CGContext的垂直翻转 [英] Vertical flip of CGContext

问题描述

我有一个UIView，我试图使用[CALayer renderInContext：]渲染到UIImage中。但是，我发现结果图像是垂直翻转的。由于坐标系不同，我有点期待这一点。然而，我尝试用仿射变换将上下文恢复正常 - 但它没有任何效果：

I have a UIView that I am trying to render into a UIImage using [CALayer renderInContext:]. However, I find that the resultant image is flipped vertically. I kind of expect this due to the different coordinate systems. However, I then try and flip the context back to normal with an affine transform - but it doesn't have any effect:

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？

推荐答案

CTM影响未来的绘图;你正在捕捉已经画出的东西。你需要在绘制之前连续转换，而不是在之后。

The CTM affects future drawing; you're capturing what you have already drawn. You need to concat that transformation before you draw, not after.

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