generateCGImagesAsynchronouslyForTimes sometimes don't generate entire thumbnails

Question

I'm working on an OS X app that is using AVAssetImageGenerator.generateCGImagesAsynchronouslyForTimes, and it normally works fine. However, once in a while the thumbnail I get back only contains the first few rows of pixels, and the rest is green, sometimes the images will be different shades of green. It's very hard to track down because it doesn't consistently happen, but when it does about half of the thumbnails are affected. This is an image of what I expect to see:

But often times this happens:

Here is the code I'm using to generate the thumbnails:

let assetGenerator = AVAssetImageGenerator(asset: AVURLAsset(URL: url))
assetGenerator.appliesPreferredTrackTransform = true
let time = CMTime(seconds: 0, preferredTimescale: 30)

let handler: AVAssetImageGeneratorCompletionHandler = { _, image, _, res, error in
    defer { dispatch_group_leave(self.waitForThumbnail!) }

    guard let image = image where res == .Succeeded else {
        if let error = error { print(error) }
        return
    }

    let s = CGSize(width: CGImageGetWidth(image), height: CGImageGetHeight(image))
    self.thumbnail = NSImage(CGImage: image, size: s)

}

waitForThumbnail = dispatch_group_create()
dispatch_group_enter(waitForThumbnail!)

assetGenerator.maximumSize = maxThumbnailSize
assetGenerator.generateCGImagesAsynchronouslyForTimes([NSValue(CMTime: time)], completionHandler: handler)

And this is how I'm retrieving the thumbnails:

dispatch_group_wait(file.waitForThumbnail!, DISPATCH_TIME_FOREVER)
dispatch_async(dispatch_get_main_queue()) {
    self.imageView.image = file.thumbnail
}

Any help is much appreciated, thanks!

Answer 1

Instead of using generateCGImagesAsynchronouslyForTimes Method you can use copyCGImageAtTime Method to get the image from the asset and save that image as you are saving before. Here is the code.

let assetGenerator = AVAssetImageGenerator(asset: AVURLAsset(URL: url))
assetGenerator.appliesPreferredTrackTransform = true
assetGenerator.maximumSize = maxThumbnailSize
let time = CMTime(seconds: 0, preferredTimescale: 30)
do {
    let cgImage = try assetGenerator.copyCGImageAtTime(time, actualTime: nil)
    let s = CGSize(width: CGImageGetWidth(cgImage), height: CGImageGetHeight(cgImage))
    self.thumbnail = NSImage(CGImage: cgImage, size: s)    
} catch let error {
    print(error)
}