How to compensate the flipped coordinate system of core graphics for easy drawing?

Problem: Origin is at lower-left corner; positive y goes upward (negative y goes downward).
Goal: Origin at upper-left corner; positive y going downward (negative y going upward).


  1. Move origin up by the view's height.
  2. Negate (multiply by -1) the y axis.

The way to do this in code is to translate up by the view bounds' height and scale by (1, -1), in that order.

There are a couple of portions of the Quartz 2D Programming Guide that are relevant to this topic, including “Drawing to a Graphics Context on iPhone OS” and the whole chapter on Transforms. Of course, you really should read the whole thing.

You can do that by apply affinetransform on the point you want to convert in UIKit related coordinates. Following is example.

// Create a affine transform object
CGAffineTransform transform = CGAffineTransformMakeScale(1, -1);
// First translate your image View according to transform
transform = CGAffineTransformTranslate(transform, 0, - imageView.bounds.size.height);
// Then whenever you want any point according to UIKit related coordinates apply this transformation on the point or rect.
// To get tranformed point
CGPoint newPointForUIKit = CGPointApplyAffineTransform(oldPointInCGKit, transform);
// To get transformed rect
CGRect newRectForUIKit = CGRectApplyAffineTransform(oldPointInCGKit, transform);

The better answer to this problem is to use the UIImage method drawInRect: to draw your image. I'm assuming you want the image to span the entire bounds of your view. This is what you'd type in your drawRect: method.

Instead of:

CGContextRef ctx = UIGraphicsGetCurrentContext();  
UIImage *myImage = [UIImage imageNamed:@"theImage.png"];  
CGImageRef img = [myImage CGImage];
CGRect bounds = [self bounds];
CGContextDrawImage(ctx, bounds, img);

Write this:

UIImage *myImage = [UIImage imageNamed:@"theImage.png"];
CGRect bounds = [self bounds];
[myImage drawInRect:bounds];