UIImage Rotation Offset

Question

I have the following code

- (void)ComputeRotationWithRadians:(CGFloat)rad {
    CGFloat width = exportImage.size.width;
    CGFloat height = exportImage.size.height;
    UIGraphicsBeginImageContext(CGSizeMake(width, height));
    CGContextRef ctxt = UIGraphicsGetCurrentContext();
    CGContextScaleCTM(ctxt, 1.0, -1.0);
    CGContextTranslateCTM(ctxt, 0.0, -exportImage.size.height);
    CGContextRotateCTM(ctxt, rad);
    CGContextDrawImage(UIGraphicsGetCurrentContext(), CGRectMake(0, 0, width, height), exportImage.CGImage);
    exportImage = UIGraphicsGetImageFromCurrentImageContext();
    UIGraphicsEndImageContext();
    [imageView setImage:exportImage];
}

The issue is that the rotation happens around the bottom left corner, when it has been calculated around the center. Is there anyway to change this?

Answer 1

Your code is translating the matrix before rotating it. Try flipping the calls to CGContextConcatCTM and CGContextTranslateCTM so you rotate first. Or perhaps you are looking for something else?