我有以下代码，它从相机捕获视频并使用AVAssetWriter将其存储为QuickMovie文件。它工作正常，但宽高比并不完美，因为AVAssetWriterInput的outputSettings中的宽度和高度是硬编码的（480 x 320）。
我宁愿找出源视频的宽高比，并指定合适的高度（480 x宽高比）。有谁知道怎么做？我应该将AssetWriterInput的创建推迟到第一个sampleBuffer吗？
// set the sessionPreset to 'medium'
self.captureSession = [[AVCaptureSession alloc] init];
self.captureSession.sessionPreset = AVCaptureSessionPresetMedium;
...
// create AVCaptureVideoDataOutput
self.captureVideo = [[AVCaptureVideoDataOutput alloc] init];
NSString* formatTypeKey = (NSString*)kCVPixelBufferPixelFormatTypeKey;
self.captureVideo.videoSettings = @{
formatTypeKey:[NSNumber numberWithInt:kCVPixelFormatType_32BGRA]
};
[self.captureVideo setSampleBufferDelegate:self queue:dispatch_get_main_queue()];
// create an AVAssetWriter
NSError* error = nil;
self.videoWriter = [[AVAssetWriter alloc] initWithURL:url
fileType:AVFileTypeQuickTimeMovie
error:&error];
...
// create AVAssetWriterInput with specified settings
NSDictionary* compression = @{
AVVideoAverageBitRateKey:[NSNumber numberWithInt:960000],
AVVideoMaxKeyFrameIntervalKey:[NSNumber numberWithInt:1]
};
self.videoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo
outputSettings:@{
AVVideoCodecKey:AVVideoCodecH264,
AVVideoCompressionPropertiesKey:compression,
AVVideoWidthKey:[NSNumber numberWithInt:480], // required
AVVideoHeightKey:[NSNumber numberWithInt:320] // required
}];
// add it to the AVAssetWriter
[self.videoWriter addInput:self.videoInput];
答案 0 :(得分：4)
这是对我有用的方式;您确定的方法不允许您的应用程序缩放。你也可以在开始时学习如何正确地做事 - 即使牺牲更多的时间和精力 - 而不是为了。
在我的应用中，创建资产编写者后......
_writer = [[AVAssetWriter alloc] initWithURL:_outURL fileType:AVFileTypeQuickTimeMovie error:outError];
...我创建了一个视频资产跟踪...
NSArray *videoTracks = [_asset tracksWithMediaType:AVMediaTypeVideo];
if ([videoTracks count] > 0)
assetVideoTrack = [videoTracks objectAtIndex:0];
...然后资产阅读器跟踪输出对象：
_readerVideoOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetVideoTrack outputSettings:decompressionVideoSettings];
[_reader addOutput:_readerVideoOutput];
然后，我将视频轨道的格式描述属性加载到一个数组中，在访问每个属性时将每个元素作为CMFormatDescriptionRef对象进行转换：
CMFormatDescriptionRef formatDescription = NULL;
NSArray *formatDescriptions = [assetVideoTrack formatDescriptions];
if ([formatDescriptions count] > 0)
formatDescription = (__bridge CMFormatDescriptionRef)[formatDescriptions objectAtIndex:0];
然后，我尝试加载干净光圈设置和像素宽高比设置，然后在最后选择它们：
if (formatDescription)
{
NSDictionary *cleanAperture = nil;
NSDictionary *pixelAspectRatio = nil;
CFDictionaryRef cleanApertureFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_CleanAperture);
if (cleanApertureFromCMFormatDescription)
{
cleanAperture = @{
AVVideoCleanApertureWidthKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureWidth),
AVVideoCleanApertureHeightKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHeight),
AVVideoCleanApertureHorizontalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHorizontalOffset),
AVVideoCleanApertureVerticalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureVerticalOffset)
};
}
CFDictionaryRef pixelAspectRatioFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_PixelAspectRatio);
if (pixelAspectRatioFromCMFormatDescription)
{
pixelAspectRatio = @{
AVVideoPixelAspectRatioHorizontalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioHorizontalSpacing),
AVVideoPixelAspectRatioVerticalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioVerticalSpacing)
};
}
// Add whichever settings we could grab from the format description to the compression settings dictionary.
