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Coordinate Systems and Transforms

Coordinate spaces simplify the drawing code required to create complex interfaces. In a standard Mac OS X application, the window represents the base coordinate system for drawing, and all content must eventually be specified in that coordinate space when it is sent to the window server. For even simple interfaces, however, it is rarely convenient to specify coordinates relative to the window origin. Even the location of fixed items can change and require recalculation when the window resizes. This is where Cocoa makes things simple.

Each Cocoa view you add to a window maintains its own local coordinate system for drawing. Rather than convert coordinate values to window coordinates, you simply draw using the local coordinate system, ignoring any changes to the position of the view. Before sending your drawing commands to the window server, Cocoa automatically corrects coordinate values and puts them in the base coordinate space.

Even with the presence of local coordinate spaces, it is often necessary to change the coordinate space temporarily to affect certain behaviors. Changing the coordinate space is done using mathematical transformations (also known as transforms). Transforms convert coordinate values from one coordinate space to another. You can use transforms to alter the coordinate system of a view in a way that affects subsequent rendering calls, or you can use them to determine the location of points in the window or another view.

The following sections provide information about how Cocoa manages the local coordinate systems of your views and how you can use transforms to affect your drawing environment.

Contents:

Coordinate Systems Basics
Transform Basics
Using Transforms in Your Code
Flipped Coordinate Systems
Doing Pixel-Exact Drawing

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