Simple projectiles have been popular in games for a long time. Angry Birds recently combined it with collision physics to great success, but long ago, games like Worms and (my personal favourite) Scorched Earth were using projectiles as their central game mechanic. A projectile, for the purpose of this post, refers to an object that is launched through the air with a certain initial velocity, but after launch the motion is only affected by gravity, which makes the projectile follow a curve through the air known as a parabola:

## The Game

Our next game will involve monkeys throwing bananas at each other. The monkeys will stand still, and our bananas will act like simple projectiles.

To implement a projectile, we must first give the projectile an initial X and Y motion — in our case, that’s decided by where the player clicks to launch a banana. The projectile then keeps its horizontal (X) motion constant — it’s always flying sideways at the same speed. What varies is its vertical (Y) motion. It starts off being a high velocity upwards. Then, each frame, the vertical velocity is reduced by gravity, until it tips past zero and the velocity becomes negative, heading downwards. Let’s illustrate this.

First, the banana is thrown, with an initial velocity — it is moving 5 squares upwards and 2 squares sideways each frame:

We will set gravity to be one square reduction in velocity per frame. So the second frame, the banana still moves 2 squares sideways — its horizontal velocity is never altered — but only four squares upwards:

The same happens in the next few frames — same sideways velocity, but upwards velocity is reduced by one square each frame, until it reaches zero:

So the vertical velocity has been 5, then 4, 3, 2, 1 and now 0 squares upwards. The pattern doesn’t alter just because the banana reached its peak — the next frame it becomes -1:

Then it becomes -2, -3, -4 and -5, until the banana hits the ground:

So in summary: you can see that the banana in the diagram starts off with an upwards velocity of five. Each turn the velocity is reduced by one, and the effect this has is to reduce the banana’s velocity to zero, then make it increasingly negative. These two simple components — high initial upwards velocity, then constant reduction each frame — give the flight this parabola.

The code to implement the banana’s flight is very short:

private void fly() { velY -= ProjectileWorld.GRAVITY; moveTo(curX + velX, curY - velY); }

I take positive Y velocity to be upwards, so I subtract the effect of gravity, but then because Greenfoot has Y going downwards, I must subtract the upwards velocity for everything to work out. You can see how this works by playing with the example scenario on the Greenfoot website.

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