XNA Game Programming: Spawning Enemies in a circle (Code)

Question

Could someone please give me some direction on how to spawn enemies in a circle (XNA Programming)?

I want the enemies to randomly spawn along the circumference of a circle that is just outside the bounds of the window. I want them to move in straight lines through the center of the window and out to the opposite side of where they started (or as close to that as possible).

Ideally this would create an environment where enemies are randomly coming across from seemingly all directions.

Here's my enemy ("Baddies") class so far. I handle the position of in the SetupTraveling game state. What I'm doing isn't really working, so any help would be greatly appreciated.

public class Baddies : Sprite
{
    public enum State
    {
        Inactive,
        SetupTraveling,
        Traveling,
        SetupInactive,
    }

    public State CurrentState
    {
        set
        {
            currentState = value;
            framesInStateCount = 0;
        }
        get
        {
            return currentState;
        }
    }

    int framesInStateCount = 0;
    State currentState = State.SetupInactive;    

    public Baddies()
    {
        Image = getImage("Bad");
        Scale = .2f;
        Rotation = 0f;
        DRotation = .05f;
        TurnedOn = true;
        BounceOn = false;
        WrapOn = false;
        Gravity = 0f;
    }

    public override void Update()
    {
        framesInStateCount++;
        switch (currentState)
        {
            case State.Inactive:
                if (RandOneIn(100)) CurrentState = State.SetupTraveling;
                break;
            case State.SetupTraveling:
                        PositionX = ((Game1.vGameWidth + 100)/2) * (float)Math.Cos(Glob.rnd(0.0, 2.0 * Math.PI));
                        PositionY = ((Game1.vGameHeight + 100)/2) * (float)Math.Sin(Glob.rnd(0.0, 2.0 * Math.PI));                        
                        DDPositionY = 0;
                        DPositionY = -1;
                        DPositionX = 1f;
                        DDPositionX = 0f;                          

                CurrentState = State.Traveling;
                break;
            case State.Traveling:
                if (PositionX > Game1.vGameWidth + (Image.Width / 2) * Scale)
                {
                    currentState = State.SetupInactive;
                }
                if (PositionX < -500f - (Image.Width / 2) * Scale)
                {
                    currentState = State.SetupInactive;
                }
                if (PositionY > Game1.vGameHeight + (Image.Height / 2) * Scale)
                {
                    currentState = State.SetupInactive;
                }
                if (PositionY < 0 - (Image.Height / 2) * Scale)
                {
                    currentState = State.SetupInactive;
                }
                break;
            case State.SetupInactive:
                PositionX = -300f;
                DPositionX = 0f;
                DPositionY = 0f;
                DDPositionX = 0f;
                CurrentState = State.Inactive;

                break;                
        }     

        base.Update();
    } 
}

score 4 · Answer 1 · answered Dec 10 '12 at 22:36

4

Other approach with trigonometry:

Decide what is your circle radius... usually is:

raqdius = sqrt(Viewport.Size.Width^2 + Viewport.Size.Height^2) / 2;

Generate a random angle

angle = (float) Random.NextDouble() * MathHelper.PI * 2;

Your coordinates are

x = ViewPort.Center.X + radius * Math.Cos(angle);
y = ViewPort.Center.Y + radius * Math.Sin(angle);

answered Dec 10 '12 at 22:36

Blau

5,742
1
18
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I think this is a better approach than mine – frankc Dec 11 '12 at 01:23
Thanks. How do I each Sprite to move towards the center of circle regardless of where it was spawned? (Velocity is handled in my program by the DPositionY and DPositionX variables). – jdewitte Dec 11 '12 at 04:21
your direction vector is (target - source), your target seems to be Viewport.Center in this case, then normalize it, and multiply it by your scalar speed factor. – Blau Dec 11 '12 at 09:07
Thanks. I tried it but I'm getting a weird error: http://stackoverflow.com/questions/13942200/xna-spawn-enemies-in-a-circle-calculation-error – jdewitte Dec 18 '12 at 22:02

frankc · Answer 2 · 2012-12-10T20:55:50.080

1

How about this:

Decide what the raidius of your circle is, call it r
generate an X value from [-r, r]
Y = sqrt(r^2 - X^2)
randomly set Y to Y or -Y. Your cooridinates would be X,Y

edited Dec 10 '12 at 20:55

answered Dec 10 '12 at 20:50

frankc

11,290
4
32
49

XNA Game Programming: Spawning Enemies in a circle (Code)

2 Answers2