contentScaleFactor

The scale factor determines how content is mapped from the logical coordinate space (measured in points) to the device coordinate space (measured in pixels). This value is typically either 1.0 or 2.0. Higher scale factors indicate that each point is represented by more than one pixel on the screen. For example, if the scale factor is 2.0 and the drawing rectangle size is 50 x 50 points, the size of the underlying area is 100 x 100 pixels.

When drawing the content for your overlays, you can use this value to determine how best to render your content.