从使用AVAssetWriter创建的video生成缩略图
我正在使用AVAssetWriter和captureOutput回调录制video+音频。 问题是，在大约5％的情况下，我无法从第二个零生成缩略图（如果我尝试在video中进一步，那么没有问题）。 我得到的错误是：
错误域= AVFoundationErrorDomain代码= -11832“无法打开”UserInfo = 0x4b31b0 {NSLocalizedFailureReason =此媒体无法使用。，NSUnderlyingError = 0x4effb0“操作无法完成。（OSStatus error -12431。）”，NSLocalizedDescription =无法打开}
这是我正在使用的代码：
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];
以下是我设置videoAVAssetWriterInput的方法 ：
- (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; }
我知道错误意味着当时没有图像，但为什么会发生这种情况呢？
我还使用AVCaptureMovieFileOutput测试了录制，并始终生成缩略图。 此外，如果我将video导出到相机胶卷，则预览在照片中也不可用。
PS：我还在某处读到这可能与关键帧间隔有关，但我不确定如何。
我解决了我的问题。 我在iOS 4.2.1上运行，并决定升级到iOS 5.1.1，问题消失了。