if (cleanAperture || pixelAspectRatio)
{
NSMutableDictionary *mutableCompressionSettings = [NSMutableDictionary dictionary];
if (cleanAperture)
[mutableCompressionSettings setObject:cleanAperture forKey:AVVideoCleanApertureKey];
if (pixelAspectRatio)
[mutableCompressionSettings setObject:pixelAspectRatio forKey:AVVideoPixelAspectRatioKey];
compressionSettings = mutableCompressionSettings;
}
}
那是你困惑的地方;一些视频轨道有一个，但不是另一个，反之亦然。因此，您尝试加载它们，然后选择哪个属性集完全恢复，并丢弃没有完成的集合。
请记住，实际上只有一种方法可以将读取和写入文件所需的所有方法嵌套到iPhone中，即使您确实看到了很多变化。你能做的最健全和最谨慎的事情是确保你正确地做到这一点。
如果你有兴趣看到它的样子，那么它就是：
#import "ExportVideo.h"
@implementation ExportVideo
@synthesize url = _url;
@synthesize renderer = _renderer;
- (id)initWithURL:(NSURL *)url usingRenderer:(GLKitView *)renderer {
NSLog(@"ExportVideo");
if (!(self = [super init])) {
return nil;
}
self.url = url;
self.renderer = renderer;
NSString *serializationQueueDescription = [NSString stringWithFormat:@"%@ serialization queue", self];
_mainSerializationQueue = dispatch_queue_create([serializationQueueDescription UTF8String], NULL);
NSString *rwAudioSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw audio serialization queue", self];
_rwAudioSerializationQueue = dispatch_queue_create([rwAudioSerializationQueueDescription UTF8String], NULL);
NSString *rwVideoSerializationQueueDescription = [NSString stringWithFormat:@"%@ rw video serialization queue", self];
_rwVideoSerializationQueue = dispatch_queue_create([rwVideoSerializationQueueDescription UTF8String], NULL);
return self;
}
- (void)startProcessing {
NSDictionary *inputOptions = [NSDictionary dictionaryWithObject:[NSNumber numberWithBool:YES] forKey:AVURLAssetPreferPreciseDurationAndTimingKey];
_asset = [[AVURLAsset alloc] initWithURL:self.url options:inputOptions];
NSLog(@"URL: %@", self.url);
_cancelled = NO;
[_asset loadValuesAsynchronouslyForKeys:[NSArray arrayWithObject:@"tracks"] completionHandler: ^{
dispatch_async(_mainSerializationQueue, ^{
if (_cancelled)
return;
BOOL success = YES;
NSError *localError = nil;
success = ([_asset statusOfValueForKey:@"tracks" error:&localError] == AVKeyValueStatusLoaded);
if (success)
{
NSFileManager *fm = [NSFileManager defaultManager];
NSString *localOutputPath = [self.url path];
if ([fm fileExistsAtPath:localOutputPath])
success = [fm removeItemAtPath:localOutputPath error:&localError];
}
if (success)
success = [self setupAssetReaderAndAssetWriter:&localError];
if (success)
success = [self startAssetReaderAndWriter:&localError];
if (!success)
[self readingAndWritingDidFinishSuccessfully:success withError:localError];
});
}];
}
- (BOOL)setupAssetReaderAndAssetWriter:(NSError **)outError
{
// Create and initialize the asset reader.
_reader = [[AVAssetReader alloc] initWithAsset:_asset error:outError];
BOOL success = (_reader != nil);
if (success)
{
// If the asset reader was successfully initialized, do the same for the asset writer.
NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
_outputURL = paths[0];
NSFileManager *manager = [NSFileManager defaultManager];
[manager createDirectoryAtPath:_outputURL withIntermediateDirectories:YES attributes:nil error:nil];
_outputURL = [_outputURL stringByAppendingPathComponent:@"output.mov"];
[manager removeItemAtPath:_outputURL error:nil];
_outURL = [NSURL fileURLWithPath:_outputURL];
_writer = [[AVAssetWriter alloc] initWithURL:_outURL fileType:AVFileTypeQuickTimeMovie error:outError];
success = (_writer != nil);
}
if (success)
{
// If the reader and writer were successfully initialized, grab the audio and video asset tracks that will be used.
