AVAssetWriterInputPixelBufferAdaptor returns null pixel buffer pool

According to the documentation:

"This property is NULL before the first call to startSessionAtTime:on the associated AVAssetWriter object."

So if you'e trying to access the pool too early, it will be NULL. I'm just learning this stuff myself so I can't really elaborate at the moment.

When the pixelBufferPool returns null, check the following:

1. the output file of the AVAssetsWriter doesn't exist.

2. use the pixelbuffer after calling startSessionAtTime: on the AVAssetsWriter.

3. the settings of AVAssetWriterInput and AVAssetWriterInputPixelBufferAdaptor are correct.

4. the present times of appendPixelBuffer uses are not the same.

I had the same problem, and I think it is possibly because you have not configured your AVAssetWriterInput correctly. My pool started working after I had done this. In particular, the pool would not give me pixel buffers unless I had provided data in AVVideoCompressionPropertiesKey. First, create and fully configure the AVAssetWriter ( Look in /Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS4.3.sdk/System/Library/Frameworks/AVFoundation.framework/Headers/AVVideoSettings.h for keys & values for outputSettings and compressionSettings):

NSError * err = 0;
AVAssetWriter * outputWriter = [AVAssetWriter
    assetWriterWithURL: [NSURL fileURLWithPath:outputPath]
              fileType: AVFileTypeAppleM4V
                 error: & err];

NSMutableDictionary * outputSettings
    = [[NSMutableDictionary alloc] init];
[outputSettings setObject: AVVideoCodecH264
                   forKey: AVVideoCodecKey];
[outputSettings setObject: [NSNumber numberWithInt: width_]
                   forKey: AVVideoWidthKey];
[outputSettings setObject: [NSNumber numberWithInt: height_]
                   forKey: AVVideoHeightKey];

NSMutableDictionary * compressionProperties
    = [[NSMutableDictionary alloc] init];
[compressionProperties setObject: [NSNumber numberWithInt: 1000000]
                          forKey: AVVideoAverageBitRateKey];
[compressionProperties setObject: [NSNumber numberWithInt: 16]
                          forKey: AVVideoMaxKeyFrameIntervalKey];
[compressionProperties setObject: AVVideoProfileLevelH264Main31
                          forKey: AVVideoProfileLevelKey];

[outputSettings setObject: compressionProperties
                   forKey: AVVideoCompressionPropertiesKey];

AVAssetWriterInput * writerInput = [AVAssetWriterInput
    assetWriterInputWithMediaType: AVMediaTypeVideo
                   outputSettings: outputSettings];

[compressionProperties release];
[outputSettings release];

Create the pixel buffer adaptor:

NSMutableDictionary * pixBufSettings = [[NSMutableDictionary alloc] init];
[pixBufSettings setObject: [NSNumber numberWithInt: kCVPixelFormatType_32BGRA]
                   forKey: (NSString *) kCVPixelBufferPixelFormatTypeKey];
[pixBufSettings setObject: [NSNumber numberWithInt: width_]
                   forKey: (NSString *) kCVPixelBufferWidthKey];
[pixBufSettings setObject: [NSNumber numberWithInt: height_]
                   forKey: (NSString *) kCVPixelBufferHeightKey];

AVAssetWriterInputPixelBufferAdaptor * outputPBA =
    [AVAssetWriterInputPixelBufferAdaptor
    assetWriterInputPixelBufferAdaptorWithAssetWriterInput: outputInput
                               sourcePixelBufferAttributes: nil];

Then retrieve pixel buffers from its pool using:

CVReturn res = CVPixelBufferPoolCreatePixelBuffer (NULL
    , [outputPBA pixelBufferPool]
    , & outputFrame);