NSImage become blurry after transformation - cocoa

Question

I rotated a NSImage at its center with the NSAffineTransform class and it works.
The code is here:
- (NSImage*)rotateImage: (NSImage*)myImage angle:(float)rotateAngle {
NSRect imageBounds = {NSZeroPoint, [myImage size]};
NSRect transformedImageBounds = imageBounds;
NSBezierPath* boundsPath = [NSBezierPath bezierPathWithRect:imageBounds];
NSAffineTransform* transform = [NSAffineTransform transform];
// calculate the bounds for the rotated image
if (rotateAngle != 0) {
NSAffineTransform* temptransform = [NSAffineTransform transform];
[temptransform rotateByDegrees:rotateAngle];
[boundsPath transformUsingAffineTransform:temptransform];
NSRect rotatedBounds = {NSZeroPoint, [boundsPath bounds].size};
// center the image within the rotated bounds
imageBounds.origin.x += (NSWidth(rotatedBounds) - NSWidth (imageBounds))/2;
imageBounds.origin.y += (NSHeight(rotatedBounds) - NSHeight (imageBounds))/2;
// set up the rotation transform
[transform translateXBy:+(NSWidth(rotatedBounds) / 2) yBy:+ (NSHeight(rotatedBounds) / 2)];
[transform rotateByDegrees:rotateAngle];
[transform translateXBy:-(NSWidth(rotatedBounds) / 2) yBy:- (NSHeight(rotatedBounds) / 2)];
transformedImageBounds = rotatedBounds;
}
// draw the original image into the new image
NSImage* transformedImage = [[NSImage alloc] initWithSize:transformedImageBounds.size];;
[transformedImage lockFocus];
[transform concat];
[myImage drawInRect:imageBounds fromRect:NSZeroRect operation:NSCompositeCopy fraction:1.0] ;
[transformedImage unlockFocus];
return transformedImage;
}
But the quality of the resulted image is very bad.
How to solve it?
Some say the "lockFocus" methods caused this but I hv no idea of how to do this without using the method.

Answer 1

I'm not sure what it's place is in this pipeline, but if you can get hold of the NSGraphicsContext (which might be as easy as:
NSGraphicsContext *myContext =
[NSGraphicsContext currentContext];
after your lockFocus), you can try
[myContext setImageInterpolation:NSImageInterpolationHigh]
to ask Cocoa to use its best algorithms for scaling the image. If you switch to using the CGImageRef API, I know you can get more control over the interpolation setting pretty easily.