IPhone RGBA到ARGB

Question

我正在使用glReadPixels抓取opengl场景的屏幕截图，然后在iOS4上使用AVAssetWriter将它们转换成视频。我的问题是，我需要将alpha通道传递给只接受kCVPixelFormatType_32ARGB和glReadPixels的视频。所以基本上我需要一种方法将我的RGBA转换成ARGB，换句话说，把alpha字节放在第一位。

int depth = 4;
unsigned char buffer[width * height * depth];  
glReadPixels(0,0,width, height, GL_RGBA, GL_UNSIGNED_BYTE, &buffer);

CGDataProviderRef ref = CGDataProviderCreateWithData(NULL, &buffer), width*height*depth, NULL );

CGBitmapInfo bitmapInfo = kCGBitmapByteOrderDefault | kCGImageAlphaPremultipliedLast;

CGImageRef image = CGImageCreate(width, height, 8, 32, width*depth, CGColorSpaceCreateDeviceRGB(), bitmapInfo, ref, NULL, true, kCGRenderingIntentDefault);

UIWindow* parentWindow = [self window];

NSDictionary *options = [NSDictionary dictionaryWithObjectsAndKeys:[NSNumber numberWithBool:YES], kCVPixelBufferCGImageCompatibilityKey, [NSNumber numberWithBool:YES], kCVPixelBufferCGBitmapContextCompatibilityKey, nil];

CVPixelBufferRef pxbuffer = NULL;
CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, width, height, kCVPixelFormatType_32ARGB, (CFDictionaryRef) options, &pxbuffer);

NSParameterAssert(status == kCVReturnSuccess);
NSParameterAssert(pxbuffer != NULL);

CVPixelBufferLockBaseAddress(pxbuffer, 0);
void *pxdata = CVPixelBufferGetBaseAddress(pxbuffer);
NSParameterAssert(pxdata != NULL);

CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pxdata, width, height, 8, depth*width, rgbColorSpace, kCGImageAlphaPremultipliedFirst);

NSParameterAssert(context);

CGContextConcatCTM(context, parentWindow.transform);
CGContextDrawImage(context, CGRectMake(0, 0, width, height), image);

CGColorSpaceRelease(rgbColorSpace);
CGContextRelease(context);

CVPixelBufferUnlockBaseAddress(pxbuffer, 0);

return pxbuffer; // chuck pixel buffer into AVAssetWriter

我想我会发布整个代码，因为我可能会帮助其他人。

干杯

Answer 1

注意:我假设每个通道8位。如果不是这样，则相应地进行调整。

要最后移动alpha位，需要执行旋转。这通常最容易通过位移位来表达。

在这种情况下，您希望将RGB位向右移动8位，将A位向左移动24位。然后应该使用逐位OR将这两个值放在一起，这样就变成了argb = (rgba >> 8) | (rgba << 24)。

Answer 2

更好的是，不要使用ARGB编码你的视频，发送你的AVAssetWriter BGRA帧。正如我在this answer中所描述的，这样做可以让你在iPhone 4上以30FPS的速度编码640x480视频，而对于720p的视频，编码速度高达20FPS。使用这种技术，iPhone 4S可以以30 FPS的速度播放1080p的视频。

此外，您还需要确保每次都使用像素缓冲池，而不是重新创建像素缓冲区。复制该答案中的代码，使用以下命令配置AVAssetWriter：

NSError *error = nil;

assetWriter = [[AVAssetWriter alloc] initWithURL:movieURL fileType:AVFileTypeAppleM4V error:&error];
if (error != nil)
{
    NSLog(@"Error: %@", error);
}


NSMutableDictionary * outputSettings = [[NSMutableDictionary alloc] init];
[outputSettings setObject: AVVideoCodecH264 forKey: AVVideoCodecKey];
[outputSettings setObject: [NSNumber numberWithInt: videoSize.width] forKey: AVVideoWidthKey];
[outputSettings setObject: [NSNumber numberWithInt: videoSize.height] forKey: AVVideoHeightKey];


assetWriterVideoInput = [AVAssetWriterInput assetWriterInputWithMediaType:AVMediaTypeVideo outputSettings:outputSettings];
assetWriterVideoInput.expectsMediaDataInRealTime = YES;

