AVAssetWriter startWriting fails on iPod Touch 3rd Gen / OS 4.2.1

Question

My app creates an mp4 file. I've verified that the code I have works on the following devices:

• iPad (OS 4.3.2)
• iPhone4 (OS 4.2.1)
• iPhone 3GS (OS 4.2.1)

.. but the init fails on my iPod Touch 3rd Gen running OS 4.2.1.

This is related to another question on here, but I'm seeing it on a a different iOS device than he was and I've included my init code here. Like the other question, I've tried different pixel formats as well as bitrates, but the AVAssetWriter's status always changes to AVAssetWriterStatusFailed after calling its startWriting function.

Any ideas would be greatly appreciated. I know that mp4 creation is possible on this device because I've downloaded another app that does it just fine on the same device that my code fails on.

Here is the minimal code to do the video setup.

#import "AVFoundation/AVFoundation.h"
#import "AVFoundation/AVAssetWriterInput.h"
#import "AVFoundation/AVAssetReader.h"
#import "Foundation/NSUrl.h"


void VideoSetupTest()
{
    int width = 320;
    int height = 480;

    // Setup the codec settings.
    int nBitsPerSecond = 100000;
    NSDictionary *codecSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                   [NSNumber numberWithInt: nBitsPerSecond], AVVideoAverageBitRateKey, 
                                   nil];        

    // Create the AVAssetWriterInput.
    NSDictionary *outputSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                    AVVideoCodecH264, AVVideoCodecKey,
                                    codecSettings, AVVideoCompressionPropertiesKey,
                                    [NSNumber numberWithInt: width], AVVideoWidthKey,
                                    [NSNumber numberWithInt: height], AVVideoHeightKey,
                                    nil];

    AVAssetWriterInput *assetWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType: AVMediaTypeVideo outputSettings: outputSettings];
    [assetWriterInput retain];

    // Create the AVAssetWriterPixelBufferAdaptor.
    NSDictionary *pixelBufferAdaptorAttribs = [NSDictionary dictionaryWithObjectsAndKeys:
                                               [NSNumber numberWithInt: kCVPixelFormatType_32BGRA], kCVPixelBufferPixelFormatTypeKey, 
                                               [NSNumber numberWithInt: width], kCVPixelBufferWidthKey,
                                               [NSNumber numberWithInt: height], kCVPixelBufferHeightKey,
                                               nil];
    AVAssetWriterInputPixelBufferAdaptor *pixelBufferAdaptor = [AVAssetWriterInputPixelBufferAdaptor alloc];
    [pixelBufferAdaptor initWithAssetWriterInput: assetWriterInput sourcePixelBufferAttributes: pixelBufferAdaptorAttribs];



    // Figure out a filename.
    NSArray *paths = NSSearchPathForDirectoriesInDomains( NSDocumentDirectory, NSUserDomainMask, YES );
    NSString *documentsDirectory = [paths objectAtIndex:0];
    char szFilename[256];
    sprintf( szFilename, "%s/test.mp4", [documentsDirectory UTF8String] );
    unlink( szFilename );
    printf( "Filename:\n%s\n\n", szFilename );


    // Create the AVAssetWriter.
    NSError *pError;
    NSURL *url = [NSURL fileURLWithPath: [NSString stringWithUTF8String: szFilename]];
    AVAssetWriter *assetWriter = [AVAssetWriter alloc];
    [assetWriter initWithURL:url  fileType:AVFileTypeMPEG4  error:&pError];


    // Bind these things together and start!
    assetWriterInput.expectsMediaDataInRealTime = YES;
    [assetWriter addInput:assetWriterInput];


    //
    // ** NOTE: Here's the call where [assetWriter status] starts returning AVAssetWriterStatusFailed
    //          on an iPod 3rd Gen running iOS 4.2.1.
    //
    [assetWriter startWriting];
}

Answer 1

I've come to the conclusion that the iPod Touch 3rd Gen just can't create .mp4 videos, period. This would make sense - why include video encoding hardware on a device that doesn't even have a camera to capture videos with?

The thing that was making me think that it could create .mp4 videos was that I had another app that was able to create them from my iPod Touch 3. But upon analyzing that app's .mp4 video, it was actually an H.263 (MPEG-4 part-2) video created with ffmpeg. (Whereas AVFoundation will create H.264 videos if it can do it at all on a device).