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    <p>I was looking for a way to get an arbitrary rectangular crop (ie., sub-image) of a UIImage. </p> <p>Most of the solutions I tried do not work if the orientation of the image is anything but UIImageOrientationUp. </p> <p>For example:</p> <p><a href="http://www.hive05.com/2008/11/crop-an-image-using-the-iphone-sdk/" rel="noreferrer">http://www.hive05.com/2008/11/crop-an-image-using-the-iphone-sdk/</a></p> <p>Typically if you use your iPhone camera, you will have other orientations like UIImageOrientationLeft, and you will not get a correct crop with the above. This is because of the use of CGImageRef/CGContextDrawImage which differ in the coordinate system with respect to UIImage. </p> <p>The code below uses UI* methods (no CGImageRef), and I have tested this with up/down/left/right oriented images, and it seems to work great. </p> <pre><code> // get sub image - (UIImage*) getSubImageFrom: (UIImage*) img WithRect: (CGRect) rect { UIGraphicsBeginImageContext(rect.size); CGContextRef context = UIGraphicsGetCurrentContext(); // translated rectangle for drawing sub image CGRect drawRect = CGRectMake(-rect.origin.x, -rect.origin.y, img.size.width, img.size.height); // clip to the bounds of the image context // not strictly necessary as it will get clipped anyway? CGContextClipToRect(context, CGRectMake(0, 0, rect.size.width, rect.size.height)); // draw image [img drawInRect:drawRect]; // grab image UIImage* subImage = UIGraphicsGetImageFromCurrentImageContext(); UIGraphicsEndImageContext(); return subImage; } </code> </pre>
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    1. POHow to crop the UIImage?
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    1. COJust be aware that you can only run the above code on the main thread. The advantage of avoiding `UIGraphicsBeginImageContext`, etc is to get around this limitation.
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      1. USWilliam Denniss
    2. CO@William Denniss are you sure? http://stackoverflow.com/questions/11528803/is-uigraphicsbeginimagecontext-thread-safe/11530675#11530675
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    3. CO@Klass, you're right, since iOS 4 you can call it on any thread. However, the code **is still not thread safe**, you may only use `UIGraphicsBeginImageContext` on one thread at a time. The docs state: "Creates a bitmap-based graphics context and makes it the current context." I believe this "current context" is global, and thus not thread safe. My app processes multiple images at once, and I had crashes when using `UIGraphicsBeginImageContext` in multiple threads, so I switched to using `CGContextRef`'s which are thread-safe.
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