// You need to use BGRA for the video in order to get realtime encoding. I use a color-swizzling shader to line up glReadPixels' normal RGBA output with the movie input's BGRA.
NSDictionary *sourcePixelBufferAttributesDictionary = [NSDictionary dictionaryWithObjectsAndKeys: [NSNumber numberWithInt:kCVPixelFormatType_32BGRA], kCVPixelBufferPixelFormatTypeKey,
                                                       [NSNumber numberWithInt:videoSize.width], kCVPixelBufferWidthKey,
                                                       [NSNumber numberWithInt:videoSize.height], kCVPixelBufferHeightKey,
                                                       nil];

assetWriterPixelBufferInput = [AVAssetWriterInputPixelBufferAdaptor assetWriterInputPixelBufferAdaptorWithAssetWriterInput:assetWriterVideoInput sourcePixelBufferAttributes:sourcePixelBufferAttributesDictionary];

[assetWriter addInput:assetWriterVideoInput];

然后使用此代码通过glReadPixels()抓取每个呈现的帧

CVPixelBufferRef pixel_buffer = NULL;

CVReturn status = CVPixelBufferPoolCreatePixelBuffer (NULL, [assetWriterPixelBufferInput pixelBufferPool], &pixel_buffer);
if ((pixel_buffer == NULL) || (status != kCVReturnSuccess))
{
    return;
}
else
{
    CVPixelBufferLockBaseAddress(pixel_buffer, 0);
    GLubyte *pixelBufferData = (GLubyte *)CVPixelBufferGetBaseAddress(pixel_buffer);
    glReadPixels(0, 0, videoSize.width, videoSize.height, GL_RGBA, GL_UNSIGNED_BYTE, pixelBufferData);
}

// May need to add a check here, because if two consecutive times with the same value are added to the movie, it aborts recording
CMTime currentTime = CMTimeMakeWithSeconds([[NSDate date] timeIntervalSinceDate:startTime],120);

if(![assetWriterPixelBufferInput appendPixelBuffer:pixel_buffer withPresentationTime:currentTime]) 
{
    NSLog(@"Problem appending pixel buffer at time: %lld", currentTime.value);
} 
else 
{
//        NSLog(@"Recorded pixel buffer at time: %lld", currentTime.value);
}
CVPixelBufferUnlockBaseAddress(pixel_buffer, 0);

CVPixelBufferRelease(pixel_buffer);

当使用glReadPixels()时，你需要调整帧的颜色，所以我使用了一个屏幕外的FBO和一个片段着色器来做这件事，代码如下：

 varying highp vec2 textureCoordinate;

 uniform sampler2D inputImageTexture;

 void main()
 {
     gl_FragColor = texture2D(inputImageTexture, textureCoordinate).bgra;
 }

然而，在iOS 5.0上，有一种比我在this answer中描述的glReadPixels()更快的方法来获取OpenGL ES内容。这个过程的好处是纹理已经存储了BGRA像素格式的内容，所以你可以直接将封装的像素缓冲区提供给AVAssetWriter，而不需要任何颜色转换，仍然可以看到很快的编码速度。

Answer 3

我知道这个问题已经得到了回答，但我想确保大家都知道vImage，它是Accelerate框架的一部分，可以在iOS和OSX中使用。我的理解是，Core Graphics使用vImage对位图进行CPU限制的向量操作。

要将ARGB转换为RGBA的特定API是vImagePermuteChannels_ARGB8888。还有一些API可以将RGB转换为ARGB/XRGB、翻转图像、覆盖通道等。这是一种隐藏的宝石！

更新: Brad Larson对基本上相同的问题here写了一个很好的答案。