How to get a rotated, zoomed and panned image from an UIImageView at its full resolution?

Question

I have an UIImageView which can be rotated, panned and scaled with gesture recognisers. As a result it is cropped in its enclosing view. Everything is worki

Answer 1

The following code creates a snapshot of the enclosing view (superview of faceImageView with clipsToBounds set to YES) using a calculated scale factor.

It assumes that the content mode of faceImageView is UIViewContentModeScaleAspectFit and that the frame of faceImageView is set to the enclosingView's bounds.

- (UIImage *)captureView {

    float imageScale = sqrtf(powf(faceImageView.transform.a, 2.f) + powf(faceImageView.transform.c, 2.f));    
    CGFloat widthScale = faceImageView.bounds.size.width / faceImageView.image.size.width;
    CGFloat heightScale = faceImageView.bounds.size.height / faceImageView.image.size.height;
    float contentScale = MIN(widthScale, heightScale);
    float effectiveScale = imageScale * contentScale;

    CGSize captureSize = CGSizeMake(enclosingView.bounds.size.width / effectiveScale, enclosingView.bounds.size.height / effectiveScale);

    NSLog(@"effectiveScale = %0.2f, captureSize = %@", effectiveScale, NSStringFromCGSize(captureSize));

    UIGraphicsBeginImageContextWithOptions(captureSize, YES, 0.0);        
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextScaleCTM(context, 1/effectiveScale, 1/effectiveScale);
    [enclosingView.layer renderInContext:context];   
    UIImage *img = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();

    return img;
}

Depending on the current transform the resulting image will have a different size. For example when you zoom in, the size gets smaller. You can also set effectiveScale to a constant value in order to get an image with a constant size.

Your gesture recognizer code does not limit the scale factor, i.e. you can zoom out/in without being limited. That can be very dangerous! My capture method can output really large images when you've zoomed out very much.

If you have zoomed out the background of the captured image will be black. If you want it to be transparent, you must set the opaque-parameter of UIGraphicsBeginImageContextWithOptions to NO.