2D Game Programming Tricks - Beginning Android Games - page 369

direction the cannon is pointing. Also, we set the speed simply by multiplying the velocity by the
distance between the touch point and the cannon. The closer the touch point to the cannon,
the more slowly the cannonball will fly.
Sounds easy enough, so now we can try implementing it. Copy over the code from the
CannonTest.java file to a new file, called CannonGravityTest.java . Rename the classes
contained in that file to CannonGravityTest and CannonGravityScreen . Listing 8-3 shows the
CannonGravityScreen class, with some comments added for clarity.
Listing 8-3. Excerpt from CannonGravityTest
class CannonGravityScreen extends Screen {
float FRUSTUM_WIDTH = 9.6f;
float FRUSTUM_HEIGHT = 6.4f;
GLGraphics glGraphics;
Vertices cannonVertices;
Vertices ballVertices;
Vector2 cannonPos = new Vector2();
float cannonAngle = 0;
Vector2 touchPos = new Vector2();
Vector2 ballPos = new Vector2(0,0);
Vector2 ballVelocity = new Vector2(0,0);
Vector2 gravity = new Vector2(0,-10);
Not a lot has changed. We simply doubled the size of the view frustum, and reflected that by
setting FRUSTUM_WIDTH and FRUSTUM_HEIGHT to 9.6 and 6.2, respectively. This means that we
can see a rectangle of 9.2×6.2 m of the world. Since we also want to draw the cannonball,
we add another Vertices instance, called ballVertices , which will hold the four vertices and
six indices of the rectangle of the cannonball. The new members ballPos and ballVelocity
store the position and velocity of the cannonball, and the member gravity is the gravitational
acceleration, which will stay at a constant (0,-10) m/s 2 over the lifetime of our program.
public CannonGravityScreen(Game game) {
super (game);
glGraphics = ((GLGame) game).getGLGraphics();
cannonVertices = new Vertices(glGraphics, 3, 0, false , false );
cannonVertices.setVertices( new float [] { -0.5f, -0.5f,
0.5f, 0.0f,
-0.5f, 0.5f }, 0, 6);
ballVertices = new Vertices(glGraphics, 4, 6, false , false );
ballVertices.setVertices( new float [] { -0.1f, -0.1f,
0.1f, -0.1f,
0.1f, 0.1f,
-0.1f, 0.1f }, 0, 8);
ballVertices.setIndices( new short [] {0, 1, 2, 2, 3, 0}, 0, 6);
}
In the constructor, we simply create the additional Vertices instance for the rectangle of the
cannonball. We define it in model space with the vertices (-0.1,-0.1), (0.1,-0.1), (0.1,0.1), and
(-0.1,0.1). We use indexed drawing, and thus specify six vertices in this case.