裁剪 UIImage [英] Cropping an UIImage

问题描述

我有一些代码可以调整图像大小，因此我可以获得图像中心的缩放块 - 我使用它来获取 UIImage 并返回一个小的方形表示图像，类似于在照片应用程序的相册视图中看到的内容.(我知道我可以使用 UIImageView 并调整裁剪模式以获得相同的结果，但这些图像有时会显示在 UIWebViews 中).

我开始注意到这段代码中有一些崩溃，我有点难住了.我有两种不同的理论，我想知道是否有一种是基于基础的.

理论 1) 我通过绘制目标尺寸的屏幕外图像上下文来实现裁剪.由于我想要图像的中心部分，因此我将传递给 drawInRect 的 CGRect 参数设置为大于图像上下文边界的值.我希望这是 Kosher，但我是不是试图勾勒出我不应该触及的其他记忆?

理论 2) 我在后台线程中执行所有这些操作.我知道 UIKit 的某些部分仅限于主线程.我假设/希望绘制到屏幕外视图不是其中之一.我错了吗?

(哦，我多么想念 NSImage 的 drawInRect:fromRect:operation:fraction: 方法.)

解决方案

更新 2014-05-28:当 iOS 3 左右是热门新事物时，我写了这个，我确信有更好的方法来做到这一点现在，可能是内置的.正如很多人提到的，这种方法没有考虑轮换；阅读一些其他答案并传播一些赞誉，以使对这个问题的回答对每个人都有帮助.

原回复:

我要将我对同一问题的回答复制/粘贴到别处:

没有一个简单的类方法可以做到这一点，但有一个函数可以用来获得所需的结果:CGImageCreateWithImageInRect(CGImageRef, CGRect) 会帮助你.>

这是一个使用它的简短示例:

CGImageRef imageRef = CGImageCreateWithImageInRect([largeImage CGImage],cropRect);//或者在任何你喜欢的地方使用 UIImage[UIImageView setImage:[UIImage imageWithCGImage:imageRef]];CGImageRelease(imageRef);

I've got some code that resizes an image so I can get a scaled chunk of the center of the image - I use this to take a UIImage and return a small, square representation of an image, similar to what's seen in the album view of the Photos app. (I know I could use a UIImageView and adjust the crop mode to achieve the same results, but these images are sometimes displayed in UIWebViews).

I've started to notice some crashes in this code and I'm a bit stumped. I've got two different theories and I'm wondering if either is on-base.

Theory 1) I achieve the cropping by drawing into an offscreen image context of my target size. Since I want the center portion of the image, I set the CGRect argument passed to drawInRect to something that's larger than the bounds of my image context. I was hoping this was Kosher, but am I instead attempting to draw over other memory that I shouldn't be touching?

Theory 2) I'm doing all of this in a background thread. I know there are portions of UIKit that are restricted to the main thread. I was assuming / hoping that drawing to an offscreen view wasn't one of these. Am I wrong?

(Oh, how I miss NSImage's drawInRect:fromRect:operation:fraction: method.)

解决方案

Update 2014-05-28: I wrote this when iOS 3 or so was the hot new thing, I'm certain there are better ways to do this by now, possibly built-in. As many people have mentioned, this method doesn't take rotation into account; read some additional answers and spread some upvote love around to keep the responses to this question helpful for everyone.

Original response:

I'm going to copy/paste my response to the same question elsewhere:

There isn't a simple class method to do this, but there is a function that you can use to get the desired results: CGImageCreateWithImageInRect(CGImageRef, CGRect) will help you out.

Here's a short example using it:

CGImageRef imageRef = CGImageCreateWithImageInRect([largeImage CGImage], cropRect);
// or use the UIImage wherever you like
[UIImageView setImage:[UIImage imageWithCGImage:imageRef]]; 
CGImageRelease(imageRef);

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