Question

When overriding drawRect I’ve found that the coordinates there use 0,0 as the upper left.

But the Apple UIView Programming Guide says this:

Some iOS technologies define default coordinate systems whose origin point and orientation differ from those used by UIKit. For example, Core Graphics and OpenGL ES use a coordinate system whose origin lies in the lower-left corner of the view or window and whose y-axis points upward relative to the screen.

I’m confused; are they talking about something different than Quartz when they refer to Core Graphics here?

Answer 1

“Core Graphics” in this documentation means “Quartz”, yes. It’s just an oversimplification.

When you create a CGContext yourself, its coordinate system has the origin in the bottom-left. When UIKit creates the CGContext for drawing into a view, it helpfully flips the coordinate system before calling -drawRect:.