Rocket Science: Acceleration

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Building the Game

The Black Hole game is based on most of the code we’ve developed in each of the previous hours of the book, and there really is no single previous hour that gets more credit than others since each hour has built upon the hours that came before. Let’s dig into the game by going over the major sections and get something up and running fairly quickly. Then we’ll continue our work on the game into the following hour, where it will get some polish and fine-tuning of the fun factor.

This game is based on the theories of Stephen Hawking. If you’re interested in black hole physics, be sure to read his popular books for more information! The Universe in a Nutshell is one of my favorites.

Gravity Well Regions

There are three regions around the black hole that affect game objects. The outer gravity well affects objects passing by, drawing them toward the black hole with relatively light force. This force is increased by an equal amount in the next two inner regions, with each region generating an equivalent gravity “tug” on objects. But the cumulative effect of all three gravity wells in the inner region of the black hole will cause objects to become hopelessly trapped.

The third and innermost region might be thought of as the event horizon, that region of a black hole where things disappear into the void, never to be seen again. It is this region that mathematics cannot penetrate, so while it appears that gravity increases toward infinity in the middle of a black hole, the truth of the matter is, there may be nothing at the very center of a black hole! The gravity might be so powerful that matter just rotates around the center of mass and no matter actually exists at that center point, which would be quite small at the center of a black hole. Then again, there might be a superdense material like a neutron star. It is at this point that physics just cannot explain it, because we don’t actually have a black hole nearby to study. Even if we did, it’s not like a spacecraft could be sent to investigate!

Figure 23.1 shows an illustration of the gravity well of the black hole in the game. The outer gravity well is quite large and draws most game objects inward at a steady “weight” or “pull,” with some help from the inner core of the black hole, also exerting force. The inner gravity well is the region where energy can be mined by the “Hawking” satellites. At any rate, that’s one of the enjoyable aspects of this game, pondering the infinite possibilities!

The code to simulate the gravitational pull of the black hole is coming up. Now let’s just take a look again at some of our earlier helper methods used in this game. All the collision code in the Black Hole game is based around this RadialCollision() method and its overloaded friend:

public bool RadialCollision(Sprite A, Sprite B)
float radius1 = A.image.Width / 2;
float radius2 = B.image.Width / 2;
return RadialCollision(A.position, B.position,
radius1, radius2);
public bool RadialCollision(Vector2 A, Vector2 B, float radius1,
float radius2)
float dist = Distance(A, B);
return (dist < radius1 + radius2);
public float Distance(Vector2 A, Vector2 B)
double diffX = A.X – B.X;
double diffY = A.Y – B.Y;
double dist = Math.Sqrt(Math.Pow(diffX, 2) +
Math.Pow(diffY, 2));
return (float)dist;

The gravity well of the black hole covers most of the Windows Phone screen.
FIGURE 23.1 The gravity well of the black hole covers most of the Windows Phone screen.

Enhancing MassiveObject

Some minor changes need to be made to MassiveObject to support some new features needed in the game that were not in the example in the preceding hour. Following is what the class now looks like, with the new variables and updated constructor:

class MassiveObject : Sprite
public string name;
public bool captured;
public double mass;
public Vector2 acceleration;
public float radius, angle;
public int lifetime, startTime;
public MassiveObject(ContentManager content,
SpriteBatch spriteBatch)
: base(content, spriteBatch)
name = “object”;
mass = 1.0f;
acceleration = Vector2.Zero;
radius = 50.0f;
angle = 0.0f;
captured = false;
lifetime = 0;
startTime = 0;
// . . . note: some code omitted here


There are no changes to be made to Game1.cs, because the main source code file is now PlayingModule.cs. In the final hour coming up, we will again use the game state modules for a more polished gameplay experience.

Gameplay Source Code

The most significant code of the game is found in PlayingModule.cs. If you skipped ahead, you may have missed Hour 21, “Finite State Gameplay,” which explained how to use states to improve a game in many ways. The PlayingModule class is the primary gameplay class where the bulk of the game code will be found. The first lines of code in the class declare all the variables, including the key objects variable, defined as a List of MassiveObjects. We also see the black hole, the super core gravity well, and the player’s ship here, among other things. Figure 23.2 shows the game as it is just getting started, and Listing 23.1 shows the source code for the class.

The Black Hole game soon after startup.
FIGURE 23.2 The Black Hole game soon after startup.

