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    <p>It would ultimately be faster, with a lot less image creation from sprite atlases, if you could set not only the image for a UIImageView, but also the top-left offset to display within that UIImage. Maybe this is possible. It would certainly eliminate a lot of effort!</p> <p>Meanwhile, I created these useful functions in a utility class that I use in my apps. It creates a UIImage from part of another UIImage, with options to rotate, scale, and flip using standard UIImageOrientation values to specify. The pixel scaling is preserved from the original image.</p> <p>My app creates a lot of UIImages during initialization, and this necessarily takes time. But some images aren't needed until a certain tab is selected. To give the appearance of quicker load I could create them in a separate thread spawned at startup, then just wait till it's done when that tab is selected.</p> <p>This code is also posted at <a href="https://stackoverflow.com/questions/8035673/most-efficient-way-to-draw-part-of-an-image-in-ios/12558366#12558366">Most efficient way to draw part of an image in iOS</a></p> <pre><code>+ (UIImage*)imageByCropping:(UIImage *)imageToCrop toRect:(CGRect)aperture { return [ChordCalcController imageByCropping:imageToCrop toRect:aperture withOrientation:UIImageOrientationUp]; } // Draw a full image into a crop-sized area and offset to produce a cropped, rotated image + (UIImage*)imageByCropping:(UIImage *)imageToCrop toRect:(CGRect)aperture withOrientation:(UIImageOrientation)orientation { // convert y coordinate to origin bottom-left CGFloat orgY = aperture.origin.y + aperture.size.height - imageToCrop.size.height, orgX = -aperture.origin.x, scaleX = 1.0, scaleY = 1.0, rot = 0.0; CGSize size; switch (orientation) { case UIImageOrientationRight: case UIImageOrientationRightMirrored: case UIImageOrientationLeft: case UIImageOrientationLeftMirrored: size = CGSizeMake(aperture.size.height, aperture.size.width); break; case UIImageOrientationDown: case UIImageOrientationDownMirrored: case UIImageOrientationUp: case UIImageOrientationUpMirrored: size = aperture.size; break; default: assert(NO); return nil; } switch (orientation) { case UIImageOrientationRight: rot = 1.0 * M_PI / 2.0; orgY -= aperture.size.height; break; case UIImageOrientationRightMirrored: rot = 1.0 * M_PI / 2.0; scaleY = -1.0; break; case UIImageOrientationDown: scaleX = scaleY = -1.0; orgX -= aperture.size.width; orgY -= aperture.size.height; break; case UIImageOrientationDownMirrored: orgY -= aperture.size.height; scaleY = -1.0; break; case UIImageOrientationLeft: rot = 3.0 * M_PI / 2.0; orgX -= aperture.size.height; break; case UIImageOrientationLeftMirrored: rot = 3.0 * M_PI / 2.0; orgY -= aperture.size.height; orgX -= aperture.size.width; scaleY = -1.0; break; case UIImageOrientationUp: break; case UIImageOrientationUpMirrored: orgX -= aperture.size.width; scaleX = -1.0; break; } // set the draw rect to pan the image to the right spot CGRect drawRect = CGRectMake(orgX, orgY, imageToCrop.size.width, imageToCrop.size.height); // create a context for the new image UIGraphicsBeginImageContextWithOptions(size, NO, imageToCrop.scale); CGContextRef gc = UIGraphicsGetCurrentContext(); // apply rotation and scaling CGContextRotateCTM(gc, rot); CGContextScaleCTM(gc, scaleX, scaleY); // draw the image to our clipped context using the offset rect CGContextDrawImage(gc, drawRect, imageToCrop.CGImage); // pull the image from our cropped context UIImage *cropped = UIGraphicsGetImageFromCurrentImageContext(); // pop the context to get back to the default UIGraphicsEndImageContext(); // Note: this is autoreleased return cropped; } </code></pre>
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    1. POHow to crop the UIImage?
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    1. USScott Lahteine
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    1. COThanks. What should be the Orientation that I send for the image if I took it from AVFoundation camera which rotates the images?
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