Any quick and dirty anti-aliasing techniques for a rotated UIImageView?

Question

I've got a UIImageView (full frame and rectangular) that i'm rotating with a CGAffineTransform. The UIImage of the UIImageView fills the entire frame. When the image is rotated and drawn the edges appear noticeably jagged. Is there anything I can do to make it look better? It's clearly not being anti-aliased with the background.

Answer 1

Remember to set the appropriate anti-alias options:

CGContextSetAllowsAntialiasing(theContext, true);
CGContextSetShouldAntialias(theContext, true);

Answer 2

The edges of CoreAnimation layers aren't antialiased by default on iOS. However, there is a key that you can set in Info.plist that enables antialiasing of the edges: UIViewEdgeAntialiasing.

https://developer.apple.com/library/content/documentation/General/Reference/InfoPlistKeyReference/Articles/iPhoneOSKeys.html

If you don't want the performance overhead of enabling this option, a work-around is to add a 1px transparent border around the edge of the image. This means that the 'edges' of the image are no longer on the edge, so don't need special treatment!

Answer 3

New API – iOS 6/7

Also works for iOS 6, as noted by @Chris, but wasn't made public until iOS 7.

Since iOS 7, CALayer has a new property allowsEdgeAntialiasing which does exactly what you want in this case, without incurring the overhead of enabling it for all views in your application! This is a property of CALayer, so to enable this for a UIView you use myView.layer.allowsEdgeAntialiasing = YES.

Answer 4

just add 1px transparent border to your image

CGRect imageRect = CGRectMake(0, 0, image.size.width, image.size.height);
UIGraphicsBeginImageContextWithOptions(imageRect.size, NO, 0.0);
[image drawInRect:CGRectMake(1,1,image.size.width-2,image.size.height-2)];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Answer 5

just add "Renders with edge antialiasing" with YES in plist and it will work.

Answer 6

I would totally recommend the following library.

http://vocaro.com/trevor/blog/2009/10/12/resize-a-uiimage-the-right-way/

It contains lots of useful extensions to UIImage that solve this problem and also include code for generating thumbnails etc.

Enjoy!

Answer 7

The best way I've found to have smooth edges and a sharp image is to do this:

CGRect imageRect = CGRectMake(0, 0, self.photo.image.size.width, self.photo.image.size.height);
UIGraphicsBeginImageContextWithOptions(imageRect.size, NO, 0.0);
[self.photo.image drawInRect:CGRectMake(1, 1, self.photo.image.size.width - 2, self.photo.image.size.height - 2)];
self.photo.image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

Adding the Info.plist key like some people describe has a big hit on performance and if you use that then you're basically applying it to everything instead of just the one place you need it.

Also, don't just use UIGraphicsBeginImageContext(imageRect.size); otherwise the layer will be blurry. You have to use UIGraphicsBeginImageContextWithOptions like I've shown.

Answer 8

I found this solution from here, and it's perfect:

+ (UIImage *)renderImageFromView:(UIView *)view withRect:(CGRect)frame transparentInsets:(UIEdgeInsets)insets {
    CGSize imageSizeWithBorder = CGSizeMake(frame.size.width + insets.left + insets.right, frame.size.height + insets.top + insets.bottom);
    // Create a new context of the desired size to render the image
    UIGraphicsBeginImageContextWithOptions(imageSizeWithBorder, NO, 0);
    CGContextRef context = UIGraphicsGetCurrentContext();

    // Clip the context to the portion of the view we will draw
    CGContextClipToRect(context, (CGRect){{insets.left, insets.top}, frame.size});
    // Translate it, to the desired position
    CGContextTranslateCTM(context, -frame.origin.x + insets.left, -frame.origin.y + insets.top);

    // Render the view as image
    [view.layer renderInContext:UIGraphicsGetCurrentContext()];

    // Fetch the image
    UIImage *renderedImage = UIGraphicsGetImageFromCurrentImageContext();

    // Cleanup
    UIGraphicsEndImageContext();

    return renderedImage;
}

usage:

UIImage *image = [UIImage renderImageFromView:view withRect:view.bounds transparentInsets:UIEdgeInsetsZero];