LISTING 23.1 Source Code for the PlayingModule Class

public class PlayingModule : IGameModule
Game1 game;
SpriteFont font;
Random rand;
Sprite background;
MassiveObject blackHole;
MassiveObject superCore;
MassiveObject ship;
float energy = 100.0f;
int startTime, lifetime;
List<MassiveObject> objects;
public PlayingModule(Game1 game)
{ = game;
rand = new Random();
startTime = 0;
lifetime = 4000;
public void LoadContent(ContentManager content)
font = content.Load<SpriteFont>(“WascoSans”);
background = new Sprite(game.Content, game.spriteBatch);
background.origin = Vector2.Zero;
blackHole = new MassiveObject(game.Content, game.spriteBatch);
blackHole.position = new Vector2(500, 240);
blackHole.scale = 2.0f;
blackHole.mass = 40;
blackHole.color = new Color(255, 100, 100, 200);
blackHole.velocityAngular = 0.1f;
superCore = new MassiveObject(game.Content, game.spriteBatch);
superCore.image = blackHole.image;
superCore.position = new Vector2(blackHole.position.X,
superCore.scale = blackHole.scale * 0.4f;
superCore.mass = 60;
superCore.color = new Color(200, 100, 100, 180);
superCore.velocityAngular = 4.0f;
superCore.origin = new Vector2(64, 64);
//create objects list
objects = new List<MassiveObject>();
//create player ship
ship = new MassiveObject(game.Content, game.spriteBatch);
ship.position = new Vector2(200, 240);
ship.mass = 100f;
ship.scale = 0.2f;
ship.rotation = MathHelper.ToRadians(90);

The Update() method is a bit monolithic at this stage, but the code is easier to follow this way than if it had been divided into several smaller methods. I usually divide a method like this when it grows too large to be easily maintained, but since the gameplay code in PlayingModule is only 300 lines long, there isn’t too much to consume here at once. There’s a lot going on here in Update(), but we won’t break up the code listing and break the flow of the code, which can be distracting.

First of all, the blackHole and superCore objects are updated. Then we go into a foreach loop that processes all the MassiveObject objects in the objects list (there’s a tongue twister!). Each object is updated, rotated, and animated. Within the foreach is where the bulk of the code is found for the game.

When one of the satellites grows to a certain size (from collecting energy), that triggers a subset of code here in the foreach block where the player’s ship actually attracts the satellite toward it, using the same code used to simulate the gravitational pull of the black hole on the same objects. The slight gravity “tug” causes the satellites to veer toward the ship and increase the chances of their being caught by it, without making it too easy for the player. After all, the ship doesn’t move yet, it only rotates in place!

Next up is the code that tugs objects inward toward the black hole, if they are in range. Figure 23.3 shows another screenshot of the game, this time with a lot of satellites in orbit. Note the addition of animated asteroids in the scene. The asteroids serve no purpose, but just fill in some detail to make the scene look more interesting. A new asteroid is added every few seconds at a random direction and velocity, and over time they do tend to add up to quite a large mass of rotation around the black hole, which only increases the fun factor. Now, there is also potential use for these asteroid sprites beyond just “for looks.”

A large number of objects are orbiting the black hole, and they tend to fall in quite frequently.
FIGURE 23.3 A large number of objects are orbiting the black hole, and they tend to fall in quite frequently.

Listing 23.2 contains the source code for the Update() method.

It’s quite a challenge to come up with mass values for the black hole, the super core, and each of the objects that not only result in a realistic simulation of gravity’s effect on objects of mass but also make the game fun. Fun is more important than realism, but we want to have a little of both if possible. But when a trade-off is required, always go with that which helps the game to sell: the fun factor.

LISTING 23.2 Source Code for the Update() Method

public void Update(TouchLocation touch, GameTime gameTime)
int time = gameTime.ElapsedGameTime.Milliseconds;
foreach (MassiveObject obj in objects)
if (!obj.alive) continue;
obj.Animate(time); //frame animation
obj.Animate(); //mod animation
//allow ship to collect energy satellites for bonus energy
if (obj.scale > 3.0f && == “satellite”)
obj.color = Color.White;
if (game.RadialCollision(obj.position, ship.position,
obj.size.X, 40))
obj.alive = false;
energy += obj.scale;
if (!obj.captured)
//attract when object is near the black hole
if (game.RadialCollision(obj.position,
blackHole.position, 10, 500))
//is object touching the outer edges of the black hole?
if (game.RadialCollision(obj.position,
blackHole.position, 10, 120))
obj.color = Color.Red;
if ( == “satellite”)
obj.scale += 0.1f;
energy += 0.5f;
obj.Attract(blackHole); //outer black hole
obj.Attract(superCore); //inner black hole
//oh no, object is caught by the black hole!
if (game.RadialCollision(obj.position,
superCore.position, 16, 60))
obj.captured = true;
obj.lifetime = 3000;
obj.startTime = (int)
OrbitalMovement anim1 = new OrbitalMovement(
blackHole.position, 10 + rand.Next(40),
obj.rotation, -0.8f);
obj.color = Color.White;
//when captured, time runs out
if (obj.lifetime > 0)
if (obj.startTime + obj.lifetime <
obj.alive = false;
//see if object has gone too far out of bounds
if (obj.position.X < -200 || obj.position.X > 1000 ||
obj.position.Y < -200 || obj.position.Y > 700)
obj.alive = false;
//update ship
if (energy <= 0)
//object is caught by the black hole
if (game.RadialCollision(ship.position, superCore.position,
64, 40))
ship.captured = true;
ship.lifetime = 3000;
ship.startTime = (int)
OrbitalMovement anim1 = new OrbitalMovement(
blackHole.position, 10 + rand.Next(40),
ship.rotation, -0.8f);
//done being squished?
if (ship.lifetime > 0)
if (ship.startTime + ship.lifetime <
ship.alive = false;
energy -= 0.05f;
ship.velocityLinear.X = 0.0f;
//check user input
if (touch.State == TouchLocationState.Released)
if (touch.Position.X > ship.position.X)
if (touch.Position.Y < 200)
ship.velocityAngular = -0.01f;
else if (touch.Position.Y > 280)
ship.velocityAngular = 0.01f;
ship.velocityAngular = 0;
//time to add another random asteroid?
if (startTime + lifetime < gameTime.TotalGameTime.
startTime = (int)gameTime.TotalGameTime.TotalMilliseconds;
//clean out the dead objects
foreach (MassiveObject obj in objects)
if (obj.alive == false)

