CGContext drawing rotated around arbitrary point - objective-c

Question

I have a really annoying issue trying to draw into a bitmap CGContext. What I am trying to do is I have a couple of images to draw into the full size of the image. One can come in at any UIImageOrientation and I've written the code to correctly rotate that properly, but I'm struggling with the second bit which is trying to draw another view at an arbitrary rotation about its centre.
The other view comprises an image drawn possibly outside of its bounds. What I am having a problem with is drawing these at a rotated angle as though it was a UIView that had an affine transform applied to it. e.g. imagine a UIView at {100, 300} of size {20, 20} and an affine transform rotating it by 45 degrees. It would be rotated about {110, 310}.
What I have tried is this:
- (void)drawOtherViewInContext:(CGContextRef)context atRect:(CGRect)rect withRotation:(CGFloat)rotation contextSize:(CGSize)contextSize {
CGRect thisFrame = <SOLVED_FEATURE_FRAME_RELATIVE_TO_RECT_SIZE>;
thisFrame.origin.y = contextSize.height - thisFrame.origin.y - thisFrame.size.height;
CGRect rotatedRect = CGRectApplyAffineTransform(CGRectMake(0.0f, 0.0f, rect.size.width, rect.size.height), CGAffineTransformMakeRotation(-rotation));
CGAffineTransform transform = CGAffineTransformIdentity;
transform = CGAffineTransformTranslate(transform, rect.origin.x, contextSize.height - rect.origin.y - rect.size.height);
transform = CGAffineTransformTranslate(transform,
+(rotatedRect.size.width/2.0f),
+(rotatedRect.size.height/2.0f));
transform = CGAffineTransformRotate(transform, -rotation);
transform = CGAffineTransformTranslate(transform,
-(rect.size.width/2.0f),
-(rect.size.height/2.0f));
CGContextConcatCTM(context, transform);
CGContextDrawImage(context, thisFrame, theCGImageToDraw);
CGContextConcatCTM(context, CGAffineTransformInvert(transform));
}
So what I am doing there, I think, is this:
Translate to the bottom left of rect which is where this view is meant to be drawn.
Translate by half the rotated size in x and y.
Rotate by the required angle.
Translate back half the original size in x and y.
I thought that this would be what I wanted to do because the first step translates the coordinate system to be such that thisFrame is drawn correctly relative to where we're being told to draw (by the rect method parameter). Then it's a pretty normal rotate about the centre of a rectangle.
The problem is that when rotated by say 45 degrees, the image is drawn slightly out of place. It's almost correct, but just not quite. When at 0, 90, 180 or 270 degrees then the position is pretty much spot on, maybe a few pixels out but when at 45, 135, 225, 315 degrees the position is too far up and to the right.
Can anyone see what I'm doing wrong here?
Update:
Silly me, it's bigger because I was passing in the wrong rect! Edited to get rid of references to it being the wrong size. It's still not quite in the right place though.

Answer 1

OK I have fixed it. The first point was that I was passing in the wrong rect at first as I was grabbing the frame from a UIView which had an affine transform applied to it, and as we all know the frame in that case is undefined. More likely it's the CGRect that comes from CGRectApplyAffineTransform(bounds, transform) but anyway, I fixed that one.
Then the main problem of drawing offset was fixed by changing my transform to this:
CGAffineTransform transform = CGAffineTransformIdentity;
transform = CGAffineTransformTranslate(transform, rect.origin.x, contextSize.height - rect.origin.y - rect.size.height);
transform = CGAffineTransformTranslate(transform,
+(rect.size.width/2.0f),
+(rect.size.height/2.0f));
transform = CGAffineTransformRotate(transform, -rotation);
transform = CGAffineTransformTranslate(transform,
-(rect.size.width/2.0f),
-(rect.size.height/2.0f));
That's what I had originally thought I should be doing, but for some reason I changed it to use the rotated CGRect.