从视频生成缩略图使用AVAssetWriter创建(Generating thumbnails fr

我记录使用视频+音频AVAssetWriter和captureOutput回调。问题是，在案件约5％，我无法生成从第二零一缩略图（如果我尝试走得更远视频，那么就没有问题）。我得到的错误是：

错误域= AVFoundationErrorDomain代码= -11832 “无法打开” 的UserInfo = 0x4b31b0 {NSLocalizedFailureReason =该媒体不能使用，NSUnderlyingError = 0x4effb0 “操作无法完成。（OSStatus错误-12431）”，NSLocalizedDescription =无法打开}

下面是我使用的代码：

AVURLAsset *asset=[[AVURLAsset alloc] initWithURL:url options:nil];
AVAssetImageGenerator *generator = [[AVAssetImageGenerator alloc] initWithAsset:asset];
generator.appliesPreferredTrackTransform=TRUE;
[asset release];
CMTime thumbTime = CMTimeMakeWithSeconds(0,30);

AVAssetImageGeneratorCompletionHandler handler = ^(CMTime requestedTime, CGImageRef im, CMTime actualTime, AVAssetImageGeneratorResult result, NSError *error)
{
    if (result != AVAssetImageGeneratorSucceeded)
    {
        [self NSLogPrint:[NSString stringWithFormat:@"couldn't generate thumbnail, error:%@", error]];
    }

    UIImage *thumbImg=[[UIImage imageWithCGImage:im] retain];   

    [image setImage:thumbImg];
    [thumbImg release];

    [generator release];
};

CGSize maxSize = CGSizeMake(177, 100);
generator.maximumSize = maxSize;
[generator generateCGImagesAsynchronouslyForTimes:[NSArray arrayWithObject:[NSValue valueWithCMTime:thumbTime]] completionHandler:handler];

下面是我如何设置我的视频AVAssetWriterInput：

- (BOOL) setupAssetWriterVideoInput:(CMFormatDescriptionRef)currentFormatDescription 
{
    float bitsPerPixel;
    CMVideoDimensions dimensions = CMVideoFormatDescriptionGetDimensions(currentFormatDescription);
    int numPixels = dimensions.width * dimensions.height;
    int bitsPerSecond;

    // Assume that lower-than-SD resolutions are intended for streaming, and use a lower bitrate
    if ( numPixels < (640 * 480) )
        bitsPerPixel = 4.05; // This bitrate matches the quality produced by AVCaptureSessionPresetMedium or Low.
    else
        bitsPerPixel = 11.04; // This bitrate matches the quality produced by AVCaptureSessionPresetHigh.

    bitsPerSecond = numPixels * bitsPerPixel;

    NSDictionary *videoCompressionSettings = [NSDictionary dictionaryWithObjectsAndKeys:
                                              AVVideoCodecH264, AVVideoCodecKey,
                                              [NSNumber numberWithInteger:dimensions.width], AVVideoWidthKey,
                                              [NSNumber numberWithInteger:dimensions.height], AVVideoHeightKey,
                                              [NSDictionary dictionaryWithObjectsAndKeys:
                                               [NSNumber numberWithInteger:bitsPerSecond], AVVideoAverageBitRateKey,
                                               [NSNumber numberWithInteger:30], AVVideoMaxKeyFrameIntervalKey,
                                               nil], AVVideoCompressionPropertiesKey,
                                              nil];
    if ([_videoWriter canApplyOutputSettings:videoCompressionSettings forMediaType:AVMediaTypeVideo]) 
    {
        self._videoWriterInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:videoCompressionSettings];
        self._videoWriterInput.expectsMediaDataInRealTime = YES;
        self._videoWriterInput.transform = [self returnOrientation];
        if ([_videoWriter canAddInput:self._videoWriterInput])
            [_videoWriter addInput:self._videoWriterInput];
        else
        {
            NSLog(@"Couldn't add asset writer video input.");
            return NO;
        }
    }
    else
    {
        NSLog(@"Couldn't apply video output settings.");
        return NO;
    }

    return YES;
}

我知道，错误意味着有当时没有图像，但为什么会出现这种情况？

我已经使用AVCaptureMovieFileOutput还测试记录和缩略图总是产生。另外，如果我出口的视频，为相机胶卷，预览还没有可用的照片。

PS：我也读的地方，这可能是与关键帧间隔，但我不知道怎么样。

我有点固定我的问题。我在iOS 4.2.1上运行，并决定升级到iOS 5.1.1和问题dissappeared。