Question

我使用glReadPixels抓取我的opengl场景的屏幕截图，然后在IOS 4上使用AVAssetWriter将它们转换成视频。我的问题是我需要将alpha通道传递给视频，该视频只接受kCVPixelFormatType_32ARGB和glReadPixels检索RGBA。所以基本上我需要一种方法将我的RGBA转换为ARGB，换句话说就是把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

以为我会发布整个代码，因为我可以帮助其他人。

干杯

Answer 1

注意：我假设每个通道有8位。如果不是这样，请相应调整。

要最后移动alpha位，您需要执行 rotation 。这通常通过位移最容易表达。

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。然后应使用按位OR将这两个值放在一起，这样就变成argb = (rgba >> 8) | (rgba << 24)。

Answer 2

更好的是，不要使用ARGB对视频进行编码，发送AVAssetWriter BGRA帧。正如我在this answer中描述的那样，这样做可以让你在iPhone 4上以30 FPS编码640x480视频，为720p视频编码高达20 FPS。 iPhone 4S可以使用此功能以30 FPS的速度一直播放到1080p视频。

此外，您还需要确保使用像素缓冲池，而不是每次都重新创建像素缓冲区。从该答案复制代码，您可以使用以下命令配置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];

然后使用此代码使用glReadPixels()抓取每个渲染帧：

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);

使用glReadPixels()时，您需要调整帧的颜色，因此我使用了以下代码的屏幕外FBO和片段着色器来执行此操作：

 varying highp vec2 textureCoordinate;

 uniform sampler2D inputImageTexture;

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

然而，在iOS 5.0上获取OpenGL ES内容比glReadPixels()更快，我在this answer中描述了{{1}}。关于该过程的好处是纹理已经以BGRA像素格式存储内容，因此您可以直接将封装像素缓冲区提供给AVAssetWriter而无需任何颜色转换，并且仍然可以看到很好的编码速度。

Answer 3

我意识到这个问题已得到解答，但我想确保人们了解vImage，它是Accelerate框架的一部分，可在iOS和OSX中使用。我的理解是，Core Graphics使用vImage在位图上执行CPU绑定的向量操作。

您希望将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，覆盖频道等等。这是一种隐藏的宝石！

更新：Brad Larson对基本相同的问题here写了一个很好的答案。

Answer 4

是每个通道的8位，所以它是这样的：

int depth = 4;
int width = 320;
int height = 480;

unsigned char buffer[width * height * depth]; 

glReadPixels(0,0,width, height, GL_RGBA, GL_UNSIGNED_BYTE, &buffer);

for(int i = 0; i < width; i++){
   for(int j = 0; j < height; j++){     
    buffer[i*j] = (buffer[i*j] >> 8) | (buffer[i*j] << 24);
    }
}

我似乎无法让它发挥作用

Answer 5

我确信可以忽略alpha值。所以你可以用像素缓冲区数组移位一个字节来做memcpy：

void *buffer = malloc(width*height*4);
glReadPixels(0, 0, width, height, GL_RGBA, GL_UNSIGNED_BYTE, &buffer);
…
memcpy(pxdata + 1, buffer, width*height*4 - 1);

Answer 6

+ (UIImage *) createARGBImageFromRGBAImage: (UIImage *)image {
    CGSize dimensions = [image size];

    NSUInteger bytesPerPixel = 4;
    NSUInteger bytesPerRow = bytesPerPixel * dimensions.width;
    NSUInteger bitsPerComponent = 8;

    unsigned char *rgba = malloc(bytesPerPixel * dimensions.width * dimensions.height);
    unsigned char *argb = malloc(bytesPerPixel * dimensions.width * dimensions.height);

    CGColorSpaceRef colorSpace = NULL;
    CGContextRef context = NULL;

    colorSpace = CGColorSpaceCreateDeviceRGB();
    context = CGBitmapContextCreate(rgba, dimensions.width, dimensions.height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedLast | kCGBitmapByteOrderDefault); // kCGBitmapByteOrder32Big
    CGContextDrawImage(context, CGRectMake(0, 0, dimensions.width, dimensions.height), [image CGImage]);
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);

    for (int x = 0; x < dimensions.width; x++) {
        for (int y = 0; y < dimensions.height; y++) {
            NSUInteger offset = ((dimensions.width * y) + x) * bytesPerPixel;
            argb[offset + 0] = rgba[offset + 3];
            argb[offset + 1] = rgba[offset + 0];
            argb[offset + 2] = rgba[offset + 1];
            argb[offset + 3] = rgba[offset + 2];
        }
    }

    colorSpace = CGColorSpaceCreateDeviceRGB();
    context = CGBitmapContextCreate(argb, dimensions.width, dimensions.height, bitsPerComponent, bytesPerRow, colorSpace, kCGImageAlphaPremultipliedFirst | kCGBitmapByteOrderDefault); // kCGBitmapByteOrder32Big
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    image = [UIImage imageWithCGImage: imageRef];
    CGImageRelease(imageRef);
    CGContextRelease(context);
    CGColorSpaceRelease(colorSpace);

    free(rgba);
    free(argb);

    return image;
}

IPhone RGBA到ARGB

6 个答案: