Question

在我正在编写的cocoa应用程序中，我从Quartz Composer渲染器（NSImage对象）获取快照图像，我想在720 * 480大小，25 fps和H264编解码器的QTMovie中对它们进行编码使用addImage：方法。以下是相应的代码：

qRenderer = [[QCRenderer alloc] initOffScreenWithSize:NSMakeSize(720,480) colorSpace:CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB) composition:[QCComposition compositionWithFile:qcPatchPath]]; // define an "offscreen" Quartz composition renderer with the right image size


imageAttrs = [NSDictionary dictionaryWithObjectsAndKeys: @"avc1", // use the H264 codec
              QTAddImageCodecType, nil];

qtMovie = [[QTMovie alloc] initToWritableFile: outputVideoFile error:NULL]; // initialize the output QT movie object

long fps = 25;
frameNum = 0;

NSTimeInterval renderingTime = 0;
NSTimeInterval frameInc = (1./fps);
NSTimeInterval myMovieDuration = 70;
NSImage * myImage;
while (renderingTime <= myMovieDuration){
    if(![qRenderer renderAtTime: renderingTime arguments:NULL])
        NSLog(@"Rendering failed at time %.3fs", renderingTime);
    myImage = [qRenderer snapshotImage];
    [qtMovie addImage:myImage forDuration: QTMakeTimeWithTimeInterval(frameInc) withAttributes:imageAttrs];
    [myImage release];
    frameNum ++;
    renderingTime = frameNum * frameInc;
}
[qtMovie updateMovieFile];
[qRenderer release];
[qtMovie release];

它有效，但我的应用程序无法在我的新MacBook Pro上实时执行此操作，而我知道QuickTime Broadcaster可以在H264中实时编码图像，其质量甚至高于我使用的图像，同一台电脑。

为什么？这是什么问题？这是硬件管理问题（多核线程，GPU，......）还是我错过了什么？让我作为Apple开发世界中的新手（2周练习），包括Objective-C，cocoa，X-code，Quicktime和Quartz Composer库等。

感谢您的帮助

Answer 1

AVFoundation是一种将QuartzComposer动画渲染为H.264视频流的更有效方法。

size_t width = 640;
size_t height = 480;

const char *outputFile = "/tmp/Arabesque.mp4";

QCComposition *composition = [QCComposition compositionWithFile:@"/System/Library/Screen Savers/Arabesque.qtz"];
QCRenderer *renderer = [[QCRenderer alloc] initOffScreenWithSize:NSMakeSize(width, height)
                                                      colorSpace:CGColorSpaceCreateWithName(kCGColorSpaceGenericRGB) composition:composition];

unlink(outputFile);
AVAssetWriter *videoWriter = [[AVAssetWriter alloc] initWithURL:[NSURL fileURLWithPath:@(outputFile)] fileType:AVFileTypeMPEG4 error:NULL];

NSDictionary *videoSettings = @{ AVVideoCodecKey : AVVideoCodecH264, AVVideoWidthKey : @(width), AVVideoHeightKey : @(height) };
AVAssetWriterInput* writerInput = [[AVAssetWriterInput alloc] initWithMediaType:AVMediaTypeVideo outputSettings:videoSettings];

[videoWriter addInput:writerInput];
[writerInput release];

AVAssetWriterInputPixelBufferAdaptor *pixelBufferAdaptor = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:writerInput sourcePixelBufferAttributes:NULL];

int framesPerSecond = 30;
int totalDuration = 30;
int totalFrameCount = framesPerSecond * totalDuration;

[videoWriter startWriting];
[videoWriter startSessionAtSourceTime:kCMTimeZero];

__block long frameNumber = 0;

dispatch_queue_t workQueue = dispatch_queue_create("com.example.work-queue", DISPATCH_QUEUE_SERIAL);

NSLog(@"Starting.");
[writerInput requestMediaDataWhenReadyOnQueue:workQueue usingBlock:^{
    while ([writerInput isReadyForMoreMediaData]) {
        NSTimeInterval frameTime = (float)frameNumber / framesPerSecond;
        if (![renderer renderAtTime:frameTime arguments:NULL]) {
            NSLog(@"Rendering failed at time %.3fs", frameTime);
            break;
        }

        CVPixelBufferRef frame = (CVPixelBufferRef)[renderer createSnapshotImageOfType:@"CVPixelBuffer"];
        [pixelBufferAdaptor appendPixelBuffer:frame withPresentationTime:CMTimeMake(frameNumber, framesPerSecond)];
        CFRelease(frame);

        frameNumber++;
        if (frameNumber >= totalFrameCount) {
            [writerInput markAsFinished];
            [videoWriter finishWriting];
            [videoWriter release];
            [renderer release];
            NSLog(@"Rendered %ld frames.", frameNumber);
            break;
        }

    }
}];

在我的测试中，这大约是使用QTKit的发布代码的两倍。最大的改进似乎来自于将H.264编码传递给GPU而不是在软件中执行。从快速浏览一下配置文件看，剩下的瓶颈似乎是组合物本身的渲染，并将渲染后的数据从GPU读回到像素缓冲区。显然，你的作文的复杂性会对此产生一些影响。

有可能通过使用QCRenderer提供快照CVOpenGLBufferRef的能力来进一步优化这一点，这可能会将帧的数据保留在GPU上，而不是将其读回来将其移除到编码器。尽管如此，我并没有看得太远。

为什么我的基于QTKit的图像编码应用程序如此之慢？

1 个答案: