];
NSDictionary* videoSettings = [NSDictionary dictionaryWithObject:value forKey:key];
AVAssetReaderTrackOutput* output = [AVAssetReaderTrackOutput
assetReaderTrackOutputWithTrack:videoTrack
outputSettings:videoSettings];
output.alwaysCopiesSampleData = NO;
[_movieReader addOutput:output];
if ([_movieReader startReading])
{
NSLog(#"reading started");
[self readNextMovieFrame];
}
else
{
NSLog(#"reading can't be started");
}
}
});
}];
}
- (void) readNextMovieFrame
{
//NSLog(#"readNextMovieFrame called");
if (_movieReader.status == AVAssetReaderStatusReading)
{
//NSLog(#"status is reading");
AVAssetReaderTrackOutput * output = [_movieReader.outputs objectAtIndex:0];
CMSampleBufferRef sampleBuffer = [output copyNextSampleBuffer]; // this is the most expensive call
if (sampleBuffer)
{
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
// Lock the image buffer
CVPixelBufferLockBaseAddress(imageBuffer,0);
// Get information of the image
uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
//
// Here's where you can process the buffer!
// (your code goes here)
//
// Finish processing the buffer!
//
// Unlock the image buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
CFRelease(sampleBuffer);
[self readNextMovieFrame];
}
else
{
NSLog(#"could not copy next sample buffer. status is %d", _movieReader.status);
NSTimeInterval scanDuration = -[startTime timeIntervalSinceNow];
float scanMultiplier = videoDuration / scanDuration;
NSString* info = [NSString stringWithFormat:#"Done\n\nvideo duration: %f seconds\nscan duration: %f seconds\nmultiplier: %f", videoDuration, scanDuration, scanMultiplier];
[self performSelectorOnMainThread:#selector(updateInfo:) withObject:info waitUntilDone:YES];
}
}
else
{
NSLog(#"status is now %d", _movieReader.status);
}
}
- (void) updateInfo: (id*)message
{
NSString* info = [NSString stringWithFormat:#"%#", message];
[infoTextView setText:info];
}

Answer 1

Just add a bool Value to your code
- (void) readMovie:(NSURL *)url
{
[self performSelectorOnMainThread:#selector(updateInfo:) withObject:#"scanning" waitUntilDone:YES];
startTime = [NSDate date];
AVURLAsset * asset = [AVURLAsset URLAssetWithURL:url options:nil];
[asset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:#"tracks"] completionHandler:
^{
dispatch_async(dispatch_get_main_queue(),
^{
AVAssetTrack * videoTrack = nil;
NSArray * tracks = [asset tracksWithMediaType:AVMediaTypeVideo];
if ([tracks count] == 1)
{
videoTrack = [tracks objectAtIndex:0];
videoDuration = CMTimeGetSeconds([videoTrack timeRange].duration);
NSError * error = nil;
// _movieReader is a member variable
_movieReader = [[AVAssetReader alloc] initWithAsset:asset error:&error];
if (error)
NSLog(#"%#", error.localizedDescription);
NSString* key = (NSString*)kCVPixelBufferPixelFormatTypeKey;
NSNumber* value = [NSNumber numberWithUnsignedInt: kCVPixelFormatType_32BGRA];
NSDictionary* videoSettings = [NSDictionary dictionaryWithObject:value forKey:key];
AVAssetReaderTrackOutput* output = [AVAssetReaderTrackOutput
assetReaderTrackOutputWithTrack:videoTrack
outputSettings:videoSettings];
output.alwaysCopiesSampleData = NO;
[_movieReader addOutput:output];
if ([_movieReader startReading])
{
NSLog(#"reading started");
[self readNextMovieFrame];
}
else
{
NSLog(#"reading can't be started");
}
}
});
}];
}
BOOL skipFrame = FALSE;
- (void) readNextMovieFrame
{
//NSLog(#"readNextMovieFrame called");
if (_movieReader.status == AVAssetReaderStatusReading)
{
//NSLog(#"status is reading");
AVAssetReaderTrackOutput * output = [_movieReader.outputs objectAtIndex:0];
CMSampleBufferRef sampleBuffer = [output copyNextSampleBuffer];
if (sampleBuffer && !skipFrame)
{
skipFrame = TRUE;
// I'm guessing this is the expensive part that we can skip if we want to skip frames
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
// Lock the image buffer
CVPixelBufferLockBaseAddress(imageBuffer,0);
// Get information of the image
uint8_t *baseAddress = (uint8_t *)CVPixelBufferGetBaseAddress(imageBuffer);
size_t bytesPerRow = CVPixelBufferGetBytesPerRow(imageBuffer);
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
//
// Here's where you can process the buffer!
// (your code goes here)
//
// Finish processing the buffer!
//
// Unlock the image buffer
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
CFRelease(sampleBuffer);
}
else
{
skipFrame = FALSE;
NSLog(#"could not copy next sample buffer. status is %d", _movieReader.status);
NSTimeInterval scanDuration = -[startTime timeIntervalSinceNow];
float scanMultiplier = videoDuration / scanDuration;
NSString* info = [NSString stringWithFormat:#"Done\n\nvideo duration: %f seconds\nscan duration: %f seconds\nmultiplier: %f", videoDuration, scanDuration, scanMultiplier];
[self performSelectorOnMainThread:#selector(updateInfo:) withObject:info waitUntilDone:YES];
}
[self readNextMovieFrame];
}
else
{
NSLog(#"status is now %d", _movieReader.status);
}
}
- (void) updateInfo: (id*)message
{
NSString* info = [NSString stringWithFormat:#"%#", message];
[infoTextView setText:info];
}