iOS occasionally writes empty video file - ios

Question

I have a Movie class that has an array of UIImage that I write to an H264 file. It works but occasionally it will write a video file that does not contain any content, but the file size is still set to a non-zero size.
Here is the code where I do the writing. I am pretty new to iOS development so this was copied from the internet so I may not fully understand what it is doing. Hopefully someone can suggest a better way of doing this.
frame is a UIImage instance in the loop
func writeAnimationToMovie() {
var error: NSError?
writer.startWriting()
writer.startSessionAtSourceTime(kCMTimeZero)
var buffer: CVPixelBufferRef
var frameCount = 0
for frame in self.photos {
buffer = createPixelBufferFromCGImage(frame.CGImage)
var appendOk = false
var j = 0
while (!appendOk && j < 30) {
if pixelBufferAdaptor.assetWriterInput.readyForMoreMediaData {
let frameTime = CMTimeMake(Int64(frameCount), Int32(fps))
appendOk = pixelBufferAdaptor.appendPixelBuffer(buffer, withPresentationTime: frameTime)
// appendOk will always be false
NSThread.sleepForTimeInterval(0.05)
} else {
NSThread.sleepForTimeInterval(0.1)
}
j++
}
if (!appendOk) {
println("Doh, frame \(frame) at offset \(frameCount) failed to append")
}
frameCount++
}
input.markAsFinished()
writer.finishWritingWithCompletionHandler({
if self.writer.status == AVAssetWriterStatus.Failed {
println("oh noes, an error: \(self.writer.error.description)")
} else {
let content = NSFileManager.defaultManager().contentsAtPath(self.fileURL.path!)
println("wrote video: \(self.fileURL.path) at size: \(content?.length)")
}
})
}
func createPixelBufferFromCGImage(image: CGImageRef) -> CVPixelBufferRef {
let options = [
"kCVPixelBufferCGImageCompatibilityKey": true,
"kCVPixelBufferCGBitmapContextCompatibilityKey": true
]
let frameSize = CGSizeMake(CGFloat(CGImageGetWidth(image)), CGFloat(CGImageGetHeight(image)))
var pixelBufferPointer = UnsafeMutablePointer<Unmanaged<CVPixelBuffer>?>.alloc(1)
var status:CVReturn = CVPixelBufferCreate(
kCFAllocatorDefault,
UInt(frameSize.width),
UInt(frameSize.height),
OSType(kCVPixelFormatType_32ARGB),
options,
pixelBufferPointer
)
var lockStatus:CVReturn = CVPixelBufferLockBaseAddress(pixelBufferPointer.memory?.takeUnretainedValue(), 0)
var pxData:UnsafeMutablePointer<(Void)> = CVPixelBufferGetBaseAddress(pixelBufferPointer.memory?.takeUnretainedValue())
let bitmapinfo = CGBitmapInfo(rawValue: CGImageAlphaInfo.NoneSkipFirst.rawValue)
let rgbColorSpace:CGColorSpace = CGColorSpaceCreateDeviceRGB()
var context:CGContextRef = CGBitmapContextCreate(
pxData,
UInt(frameSize.width),
UInt(frameSize.height),
8,
4 * CGImageGetWidth(image),
rgbColorSpace,
bitmapinfo
)
CGContextDrawImage(context, CGRectMake(0, 0, frameSize.width, frameSize.height), image)
CVPixelBufferUnlockBaseAddress(pixelBufferPointer.memory?.takeUnretainedValue(), 0)
return pixelBufferPointer.memory!.takeUnretainedValue()
}
EDIT The files that do not seem to have any content won't play like the other movies that work. If I view a working file with exiftool this is what I get
$ exiftool 20242697651186-o.mp4
ExifTool Version Number : 9.76
File Name : 20242697651186-o.mp4
Directory : .
File Size : 74 kB
File Modification Date/Time : 2014:12:05 11:07:29-05:00
File Access Date/Time : 2014:12:08 10:12:29-05:00
File Inode Change Date/Time : 2014:12:05 11:07:29-05:00
File Permissions : rw-r--r--
File Type : MP4
MIME Type : video/mp4
Major Brand : MP4 v2 [ISO 14496-14]
Minor Version : 0.0.1
Compatible Brands : mp41, mp42, isom
Movie Data Size : 74741
Movie Data Offset : 44
Movie Header Version : 0
Create Date : 2014:12:05 16:07:32
Modify Date : 2014:12:05 16:07:32
Time Scale : 600
Duration : 0.50 s
Preferred Rate : 1
Preferred Volume : 100.00%
Preview Time : 0 s
Preview Duration : 0 s
Poster Time : 0 s
Selection Time : 0 s
Selection Duration : 0 s
Current Time : 0 s
Next Track ID : 2
Track Header Version : 0
Track Create Date : 2014:12:05 16:07:32
Track Modify Date : 2014:12:05 16:07:32
Track ID : 1
Track Duration : 0.50 s
Track Layer : 0
Track Volume : 0.00%
Matrix Structure : 1 0 0 0 1 0 0 0 1
Image Width : 640
Image Height : 480
Media Header Version : 0
Media Create Date : 2014:12:05 16:07:32
Media Modify Date : 2014:12:05 16:07:32
Media Time Scale : 600
Media Duration : 0.50 s
Media Language Code : und
Handler Type : Video Track
Handler Description : Core Media Video
Graphics Mode : srcCopy
Op Color : 0 0 0
Compressor ID : avc1
Source Image Width : 640
Source Image Height : 480
X Resolution : 72
Y Resolution : 72
Bit Depth : 24
Video Frame Rate : 8
Avg Bitrate : 1.2 Mbps
Image Size : 640x480
Rotation : 0
And here is a file that doesn't work. It doesn't have all that metadata in it
$ exiftool 20242891987099-o.mp4
ExifTool Version Number : 9.76
File Name : 20242891987099-o.mp4
Directory : .
File Size : 75 kB
File Modification Date/Time : 2014:12:05 11:07:37-05:00
File Access Date/Time : 2014:12:08 10:12:36-05:00
File Inode Change Date/Time : 2014:12:05 11:07:37-05:00
File Permissions : rw-r--r--
File Type : MP4
MIME Type : video/mp4
Major Brand : MP4 v2 [ISO 14496-14]
Minor Version : 0.0.1
Compatible Brands : mp41, mp42, isom
Movie Data Size : 76856
Movie Data Offset : 44

Answer 1

You have a few issues. I'm not sure exactly what you mean by "does not contain any content", but hopefully one of these will help (and if not, you should implement them anyway):
You're blocking the thread with your calls to sleepForTimeInterval(), which could cause the problem. This answer suggests moving the run loop along instead of sleeping, which is a slightly better solution, but the readyForMoreMediaData documentation has an even better suggestion:
This property is observable using key-value observing (see Key-Value Observing Programming Guide). Observers should not assume that they will be notified of changes on a specific thread.
Instead of running a loop and asking if it's available, just get the object to tell you when it's ready for more using KVO.
In your createPixelBufferFromCGImage method, you're not checking for failure at any point. For example, you should handle the possibility that pixelBufferPointer.memory? is nil.
Basically, I'm hypothesizing that one of these things is happening:
j hits 30 and nothing's been written yet because a thread is blocked. In this case you write an empty file with the correct file size.
createPixelBufferFromCGImage is returning unexpected data, which you're then writing to disk