iOS里生成灰化（黑白）图像

代码的目的是根据一个UIImage生成对应的灰化（黑白）UIImage.

以下代码源自stackoverflow里的一个回复，可惜原文链接已经找不到了。

- (UIImage*) convertImageToGreyScale:(UIImage*) image
{
    // Create image rectangle with current image width/height
    CGRect imageRect = CGRectMake(0, 0, image.size.width, image.size.height);
    
    // Grayscale color space
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceGray();
    
    // Create bitmap content with current image size and grayscale colorspace
    CGContextRef context = CGBitmapContextCreate(nil, image.size.width, image.size.height, 8, 0, colorSpace, kCGImageAlphaNone);
    
    // Draw image into current context, with specified rectangle
    // using previously defined context (with grayscale colorspace)
    CGContextDrawImage(context, imageRect, [image CGImage]);
    
    // Create bitmap image info from pixel data in current context
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    
    // Create a new UIImage object
    UIImage *newImage = [UIImage imageWithCGImage:imageRef];
    
    // Release colorspace, context and bitmap information
    CGColorSpaceRelease(colorSpace);
    CGContextRelease(context);
    CFRelease(imageRef);
    
    // Return the new grayscale image
    return newImage;
}