如何从视频中提取图片框架（缩略图）

我要从录制视频中提取一张图片，用了下面代码，没实现。

AVURLAsset* asset = [AVURLAsset URLAssetWithURL:[NSURL URLWithString:videoPath] options:nil];
AVAssetImageGenerator* imageGenerator = [AVAssetImageGenerator assetImageGeneratorWithAsset:asset];
UIImage* image = [UIImage imageWithCGImage:[imageGenerator copyCGImageAtTime:CMTimeMake(0, 1) actualTime:nil error:nil]];
[videoFrame setImage:image];

请高手帮忙看一下，怎么从视频中提取图片？

   NSURL *videoURl = [NSURL fileURLWithPath:videoPath];
    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURl options:nil];
    AVAssetImageGenerator *generate = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    generate.appliesPreferredTrackTransform = YES;
    NSError *err = NULL;
    CMTime time = CMTimeMake(1, 60);
    CGImageRef imgRef = [generate copyCGImageAtTime:time actualTime:NULL error:&err];
     UIImage *img = [[UIImage alloc] initWithCGImage:imgRef];
    [videoFrame setImage:img];
    [img release];

试试这段代码吧。

生成缩略图的代码：

#pragma mark -
#pragma mark THUMBNAIL GENARATION
- (UIImage*)testGenerateThumbNailDataWithVideo:(NSURL *)videoURL {

    AVURLAsset *asset = [[AVURLAsset alloc] initWithURL:videoURL options:nil];
    AVAssetImageGenerator *gen = [[AVAssetImageGenerator alloc] initWithAsset:asset];
    gen.appliesPreferredTrackTransform = YES;
    CMTime time = CMTimeMakeWithSeconds(0.0, 600);
    NSError *error = nil;
    CMTime actualTime;

    CGImageRef image = [gen copyCGImageAtTime:time actualTime:&actualTime error:&error];
    UIImage *currentImg = [[UIImage alloc] initWithCGImage:image];
    CGImageRelease(image);

    return currentImg;
}