AVAssetTrack *assetAudioTrack = nil, *assetVideoTrack = nil;
NSArray *audioTracks = [_asset tracksWithMediaType:AVMediaTypeAudio];
if ([audioTracks count] > 0)
assetAudioTrack = [audioTracks objectAtIndex:0];
NSArray *videoTracks = [_asset tracksWithMediaType:AVMediaTypeVideo];
if ([videoTracks count] > 0)
assetVideoTrack = [videoTracks objectAtIndex:0];
if (assetAudioTrack)
{
// If there is an audio track to read, set the decompression settings to Linear PCM and create the asset reader output.
NSDictionary *decompressionAudioSettings = @{ AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatLinearPCM] };
_readerAudioOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetAudioTrack outputSettings:decompressionAudioSettings];
[_reader addOutput:_readerAudioOutput];
// Then, set the compression settings to 128kbps AAC and create the asset writer input.
AudioChannelLayout stereoChannelLayout = {
.mChannelLayoutTag = kAudioChannelLayoutTag_Stereo,
.mChannelBitmap = 0,
.mNumberChannelDescriptions = 0
};
NSData *channelLayoutAsData = [NSData dataWithBytes:&stereoChannelLayout length:offsetof(AudioChannelLayout, mChannelDescriptions)];
NSDictionary *compressionAudioSettings = @{
AVFormatIDKey : [NSNumber numberWithUnsignedInt:kAudioFormatMPEG4AAC],
AVEncoderBitRateKey : [NSNumber numberWithInteger:128000],
AVSampleRateKey : [NSNumber numberWithInteger:44100],
AVChannelLayoutKey : channelLayoutAsData,
AVNumberOfChannelsKey : [NSNumber numberWithUnsignedInteger:2]
};
_writerAudioInput = [AVAssetWriterInput assetWriterInputWithMediaType:[assetAudioTrack mediaType] outputSettings:compressionAudioSettings];
[_writer addInput:_writerAudioInput];
}
if (assetVideoTrack)
{
// If there is a video track to read, set the decompression settings for YUV and create the asset reader output.
NSDictionary *decompressionVideoSettings = @{
(id)kCVPixelBufferPixelFormatTypeKey : [NSNumber numberWithUnsignedInt:kCVPixelFormatType_420YpCbCr8BiPlanarVideoRange],
(id)kCVPixelBufferIOSurfacePropertiesKey : [NSDictionary dictionary]
};
_readerVideoOutput = [AVAssetReaderTrackOutput assetReaderTrackOutputWithTrack:assetVideoTrack outputSettings:decompressionVideoSettings];
[_reader addOutput:_readerVideoOutput];
CMFormatDescriptionRef formatDescription = NULL;
// Grab the video format descriptions from the video track and grab the first one if it exists.
NSArray *formatDescriptions = [assetVideoTrack formatDescriptions];
if ([formatDescriptions count] > 0)
formatDescription = (__bridge CMFormatDescriptionRef)[formatDescriptions objectAtIndex:0];
CGSize trackDimensions = {
.width = 0.0,
.height = 0.0,
};
// If the video track had a format description, grab the track dimensions from there. Otherwise, grab them direcly from the track itself.
if (formatDescription)
trackDimensions = CMVideoFormatDescriptionGetPresentationDimensions(formatDescription, false, false);
else
trackDimensions = [assetVideoTrack naturalSize];
NSDictionary *compressionSettings = nil;
// If the video track had a format description, attempt to grab the clean aperture settings and pixel aspect ratio used by the video.
