iphone - CGContext的垂直翻转

Question

我有一个 UIView，我正在尝试使用 [CALayer renderInContext:] 将其渲染为 UIImage。然而，我发现生成的图像是垂直翻转的。由于坐标系不同，我有点期待这一点。但是，然后我尝试使用仿射变换将上下文翻转回正常状态 - 但它没有任何效果:

CGAffineTransform flipVertical = CGAffineTransformMake(
    1, 0, 0, -1, 0, imageContextHeight
);
CGContextConcatCTM(imageContext, flipVertical);
CGImageRef cgImage = CGBitmapContextCreateImage(imageContext);
UIImage* uiImage = [[UIImage imageWithCGImage:cgImage] retain];
CGImageRelease(cgImage);

我做错了什么？

Answer 1

这是我根据这个答案编写的一些代码，以防对任何人有帮助:

#import <QuartzCore/QuartzCore.h>
...
+ (UIImage *) flipImageVertically:(UIImage *)originalImage {
    UIImageView *tempImageView = [[UIImageView alloc] initWithImage:originalImage];

    UIGraphicsBeginImageContext(tempImageView.frame.size);
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGAffineTransform flipVertical = CGAffineTransformMake(
            1, 0, 0, -1, 0, tempImageView.frame.size.height
    );
    CGContextConcatCTM(context, flipVertical);  

    [tempImageView.layer renderInContext:context];

    UIImage *flipedImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [tempImageView release];

    return flipedImage;
}