Ekulelu's Blog

iOS视频制作

Posted on | In | | Visitors

#
这里说的视频制作并不是从摄像头录制视频，而是如何将一帧一帧的图片合成一个视频文件。这里我们所使用到的类是AVAssetWriter，它的使用方式比较固定，且代码也比较多。
首先是设定好AVAssetWriter

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-(void)setUpWriter
{
    _rgbColorSpace = CGColorSpaceCreateDeviceRGB();
    
    NSDictionary *bufferAttributes = @{(id)kCVPixelBufferPixelFormatTypeKey : @(kCVPixelFormatType_32BGRA),
                                       (id)kCVPixelBufferCGBitmapContextCompatibilityKey : @YES,
                                       (id)kCVPixelBufferWidthKey : @(_viewSize.width),
                                       (id)kCVPixelBufferHeightKey : @(_viewSize.height),
                                       (id)kCVPixelBufferBytesPerRowAlignmentKey : @(_viewSize.width * 4)
                                       };
    
    _outputBufferPool = NULL;
    CVPixelBufferPoolCreate(NULL, NULL, (__bridge CFDictionaryRef)(bufferAttributes), &_outputBufferPool);
    
    
    NSError* error = nil;
    _videoWriter = [[AVAssetWriter alloc] initWithURL: [self tempFileURL]
                                             fileType:AVFileTypeQuickTimeMovie
                                                error:&error];
    NSParameterAssert(_videoWriter);
    
    NSInteger pixelNumber = _viewSize.width * _viewSize.height;
    NSDictionary* videoCompression = @{AVVideoAverageBitRateKey: @(pixelNumber * 1),
                                       
                                       };
    
    NSDictionary* videoSettings = @{AVVideoCodecKey: AVVideoCodecH264,
                                    AVVideoWidthKey: [NSNumber numberWithInt:_viewSize.width],
                                    AVVideoHeightKey: [NSNumber numberWithInt:_viewSize.height],
                                    AVVideoCompressionPropertiesKey: videoCompression};
    
    _videoWriterInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];
    NSParameterAssert(_videoWriterInput);
    
    _videoWriterInput.expectsMediaDataInRealTime = YES;
    _videoWriterInput.transform = CGAffineTransformIdentity;
    
    _avAdaptor = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:_videoWriterInput sourcePixelBufferAttributes:nil];
    
    [_videoWriter addInput:_videoWriterInput];
    
    [_videoWriter startWriting];
    [_videoWriter startSessionAtSourceTime:CMTimeMake(0, 1000)];
}

配置好AVAssetWriter后，就可以往它写入每一帧的图像数据了。写入的方法如下。代码里面使用了代理模式，将每一帧的context传递给了代理，由代理去完成绘制后，把这个context写入到视频。

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- (void)writeVideoFrame
{   
    if (dispatch_semaphore_wait(_pixelAppendSemaphore, DISPATCH_TIME_NOW) != 0) {
        return;
    }
    dispatch_async(_append_pixelBuffer_queue, ^{
        
        if (![_videoWriterInput isReadyForMoreMediaData]) return;
        if (self.currentTime > self.totalTime && self.isExporting) {
            [self stopExporting];
            return;
        }
        DLog(@"time %f %f", self.currentTime, self.currentTime - self.totalTime);
        
        CMTime time = CMTimeMakeWithSeconds(self.currentTime, 1000);
        
        CVPixelBufferRef pixelBuffer = NULL;
        CGContextRef bitmapContext = [self createPixelBufferAndBitmapContext:&pixelBuffer];
        
        RYExporterNextOperation nextOpr = RYExporterNextOperationContinue;
        //通过代理去绘制图像
        if (self.delegate) {
            if ([self.delegate respondsToSelector:@selector(exporter:getFrameInContext:time:totalTime:nextOperation:)] == YES) {
                [self.delegate exporter:self getFrameInContext:bitmapContext time:self.currentTime totalTime:self.totalTime nextOperation:&nextOpr];
            }
        }
        
        BOOL success = [_avAdaptor appendPixelBuffer:pixelBuffer withPresentationTime:time];
        if (!success) {
            DLog(@"Warning: Unable to write buffer to video");
        }
        CGContextRelease(bitmapContext);
        CVPixelBufferUnlockBaseAddress(pixelBuffer, 0);
        CVPixelBufferRelease(pixelBuffer);
        
        dispatch_semaphore_signal(_pixelAppendSemaphore);
        
        if (nextOpr == RYExporterNextOperationCancel) {
            [self cancelExporting];
            return;
        } else if (nextOpr == RYExporterNextOperationEnd) {
            [self stopExporting];
            return;
        }
        self.currentTime += 1/self.fps;
        
    });
}
- (CGContextRef)createPixelBufferAndBitmapContext:(CVPixelBufferRef *)pixelBuffer
{
    CVPixelBufferPoolCreatePixelBuffer(NULL, _outputBufferPool, pixelBuffer);
    CVPixelBufferLockBaseAddress(*pixelBuffer, 0);
    
