The antialias of rotated CGImage/CGlayer seems jaggy, UIImageView's is not

I need to mask a "texture" image with a rotated greyscale image.

I found out, that I have to do it with CGImages or CGlayers (if there is a simplier way using UIImageViews only, please let me know about it).

My problem is simple:

The antialias of any rotation-transformed CG stuff is quiet jaggy...

... but the antialias of a rotation-transformed UIImageView is kinda perfect. How can I produce that beautiful ant开发者_JAVA百科ialiased rotations?

I've uploaded a "proof" involving actual iPhone Simulator screenshots, to see what am I talkin' about: http://gotoandplay.freeblog.hu/files/Proof.png

I've tried to use CGImages, CGLayers, UIImageViews "captured" with renderInContext, I've tried to CGContextSetInterpolationQuality to high, and also tried to set CGContextSetAllowsAntialiasing - CGContextSetShouldAntialias, but every case returned the same jaggy result.

I'm planning to learn using OpenGL next year, but this development should released using CoreGraphics only. Please let me know how to get a perfectly rendered rotated image, I just can't accept it's impossible.

---

To add 1px transparent border to your image use this

CGRect imageRect = CGRectMake(0, 0, image.size.width, image.size.height);
UIGraphicsBeginImageContext( imageRect.size ); 
[image drawInRect:CGRectMake(1,1,image.size.width-2,image.size.height-2)];
image = UIGraphicsGetImageFromCurrentImageContext();
UIGraphicsEndImageContext();

---

Have you tried adding a clear, 1 pixel border around your image? I've heard of that recommended as a trick to avoid aliasing, by giving CoreGraphics some room to work with when blending the edges.

---

I am having a similar problem, looks like I'm going to move it over to OpenGL ES as well. I can't nail down an effective solution that doesn't hurt performance.

For reference of future CoreGraphics explorers, putting a 1-pixel transparent border did make for a noticeable improvement in my experiments, but it appears that as Eonil mentioned, you end up with multiple stages of antialiasing/smoothing/interpolation working against each other. IE: CGLayer does some interpolation for it's rotation, then context it's being drawn to will do some interpolation/antialiasing, so on so forth until it ends up looking pretty rough.

I actually ended up with better results by disabling interpolation and antialiasing on the destination context, though it was still obviously jaggy (less artifacts overall though). I was able to achieve the best overall appearance by enabling interpolation and antialiasing when constructing the CGLayer, and disabling it for the destination context when re-drawing it. This approach, obviously, is fraught with other problems.