ios – 如何将UIImage / CGImageRef的Alpha通道转换为掩码？

这是完成这个的一种方法.拿你原来的RBGA图像,并通过以下处理：

>将其绘制为仅Alpha通道位图图像
>反转每个像素的alpha值得到与上述相反的行为
>将此倒置的Alpha图像转换为实际的掩码
>使用它

例如.

#define ROUND_UP(N,S) ((((N) + (S) - 1) / (S)) * (S))

// Original RGBA image
CGImageRef originalMaskImage = [[UIImage imageNamed:@"masktest.png"] CGImage];
float width = CGImageGetWidth(originalMaskImage);
float height = CGImageGetHeight(originalMaskImage);

// Make a bitmap context that's only 1 alpha channel
// WARNING: the bytes per row probably needs to be a multiple of 4 
int strideLength = ROUND_UP(width * 1,4);
unsigned char * alphaData = calloc(strideLength * height,sizeof(unsigned char));
CGContextRef alphaOnlyContext = CGBitmapContextCreate(alphaData,width,height,8,strideLength,NULL,kCGImageAlphaOnly);

// Draw the RGBA image into the alpha-only context.
CGContextDrawImage(alphaOnlyContext,CGRectMake(0,height),originalMaskImage);

// Walk the pixels and invert the alpha value. This lets you colorize the opaque shapes in the original image.
// If you want to do a traditional mask (where the opaque values block) just get rid of these loops.
for (int y = 0; y < height; y++) {
    for (int x = 0; x < width; x++) {
        unsigned char val = alphaData[y*strideLength + x];
        val = 255 - val;
        alphaData[y*strideLength + x] = val;
    }
}

CGImageRef alphaMaskImage = CGBitmapContextCreateImage(alphaOnlyContext);
CGContextRelease(alphaOnlyContext);
free(alphaData);

// Make a mask
CGImageRef finalMaskImage = CGImageMaskCreate(CGImageGetWidth(alphaMaskImage),CGImageGetHeight(alphaMaskImage),CGImageGetBitsPerComponent(alphaMaskImage),CGImageGetBitsPerPixel(alphaMaskImage),CGImageGetBytesPerRow(alphaMaskImage),CGImageGetDataProvider(alphaMaskImage),false);
CGImageRelease(alphaMaskImage);

现在,您可以使用finalMaskImage作为CGContextClipToMask等中的掩码等.