Pitfalls and Errors When Using AVAssetWriter to Save the OpenGL Backbuffer Into a Video.

For my current game I need to record the OpenGL back buffer of GLKView into a video using AVAssetWriter.

I will not go over all the code to do this(which is not that much).

However, I will go quickly over some of the pitfalls that might make you go scratching your head when you try to create a video on iOS.

The first thing to consider is that AVAssetWriter might fail without throwing an exception and without returning any value pointing that out.

To test if there was an error within AVAssetWriter methods you need to read the status and error properties of AVAssetWriter.

You can print the status and error like so:

NSLog(@"%ld %@", (long)videoWriter.status, videoWriter.error);

The first pitfall I encountered is that after startWritting my AVAssetWriter status change to AVAssetWriterStatusFailed.

(Or the number 3)

The reason I was getting this error is because I didn’t delete the file I tried to write into from the previous run session.

That was an easy error but after that I was getting an error when trying to append the first(and following) CVPixelBuffer.

I got AVAssetWriterStatusFailed again but the error value was Unknown Error or the OSStatus was -12902.

After a while I have found out that nothing was wrong with my code except for the fact that something was wrong with the resolution.

Since I was running my app on an iPad3 the resolution was 2048×1536. I don’t know why but the codec or AVAssetWriter couldn’t handle this resolution.

Simply reducing the resolution to something smaller allowed me to write the video without getting any error.


For the sake of completion here is part of the code I was using:


- (void)glkView:(GLKView *)view drawInRect:(CGRect)rect
    @synchronized(self) {
        if (Recording)
 //           sample = [self sampleBufferFromCGImage:[self snapshotRenderBuffer]];
            GLubyte * data = [self snapshotRenderBufferToData];
            CVPixelBufferRef pixelBuffer = [self pixelBufferFromBytes: &data withSize:CGSizeMake(videoWidth, videoHeight) withSourceSize: CGSizeMake(screenWidth, screenHeight)];
            if (adaptor.assetWriterInput.readyForMoreMediaData)
                [adaptor appendPixelBuffer:pixelBuffer withPresentationTime:CMTimeMake(RecordFrame*10, 600)];
            //                [writerInput appendSampleBuffer: sample];
            free (data);
            NSLog(@"%ld %@", (long)videoWriter.status, videoWriter.error);
            //            NSLog(@"%ld", (long)videoWriter.status);
    if (DoneRecord)
    @synchronized(self) {
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectory = [paths objectAtIndex:0];
        NSString *sourcePath = [documentsDirectory stringByAppendingPathComponent:@"video.mov"];
        if (Recording)
            [writerInput markAsFinished];
            NSLog(@"%ld", (long)videoWriter.status);
            [videoWriter finishWritingWithCompletionHandler:^{
            DoneRecord = YES;
            videoWidth = screenWidth/2;
            videoHeight = screenHeight/2;
            NSError * error = nil;
            NSFileManager *fileManager = [NSFileManager defaultManager];
            BOOL success = [fileManager removeItemAtPath:sourcePath error:&error];
            if (success) {

                NSLog(@"Could not delete file -:%@ ",[error localizedDescription]);
            RecordFrame = 0;
            videoWriter = [[AVAssetWriter alloc] initWithURL:
                                          [NSURL fileURLWithPath:sourcePath] fileType:AVFileTypeQuickTimeMovie
            NSLog(@"%ld", (long)videoWriter.status);

            NSDictionary *videoSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                           AVVideoCodecH264, AVVideoCodecKey,
                                           [NSNumber numberWithInt:videoWidth], AVVideoWidthKey,
                                           [NSNumber numberWithInt:videoHeight], AVVideoHeightKey,
            writerInput = [AVAssetWriterInput
                                                outputSettings:videoSettings]; //retain should be removed if ARC

            NSParameterAssert([videoWriter canAddInput:writerInput]);
            writerInput.expectsMediaDataInRealTime = YES;
            [videoWriter addInput:writerInput];

            adaptor = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:writerInput sourcePixelBufferAttributes:nil];

            [videoWriter startWriting];
//            NSLog(@"%ld %@", (long)videoWriter.status, videoWriter.error);
            [videoWriter startSessionAtSourceTime:kCMTimeZero];
        Recording = !Recording;