如何在 iOS 8 的 Accelerate 框架中使用 vImage 缩放图像？

Question

我正在尝试在 iOS 8 设备上尽快调整 CMSampleBufferRef 的大小以用于图像处理。从我在网上找到的内容来看，执行此操作的方法似乎是在 Accelerate 框架中使用 vImage API。但是，我对 Accelerate 框架所做的工作并不多，而且我不太清楚如何做到这一点。到目前为止，这是我将图像缩放到 200x200 的内容：

- (void)captureOutput:(AVCaptureOutput *)captureOutput didOutputSampleBuffer:(CMSampleBufferRef)sampleBuffer fromConnection:(AVCaptureConnection *)connection
{
    CVImageBufferRef cvimgRef = CMSampleBufferGetImageBuffer(sampleBuffer);
    CVPixelBufferLockBaseAddress(cvimgRef,0);
    void *imageData = CVPixelBufferGetBaseAddress(cvimgRef);
    NSInteger width = CVPixelBufferGetWidth(cvimgRef);
    NSInteger height = CVPixelBufferGetHeight(cvimgRef);

    unsigned char *newData= // NOT SURE WHAT THIS SHOULD BE...
    vImage_Buffer  inBuff = { imageData, height, width, 4*width };
    vImage_Buffer  outBuff = { newData, 200, 200, 4*200 };

    // NOT SURE IF THIS IS THE CORRECT METHOD... video output settings for kCVPixelBufferPixelFormatTypeKey is set to kCVPixelFormatType_32BGRA
    // This seems wrong since the image scale is ARGB, not BGRA.
    vImageScale_ARGB8888(inBuffer, outBuffer, NULL, kvImageNoFlags);
    CVPixelBufferUnlockBaseAddress(cvimgRef,0);
}

其中 outBuffer 是结果。之后，我也不确定如何将 outBuffer 转换回 CVImageBufferRef 以进行进一步的图像处理。任何建议将不胜感激！

Answer 1

- (CVPixelBufferRef)copyRenderedPixelBuffer:(CVPixelBufferRef)pixelBuffer {

CVPixelBufferLockBaseAddress( pixelBuffer, 0 );

// vImage processing
vImage_Error err;
vImage_Buffer buffer;
buffer.data = (unsigned char *)CVPixelBufferGetBaseAddress( pixelBuffer );
buffer.rowBytes = CVPixelBufferGetBytesPerRow( pixelBuffer );
buffer.width = CVPixelBufferGetWidth( pixelBuffer );
buffer.height = CVPixelBufferGetHeight( pixelBuffer );
vImageCVImageFormatRef vformat = vImageCVImageFormat_CreateWithCVPixelBuffer( pixelBuffer );
vImage_CGImageFormat cgformat = {
    .bitsPerComponent = 8,
    .bitsPerPixel = 32,
    .bitmapInfo = kCGBitmapByteOrderDefault,
    .colorSpace = NULL,    //sRGB
};
const CGFloat bgColor[3] = {0.0, 0.0, 0.0};
vImageBuffer_InitWithCVPixelBuffer(&buffer, &cgformat, pixelBuffer, vformat, bgColor, kvImageNoAllocate);

vImage_Buffer outbuffer;
void *tempBuffer;
tempBuffer = malloc(CVPixelBufferGetBytesPerRow( pixelBuffer ) * CVPixelBufferGetHeight( pixelBuffer ));
outbuffer.data = tempBuffer;
outbuffer.rowBytes = CVPixelBufferGetBytesPerRow( pixelBuffer );
outbuffer.width = CVPixelBufferGetWidth( pixelBuffer );
outbuffer.height = CVPixelBufferGetHeight( pixelBuffer );

// 在此处处理图像

    err = vImageBuffer_CopyToCVPixelBuffer(&outbuffer, &cgformat, pixelBuffer, vformat, bgColor, kvImageNoFlags);

if(err != -1)
    free(tempBuffer);

CVPixelBufferUnlockBaseAddress( pixelBuffer, 0 );

return (CVPixelBufferRef)CFRetain( pixelBuffer );

}

Answer 2

您必须将重采样过滤器与任何改变图像几何形状的 vImage 操作结合使用：第 32 页，vImage 编程指南。

Answer 3

vImageScale 只返回一个缓冲区数据，注意缓冲区需要被释放。
我不知道是否有更快的方法只使用该输出缓冲区，但我会将缓冲区转换为CGImage。类似的东西取自here，所以把它作为参考

vImage_CGImageFormat format = {
        .bitsPerComponent = 8,
        .bitsPerPixel = 32,
        .colorSpace = NULL,
        .bitmapInfo = (CGBitmapInfo)kCGImageAlphaFirst,
        .version = 0,
        .decode = NULL,
        .renderingIntent = kCGRenderingIntentDefault,
    };
ret = kvImageNoError;
    CGImageRef destRef = vImageCreateCGImageFromBuffer(&dstBuffer, &format, NULL, NULL, kvImageNoFlags, &ret)

稍后我会将其转换为CVPixelBuffer。

- (CVPixelBufferRef) pixelBufferFromCGImage: (CGImageRef) image
{
    NSDictionary *options = @{
                              (NSString*)kCVPixelBufferCGImageCompatibilityKey : @YES,
                              (NSString*)kCVPixelBufferCGBitmapContextCompatibilityKey : @YES,
                              };

CVPixelBufferRef pxbuffer = NULL;
CVReturn status = CVPixelBufferCreate(kCFAllocatorDefault, CGImageGetWidth(image),
                    CGImageGetHeight(image), kCVPixelFormatType_32ARGB, (__bridge CFDictionaryRef) options,
                    &pxbuffer);
if (status!=kCVReturnSuccess) {
    DLog(@"Operation failed");
}
NSParameterAssert(status == kCVReturnSuccess && pxbuffer != NULL);

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

CGColorSpaceRef rgbColorSpace = CGColorSpaceCreateDeviceRGB();
CGContextRef context = CGBitmapContextCreate(pxdata, CGImageGetWidth(image),
                                             CGImageGetHeight(image), 8, 4*CGImageGetWidth(image), rgbColorSpace,
                                             kCGImageAlphaNoneSkipFirst);
NSParameterAssert(context);

CGContextConcatCTM(context, CGAffineTransformMakeRotation(0));

CGContextDrawImage(context, CGRectMake(0, 0, CGImageGetWidth(image),
                                       CGImageGetHeight(image)), image);
CGColorSpaceRelease(rgbColorSpace);
CGContextRelease(context);

CVPixelBufferUnlockBaseAddress(pxbuffer, 0);
return pxbuffer;
}

我很确定可以避免转换为 CGImage 并开始使用缓冲区，但我从未尝试过。