Cocoa NSPoint to Quartz NSPoint - Flip Y cordinate

Question

In macOS programming, We know that

• Quartz uses a coordinate space where the origin (0, 0) is at the top-left of the primary display. Increasing y goes down.
• Cocoa uses a coordinate space where the origin (0, 0) is the bottom-left of the primary display and increasing y goes up.

Now am using a Quartz API - CGImageCreateWithImageInRect to crop an image , which takes a rectangle as a param. The rect has the Y origin coming from Cocoa's mousedown events.

Thus i get crops at inverted locations...

I tried this code to flip my Y co-ordinate in my cropRect

 //Get the point in MouseDragged event
 NSPoint currentPoint = [self.view convertPoint:[theEvent locationInWindow] fromView:nil];
 CGRect nsRect = CGRectMake(currentPoint.x , currentPoint.y,   
                                  circleSizeW,  circleSizeH);
 //Now Flip the Y please!
 CGFloat flippedY = self.imageView.frame.size.height - NSMaxY(nsRectFlippedY);
 CGRect cropRect = CGRectMake(currentPoint.x, flippedY, circleSizeW, circleSizeH);

But for the areas on the top, i wrong FlippedY coordinates. If i click near top edge of the view, i get flippedY = 510 to 515 At the top edge it should be between 0 to 10 :-|

Can someone point me to the correct and reliable way to Flip the Y coordinate in such circumstances? Thank you!

Here is sample project in GitHub highlighting the issue https://github.com/kamleshgk/SampleMacOSApp

Answer 1

As Charles mentioned, the Core Graphics API you are using requires coordinates relative to the image (not the screen). The important thing is to convert the event location from window coordinates to the view which most closely corresponds to the image's location and then flip it relative to that same view's bounds (not frame). So:

NSView *relevantView = /* only you know which view */;
NSPoint currentPoint = [relevantView convertPoint:[theEvent locationInWindow] fromView:nil];
// currentPoint is in Cocoa's y-up coordinate system, relative to relevantView, which hopefully corresponds to your image's location

currentPoint.y = NSMaxY(relevantView.bounds) - currentPoint.y;
// currentPoint is now flipped to be in Quartz's y-down coordinate system, still relative to relevantView/your image

Answer 2

The rect you pass to CGImageCreateWithImageInRect should be in coordinates relative to the input image's size, not screen coordinates. Assuming the size of the input image matches the size of the view to which you've converted your point, you should be able to achieve this by subtracting the rect's corner from the image's height, rather than the screen height.