Decoding H264: VTDecompressionSessionCreate fails with error code -12910 (kVTVideoDecoderUnsupportedDataFormatErr) - ios

Question

I'm getting error -12910 (kVTVideoDecoderUnsupportedDataFormatErr) using VTDecompressionSessionCreate when running code on my iPad, but not on the sim. I'm using Avios (https://github.com/tidwall/Avios) and this is the relevant section:
private func initVideoSession() throws {
formatDescription = nil
var _formatDescription : CMFormatDescription?
let parameterSetPointers : [UnsafePointer<UInt8>] = [ pps!.buffer.baseAddress, sps!.buffer.baseAddress ]
let parameterSetSizes : [Int] = [ pps!.buffer.count, sps!.buffer.count ]
var status = CMVideoFormatDescriptionCreateFromH264ParameterSets(kCFAllocatorDefault, 2, parameterSetPointers, parameterSetSizes, 4, &_formatDescription);
if status != noErr {
throw H264Error.CMVideoFormatDescriptionCreateFromH264ParameterSets(status)
}
formatDescription = _formatDescription!
if videoSession != nil {
VTDecompressionSessionInvalidate(videoSession)
videoSession = nil
}
var videoSessionM : VTDecompressionSession?
let decoderParameters = NSMutableDictionary()
let destinationPixelBufferAttributes = NSMutableDictionary()
destinationPixelBufferAttributes.setValue(NSNumber(unsignedInt: kCVPixelFormatType_32BGRA), forKey: kCVPixelBufferPixelFormatTypeKey as String)
var outputCallback = VTDecompressionOutputCallbackRecord()
outputCallback.decompressionOutputCallback = callback
outputCallback.decompressionOutputRefCon = UnsafeMutablePointer<Void>(unsafeAddressOf(self))
status = VTDecompressionSessionCreate(nil, formatDescription, decoderParameters, destinationPixelBufferAttributes, &outputCallback, &videoSessionM)
if status != noErr {
throw H264Error.VTDecompressionSessionCreate(status)
}
self.videoSession = videoSessionM;
}
Here pps and sps are buffers containing PPS and SPS frames.
As mentioned above, the strange thing is that it works completely fine on the simulator, but not on an actual device. Both are on iOS 9.3, and I'm simulating the same hardware as the device.
What could cause this error?
And, more generally, where can I go for API reference and error docs for VideoToolbox? Genuinely can't find anything of relevance on Apple's site.

Answer 1

The answer turned out to be that the stream resolution was greater than 1920x1080, which is the maximum that the iPad supports. This is a clear difference with the simulator which supports beyond that resolution (perhaps it just uses the Mac VideoToolbox libraries rather than simulating the iOS ones).
Reducing the stream to fewer pixels than 1080p solved the problem.
This is the response from a member of Apple staff which pointed me in the right direction: https://forums.developer.apple.com/thread/11637
As for proper VideoToolbox reference - still nothing of value exists, which is a massive disadvantage. One wonders how the tutorial writers first got their information.
Edit: iOS 10 now appears to support streams greater than 1080p.