if (formatDescription)
{
NSDictionary *cleanAperture = nil;
NSDictionary *pixelAspectRatio = nil;
CFDictionaryRef cleanApertureFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_CleanAperture);
if (cleanApertureFromCMFormatDescription)
{
cleanAperture = @{
AVVideoCleanApertureWidthKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureWidth),
AVVideoCleanApertureHeightKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHeight),
AVVideoCleanApertureHorizontalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureHorizontalOffset),
AVVideoCleanApertureVerticalOffsetKey : (id)CFDictionaryGetValue(cleanApertureFromCMFormatDescription, kCMFormatDescriptionKey_CleanApertureVerticalOffset)
};
}
CFDictionaryRef pixelAspectRatioFromCMFormatDescription = CMFormatDescriptionGetExtension(formatDescription, kCMFormatDescriptionExtension_PixelAspectRatio);
if (pixelAspectRatioFromCMFormatDescription)
{
pixelAspectRatio = @{
AVVideoPixelAspectRatioHorizontalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioHorizontalSpacing),
AVVideoPixelAspectRatioVerticalSpacingKey : (id)CFDictionaryGetValue(pixelAspectRatioFromCMFormatDescription, kCMFormatDescriptionKey_PixelAspectRatioVerticalSpacing)
};
}
// Add whichever settings we could grab from the format description to the compression settings dictionary.
if (cleanAperture || pixelAspectRatio)
{
NSMutableDictionary *mutableCompressionSettings = [NSMutableDictionary dictionary];
if (cleanAperture)
[mutableCompressionSettings setObject:cleanAperture forKey:AVVideoCleanApertureKey];
if (pixelAspectRatio)
[mutableCompressionSettings setObject:pixelAspectRatio forKey:AVVideoPixelAspectRatioKey];
compressionSettings = mutableCompressionSettings;
}
}
// Create the video settings dictionary for H.264.
NSMutableDictionary *videoSettings = (NSMutableDictionary *) @{
AVVideoCodecKey : AVVideoCodecH264,
AVVideoWidthKey : [NSNumber numberWithDouble:trackDimensions.width],
AVVideoHeightKey : [NSNumber numberWithDouble:trackDimensions.height]
};
// Put the compression settings into the video settings dictionary if we were able to grab them.
if (compressionSettings)
[videoSettings setObject:compressionSettings forKey:AVVideoCompressionPropertiesKey];
// Create the asset writer input and add it to the asset writer.
_writerVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:[assetVideoTrack mediaType] outputSettings:videoSettings];
[_writer addInput:_writerVideoInput];
}
}
return success;
}
- (BOOL)startAssetReaderAndWriter:(NSError **)outError
{
BOOL success = YES;
// Attempt to start the asset reader.
success = [_reader startReading];
if (!success) {
*outError = [_reader error];
NSLog(@"Reader error");
}
if (success)
{
// If the reader started successfully, attempt to start the asset writer.
success = [_writer startWriting];
if (!success) {
*outError = [_writer error];
NSLog(@"Writer error");
}
}
if (success)
{
// If the asset reader and writer both started successfully, create the dispatch group where the reencoding will take place and start a sample-writing session.
_dispatchGroup = dispatch_group_create();
[_writer startSessionAtSourceTime:kCMTimeZero];
_audioFinished = NO;
_videoFinished = NO;
if (_writerAudioInput)
{
// If there is audio to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(_dispatchGroup);
// Specify the block to execute when the asset writer is ready for audio media data, and specify the queue to call it on.
[_writerAudioInput requestMediaDataWhenReadyOnQueue:_rwAudioSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (_audioFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([_writerAudioInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next audio sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [_readerAudioOutput copyNextSampleBuffer];
if (sampleBuffer != NULL)
{
BOOL success = [_writerAudioInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the audio work has finished).
BOOL oldFinished = _audioFinished;
_audioFinished = YES;
if (oldFinished == NO)
{
[_writerAudioInput markAsFinished];
}
dispatch_group_leave(_dispatchGroup);
}
}];
}
if (_writerVideoInput)
{
// If we had video to reencode, enter the dispatch group before beginning the work.
dispatch_group_enter(_dispatchGroup);
// Specify the block to execute when the asset writer is ready for video media data, and specify the queue to call it on.
[_writerVideoInput requestMediaDataWhenReadyOnQueue:_rwVideoSerializationQueue usingBlock:^{
// Because the block is called asynchronously, check to see whether its task is complete.
if (_videoFinished)
return;
BOOL completedOrFailed = NO;
// If the task isn't complete yet, make sure that the input is actually ready for more media data.
while ([_writerVideoInput isReadyForMoreMediaData] && !completedOrFailed)
{
// Get the next video sample buffer, and append it to the output file.
