Introduction to Game Physics with Box2D

7

The Cannon Game

The Cannon Game gives the player control of a cannon in a world with

a tempting tower of books. The player’s job is to knock down the tower

in 60 seconds or less by firing cannonballs at it. The temperature of the

cannon goes up every time it is fired, and it cools down slowly afterwards.

The current temperature of the cannon barrel and the maximum temper-

ature reached so far are indicated on a temperature gauge (as shown in

Figure 7.1). If the cannon barrel is overstressed by heating it above the

maximum allowable temperature, then it will explode the next time it is

fired. (See Figures 7.2–7.6.) The player controls the cannon using the

keyboard. Table 7.1 shows the keyboard bindings.

Although this 60-second minigame is very basic, it exhibits in rudi-

mentary form all of the required characteristics of a game, which are the

following:

1. It has a virtual world.

2. The world has stuff in it.

3. The player can interact with the stuff.

4. The player can win.

5. The player can lose.

The “virtual world” is the screen space, the “stuff” is the cannon and the

books, the “interaction” is firing the cannon and knocking down the books,

the player “wins” when the books are knocked down and “loses” when the

cannon overheats. What’s not to like?

161

162 7 • The Cannon Game

Figure 7.1

•

The temperature gauge.

Key Action

ESC Quit

Enter Restart

↑ Barrel up

↓ Barrel down

← Move left

→ Move right

Backspace Stop moving

Space Fire

Table 7.1

•

The keys used in the Cannon Game.

Figure 7.2

•

The start of the cannon game.

7.1 • The Platform and the Tower 163

Figure 7.3

•

The cannon game is underway, the barrel is starting to heat up.

Figure 7.4

•

The tower of books is toppled.

164 7 • The Cannon Game

Figure 7.5

•

The tower has been vanquished.

Figure 7.6

•

This is what happens when you let the cannon overheat.

7.1 • The Platform and the Tower 165

Figure 7.7

•

The ledge is 512 × 62 pixels.

• 7.1 The Platform and the Tower

I began by taking the Getting Started app from Section 5.4 and stripping

out the code in NonPlayerObjects.cpp. We’ll replace it with code to

create the cannon and the tower of books.

I want a platform for the cannon to sit on. In my favorite image editing

program,

1

I create a ledge that is half the width of the image in Physics

World units, and I see in Figure 7.7 that it is 62 pixels high, which is

RW2PW(62) in Physics World units. There’s no need to make it a sprite,

so I just paint it into the background image background.png,asshownin

Figure 7.8. This code gets added to function CreateWorldEdges to add the

corresponding edge to Physics World, as shown in Figure 7.9 (remembering

that it has already computed w, the world width in Physics World units):

const float lh = RW2PW(62);

shape. Set( b2Vec2(0, lh), b2Vec2(w/2, lh ));

edge - > CreateFixture (&shape , 0);

While we were a little cavalier about how we created the edge bodies for

the boundaries of our Physics World, it’s time we got a little more careful

about how we structure our code. We’re going to create the rest of the

physics bodies for our game in the following order:

1. Shape: b2Shape or any of the shapes derived from it, see Section 6.4.

2. Fixture: b2FixtureDef, see Section 6.5.2.

3. Body definition: b2BodyDef, see Section 6.5.3.

4. Body: b2Body, see Section 6.5.3.

...........................

1

I like to use either Gimp or paint.net, depending on the task. They are both Open

Source, and between them, I have all the functionality that a lowly programmer could

need, for free.