    CGContextRef bitmapContext = NULL;
    bitmapContext = CGBitmapContextCreate(CVPixelBufferGetBaseAddress(*pixelBuffer),
                                          CVPixelBufferGetWidth(*pixelBuffer),
                                          CVPixelBufferGetHeight(*pixelBuffer),
                                          8, CVPixelBufferGetBytesPerRow(*pixelBuffer), _rgbColorSpace,
                                          kCGBitmapByteOrder32Little | kCGImageAlphaPremultipliedFirst
                                          );
    CGContextScaleCTM(bitmapContext, _scale, _scale);
    CGAffineTransform flipVertical = CGAffineTransformMake(1, 0, 0, -1, 0, _viewSize.height);
    CGContextConcatCTM(bitmapContext, flipVertical);
    
    return bitmapContext;
}

留意到上面的代码是运行在一个串行队列中，但是这个串行队列是运行在全局队列中，配置如下，写在了init方法里面。

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_append_pixelBuffer_queue = dispatch_queue_create("RYExporter.append_queue", DISPATCH_QUEUE_SERIAL);
dispatch_set_target_queue(_append_pixelBuffer_queue, dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_HIGH, 0));
_pixelAppendSemaphore = dispatch_semaphore_create(0); //初始化一个信号量

为了能自动调用writeVideoFrame方法，使用了一个DisplayLink，在开始导出视频的时候，初始化这个DisplayLink并将它加入到runloop。使用DisplayLink的好处是能获得和屏幕刷新率一样的频率。在录屏的时候也可以使用这个视频导出类。

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- (BOOL)startExportingWithTotalTime:(CGFloat)totalTime viewSize:(CGSize)viewSize videoPath:(NSString*)videoPath fps:(NSInteger)fps
{
    if (!_isExporting) {
        self.totalTime = totalTime;
        self.viewSize = viewSize;
        _videoPath = videoPath;
        self.fps = fps;
        [self setUpWriter];
        _isExporting = (_videoWriter.status == AVAssetWriterStatusWriting);
        _displayLink = [CADisplayLink displayLinkWithTarget:self selector:@selector(writeVideoFrame)];
        [_displayLink addToRunLoop:[NSRunLoop mainRunLoop] forMode:NSRunLoopCommonModes];
        self.beginTime = [NSDate date].timeIntervalSince1970;
        dispatch_semaphore_signal(_pixelAppendSemaphore);
        
    }
    return _isExporting;
}

绘制完成或者取消后，必须对一些用到的对象进行清理，避免内存泄露。

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- (void)completeExportingSession
{
    @weakify(self)
    dispatch_async(_append_pixelBuffer_queue, ^{
        @strongify(self)
        [_videoWriterInput markAsFinished];
        [_videoWriter finishWritingWithCompletionHandler:^{
            void (^completion)(NSError* error) = ^(NSError* error) {
                [self cleanup];
                dispatch_async(dispatch_get_main_queue(), ^{
                    if (self.delegate && [self.delegate respondsToSelector:@selector(exporterVideoCompleted:videoPath:error:)]) {
                        [self.delegate exporterVideoCompleted:self videoPath:self.videoPath error:error];
                    }
                });
            };
            
            if (self.videoPath) { //如果有设定目标路径，则移动到目标路径
                NSFileManager* fileManager = [NSFileManager defaultManager];
                NSError *error;
                if ([fileManager fileExistsAtPath:self.videoPath isDirectory:nil]) {
                    [fileManager removeItemAtPath:self.videoPath error:&error];
                }
                [fileManager moveItemAtPath:self.tmpFilePath toPath:self.videoPath error:&error];
                completion(error);
            } else {
                ALAssetsLibrary *library = [[ALAssetsLibrary alloc] init];
                [library writeVideoAtPathToSavedPhotosAlbum:_videoWriter.outputURL completionBlock:^(NSURL *assetURL, NSError *error) {
                    if (error) {
                        NSLog(@"Error copying video to camera roll:%@", [error localizedDescription]);
                    } else {
                        [self removeTempFilePath:_videoWriter.outputURL.path];
                    }
                    completion(error);
                }];
            }
        }];
    });
}
- (void)cancelExportingSession {
    @weakify(self)
    dispatch_async(_append_pixelBuffer_queue, ^{
        @strongify(self)
        [_videoWriterInput markAsFinished];
        [_videoWriter finishWritingWithCompletionHandler:^{
            void (^completion)(void) = ^() {
                [self cleanup];
                dispatch_async(dispatch_get_main_queue(), ^{
                    if (self.delegate && [self.delegate respondsToSelector:@selector(exporterVideoCompleted:videoPath:error:)]) {
                        [self.delegate exporterVideoCompleted:self videoPath:nil error:nil];
                    }
                });
            };
            
            [self removeTempFilePath:_videoWriter.outputURL.path];
            completion();
        }];
    });
}
- (void)cleanup
{
    self.avAdaptor = nil;
    self.videoWriterInput = nil;
    self.videoWriter = nil;
    self.outputBufferPoolAuxAttributes = nil;
    CGColorSpaceRelease(_rgbColorSpace);
    CVPixelBufferPoolRelease(_outputBufferPool);
    
    self.currentTime = 0;
    self.totalTime = 0;
    self.isExporting = NO;
}