Chapter 10

3D Graphics in OpenGL

Two-dimensional graphics are great for most programs, but sometimes you need an extra level of depth, interactivity, or realism that isn’t possible in 2D. For these times, Android provides a three-dimensional graphics library based on the OpenGL ES standard. In this chapter, we’ll explore 3D concepts and build up a sample program that uses OpenGL.

10.1Understanding 3D Graphics

The world is three-dimensional, yet we routinely view it in two dimensions. When you watch television or look at a picture in a book, the 3D images are flattened out, or projected, onto a 2D surface (the TV panel or book page).

Try this simple experiment: cover one eye and look out the window. What do you see? Light from the sun bounces off objects outside, passes through the window, and travels to your eye so you can perceive it. In graphics terms, the scene outside is projected onto the window (or viewport). If someone replaced your window with a high-quality photograph, it would look the same until you moved.

Based on how close your eye is to the window and how big the window is, you can see a limited amount of the world outside. This is called your field of view. If you draw a line from your eye to the four corners of the window and beyond, you would get the pyramid in Figure 10.1, on the next page. This is called the view frustum (Latin for a “piece broken off”). For performance reasons, the frustum is usually bounded by near and far clipping planes as well. You can see everything inside the frustum but nothing outside of it.

INTRODUCING OPENGL 199

Figure 10.1: Viewing a three-dimensional scene

In 3D computer graphics, your computer screen acts as the viewport. Your job is to fool the user into thinking it’s a window into another world just on the other side of the glass. The OpenGL graphics library is the API you use to accomplish that.

10.2Introducing OpenGL

OpenGL1 was developed by Silicon Graphics in 1992. It provides a unified interface for programmers to take advantage of hardware from any manufacturer. At its core, OpenGL implements familiar concepts such as viewports and lighting and tries to hide most of the hardware layer from the developer.

Because it was designed for workstations, OpenGL is too large to fit on a mobile device. So, Android implements a subset of OpenGL called OpenGL for Embedded Systems (OpenGL ES).2 This standard was cre-

1. http://www.opengl.org

2.http://www.khronos.org/opengles

BUILDING AN OPENGL PROGRAM 201

Joe Asks. . .

Will Every Phone Have 3D?

Yes and no. Some low-end devices running Android may not actually have 3D hardware. However, the OpenGL programming interface will still be there. All the 3D functions will be emulated in software. Your program will still run, but it will be much slower than a hardware-accelerated device. For this reason, it’s a good idea to provide options for users to turn off certain details and special effects that take time to draw but aren’t absolutely necessary for the program. That way, if the user is running your program on a slower device, they can disable some of your eye candy to get better performance.

This will create OpenGL.java to contain your main activity. Edit this, and change it to refer to a custom view named GLView, as shown here:

Download OpenGL/src/org/example/opengl/OpenGL.java

package org.example.opengl;

import android.app.Activity; import android.os.Bundle;

public class OpenGL extends Activity { GLView view;

@Override

public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState);

view = new GLView(this); setContentView(view);

}

@Override

protected void onPause() { super.onPause(); view.onPause();

}

@Override

protected void onResume() { super.onResume(); view.onResume();

}

}