The Draw() method is next, with its source code in Listing 23.3. This is a rather small method because the gameplay objects are managed.

LISTING 23.3 Source Code for the Draw() Method

public void Draw(GameTime gameTime)
foreach (MassiveObject obj in objects)
if (obj.alive)
string text = “Ship rot “ + MathHelper.ToDegrees(
game.spriteBatch.DrawString(font, text, new Vector2(0, 0),
text = “Objects “ + objects.Count.ToString();
game.spriteBatch.DrawString(font, text, new Vector2(0, 20),
text = “Energy “ + energy.ToString(“N0”);
game.spriteBatch.DrawString(font, text, new Vector2(650, 0),

Finally, we have two helper methods, CreateAsteroid() and CreateSatellite(), that generate a random asteroid and random satellite, respectively. These two methods, shown in Listing 23.4, are quite important to the gameplay because they determine whether the objects will actually move reasonably on the screen. I say reasonably rather than realistically because, again, we don’t want absolute realism; we want some realism with gobs of fun gameplay. The asteroids aren’t important to the gameplay, because they are just for looks, but we do want them to start off in such a way that they end up rotating around the black hole. Likewise, the satellite must be launched in such a way that it moves reasonably well. At this stage, our satellites move at a constant speed, but in the next (and final) hour, we will add GUI controls that allow the player to adjust the power.

LISTING 23.4 Source Code for the CreateAsteroid() and CreateSatellite() Methods

public void CreateAsteroid()
MassiveObject obj = new MassiveObject(game.Content, game.spriteBatch);
obj.columns = 8;
obj.totalFrames = 64;
obj.scale = 0.1f + (float)rand.NextDouble();
obj.size = new Vector2(60, 60);
obj.radius = 80;
//randomly place at top or bottom of screen
obj.position = new Vector2(rand.Next(100, 800), -100);
obj.velocityLinear = new Vector2(4.0f, (float)(rand.NextDouble() *
if (rand.Next(2) == 1)
obj.position.Y = -obj.position.Y;
obj.velocityLinear.Y = -obj.velocityLinear.Y;
obj.scale = 0.4f;
obj.mass = 1;
obj.velocityAngular = 0.001f;
obj.lifetime = 0; = “asteroid”;
public void CreateSatellite()
MassiveObject obj;
obj = new MassiveObject(game.Content, game.spriteBatch);
obj.position = ship.position;
obj.mass = 1;
obj.scale = 0.5f;
obj.lifetime = 0; = “satellite”;
//calculate velocity based on ship’s angle
float accel = 4.0f;
float angle = ship.rotation – MathHelper.ToRadians(90);
float x = (float)Math.Cos(angle) * accel;
float y = (float)Math.Sin(angle) * accel;
obj.velocityLinear = new Vector2(x,y);
//load energy to launch
energy -= 1;
This all sounds like fun, but is there even a way to lose the game? Certainly! If the player runs out of energy, the ship will fall into the black hole! At this stage, the ship just loses its “traction” or station-keeping thrusters and is drawn into the black hole, only to be whipped around by the acceleration code. Some sort of fantastic animation will have to be added so that the ship gets sucked into the black hole like the other objects—a task for the next hour! Figure 23.4 shows what happens now if the player runs out of energy. Another improvement to be made in the next hour is an energy bar rather than just a text display. We have a lot of work yet to do on this game, but it’s already showing promise.

Running out of energy spells doom for the poor ship and its crew!
FIGURE 23.4 Running out of energy spells doom for the poor ship and its crew!


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