CMSampleBufferRef sampleBuffer = [_readerVideoOutput copyNextSampleBuffer];
/* PROCESS FRAME HERE */
//CVImageBufferRef pixelBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
//_currentBuffer = pixelBuffer;
//[self performSelectorOnMainThread:@selector(processFrame) withObject:nil waitUntilDone:YES];
/* ------------------ */
if (sampleBuffer != NULL)
{
BOOL success = [_writerVideoInput appendSampleBuffer:sampleBuffer];
CFRelease(sampleBuffer);
sampleBuffer = NULL;
completedOrFailed = !success;
}
else
{
completedOrFailed = YES;
}
}
if (completedOrFailed)
{
// Mark the input as finished, but only if we haven't already done so, and then leave the dispatch group (since the video work has finished).
BOOL oldFinished = _videoFinished;
_videoFinished = YES;
if (oldFinished == NO)
{
[_writerVideoInput markAsFinished];
}
dispatch_group_leave(_dispatchGroup);
}
}];
}
// Set up the notification that the dispatch group will send when the audio and video work have both finished.
dispatch_group_notify(_dispatchGroup, _mainSerializationQueue, ^{
BOOL finalSuccess = YES;
NSError *finalError = nil;
// Check to see if the work has finished due to cancellation.
if (_cancelled)
{
// If so, cancel the reader and writer.
[_reader cancelReading];
[_writer cancelWriting];
}
else
{
// If cancellation didn't occur, first make sure that the asset reader didn't fail.
if ([_reader status] == AVAssetReaderStatusFailed)
{
finalSuccess = NO;
finalError = [_reader error];
NSLog(@"_reader finalError: %@", finalError);
}
// If the asset reader didn't fail, attempt to stop the asset writer and check for any errors.
[_writer finishWritingWithCompletionHandler:^{
UISaveVideoAtPathToSavedPhotosAlbum(_outputURL, nil, nil, nil);
[self readingAndWritingDidFinishSuccessfully:finalSuccess withError:[_writer error]];
}];
}
// Call the method to handle completion, and pass in the appropriate parameters to indicate whether reencoding was successful.
});
}
// Return success here to indicate whether the asset reader and writer were started successfully.
return success;
}
- (void)readingAndWritingDidFinishSuccessfully:(BOOL)success withError:(NSError *)error
{
if (!success)
{
// If the reencoding process failed, we need to cancel the asset reader and writer.
[_reader cancelReading];
[_writer cancelWriting];
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to failure.
});
}
else
{
// Reencoding was successful, reset booleans.
_cancelled = NO;
_videoFinished = NO;
_audioFinished = NO;
dispatch_async(dispatch_get_main_queue(), ^{
// Handle any UI tasks here related to success.
});
}
NSLog(@"readingAndWritingDidFinishSuccessfully success = %@ : Error = %@", (success == 0) ? @"NO" : @"YES", error);
}
@end
答案 1 :(得分：1)
一个简单的解决方案是使用固定大小的AVCaptureSessionPresets之一，例如AVCaptureSessionPreset640x480。在捕获开始之前似乎没有公共API来获取AVCaptureSession解析：
答案 2 :(得分：0)
谢谢你快速回答，塔克。根据你的回答，我决定将AVAssetWriterInput的创建推迟到第一帧，并且效果很好。这是代码。
CVImageBufferRef imageBuffer = CMSampleBufferGetImageBuffer(sampleBuffer);
CVPixelBufferLockBaseAddress(imageBuffer, 0);
size_t width = CVPixelBufferGetWidth(imageBuffer);
size_t height = CVPixelBufferGetHeight(imageBuffer);
CVPixelBufferUnlockBaseAddress(imageBuffer,0);
NSLog(@"NVW size=%zd, %zd", width, height);
NSDictionary* compression = @{
AVVideoAverageBitRateKey:[NSNumber numberWithInt:960000],
AVVideoMaxKeyFrameIntervalKey:[NSNumber numberWithInt:1]
};
self.videoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo
outputSettings:@{
AVVideoCodecKey:AVVideoCodecH264,
AVVideoCompressionPropertiesKey:compression,
AVVideoWidthKey:[NSNumber numberWithInt:width],
AVVideoHeightKey:[NSNumber numberWithInt:height]
}];
self.videoInput.expectsMediaDataInRealTime = YES;