Android 9 Development Cookbook - Third Edition

Drawing shapes on GLSurfaceView

The previous recipe set up the activity to use OpenGL. This recipe will continue by showing how to draw on OpenGLSurfaceView.

First, we need to define the shape. With OpenGL, it is important to realize that the order in which the vertices of a shape are defined is very important, as they determine the front (face) and back of the shape. It's customary (and the default behavior) to define vertices counterclockwise. (Although this behavior can be changed, it requires additional code and is not standard practice.)

It's also important to understand the OpenGL screen coordinate system, as it differs from the Android canvas. The default coordinate system defines (0,0,0) as the center of the screen. The four edge ...