Improvement to Trevor Harmon’s UIImage+Resize.m

from the “Resize a UIImage the right way” post on Trevor’s Bike Shed

Trevor says:

December 24, 2009 at 9:18 pm

True; I didn’t bother with handling the imageRotation setting in croppedImage. I changed the method to include a source code comment explaining this.

If anyone would like to contribute an improved croppedImage function that fixes this oversight, I’d be happy to include it in the distribution.

I found a good post on Robert Clark’s Niftybean blog, “Selecting regions from rotated EXIF images on iPhone” with code for adapting the a crop rect based on a UIImage’s imageRotation. Adding this to croppedImage I then only needed to finally rotate the resulting CGImageRef, for I only needed to slightly generalize the private resizeImage:transform:… helper method and then use it to rotate the resulting cropped image to match imageRotation. It would be much better if the original UIImage’s metadata, imageRotation included, could be retained so the final rotation step could be avoided.

// Returns a copy of this image that is cropped to the given bounds.
// The bounds will be adjusted using CGRectIntegral.
// JPMH-This method no long ignores the image's imageOrientation setting.
- (UIImage *)croppedImage:(CGRect)bounds {
    CGAffineTransform txTranslate;
    CGAffineTransform txCompound;
    CGRect adjustedBounds;
    BOOL drawTransposed;
    
    switch (self.imageOrientation) {
        case UIImageOrientationDown:
        case UIImageOrientationDownMirrored:
            txTranslate = CGAffineTransformMakeTranslation(self.size.width, self.size.height);
            txCompound = CGAffineTransformRotate(txTranslate, M_PI);
            adjustedBounds = CGRectApplyAffineTransform(bounds, txCompound);
            drawTransposed = NO;
            break;
        case UIImageOrientationLeft:
        case UIImageOrientationLeftMirrored:
            txTranslate = CGAffineTransformMakeTranslation(self.size.height, 0.0);
            txCompound = CGAffineTransformRotate(txTranslate, M_PI_2);
            adjustedBounds = CGRectApplyAffineTransform(bounds, txCompound);
            drawTransposed = YES;
            break;
        case UIImageOrientationRight:
        case UIImageOrientationRightMirrored:
            txTranslate = CGAffineTransformMakeTranslation(0.0, self.size.width);
            txCompound = CGAffineTransformRotate(txTranslate, M_PI + M_PI_2);
            adjustedBounds = CGRectApplyAffineTransform(bounds, txCompound);
            drawTransposed = YES;
            break;
        default:
            adjustedBounds = bounds;
            drawTransposed = NO;
    }
    
    CGImageRef imageRef = CGImageCreateWithImageInRect([self CGImage], adjustedBounds);
    UIImage *croppedImage;
    if (CGRectEqualToRect(adjustedBounds, bounds))
        croppedImage = [UIImage imageWithCGImage:imageRef];
    else
        croppedImage = [self resizedImage:imageRef
                                     size:bounds.size
                                transform:[self transformForOrientation:bounds.size]
                           drawTransposed:drawTransposed
                     interpolationQuality:kCGInterpolationHigh];
    CGImageRelease(imageRef);
    return croppedImage;
}

Where the resizeImage: utility method was change to take a CGImageRef as a parameter instead of always using self.CGImage:

- (UIImage *)resizedImage:(CGImageRef)imageRef
                     size:(CGSize)newSize
                transform:(CGAffineTransform)transform
           drawTransposed:(BOOL)transpose
     interpolationQuality:(CGInterpolationQuality)quality;
- (CGAffineTransform)transformForOrientation:(CGSize)newSize;

---