using loop to detect collision in XNA

Question

Got a problem, with collision detection, I need to detect collision between bullets and meteors using nested loop for this, but it isn't working.

                            } 
                }
                    check();
                    LoadMeteors();      
        base.Update(gameTime);
    }





    public void LoadMeteors()
    {

        int randY = random.Next(100, 400);
        if (spawn >= 0.5)
        {
            spawn = 0;

                meteors.Add(new Meteor(Content.Load<Texture2D>("Backgrounds/meteori"), new Vector2(800, randY), new Rectangle(0,0,50,50)));
        }
    }

    public void SpawnBullet()
    {

        bullets.Add(new Bullet(Content.Load<Texture2D>("Backgrounds/pula"), new Vector2(rocket.rectangle.X + 20, rocket.rectangle.Y + 20), new Rectangle(0, 0, 10, 5)));
        soundEffect.Play();

    }
    public void check()
    {

        for (k = 0; k < bullets.Count; k++)
        {
            for (i = 0; i < meteors.Count; i++)
            {
                if (bullets[k].rectangle.Intersects(meteors[i].rectangle))
                {
                    meteors[i].isVisible = false;
                }
            }
        }
    }

that is my meteor class:

using System;
using System.Collections.Generic;
using System.Linq;
using Microsoft.Xna.Framework;
using Microsoft.Xna.Framework.Audio;
using Microsoft.Xna.Framework.Content;
using Microsoft.Xna.Framework.GamerServices;
using Microsoft.Xna.Framework.Graphics;
using Microsoft.Xna.Framework.Input;
using Microsoft.Xna.Framework.Media;

namespace Rocket
{
   class Meteor
 {
    public Texture2D texture;
    public Rectangle rectangle;
    public Vector2 position;
    public Vector2 velocity;

    public bool isVisible = true;

    Random random = new Random();
    int randX, randY;

    public Meteor(Texture2D newTexture, Vector2 newPosition, Rectangle newRectangle)
    {

        texture = newTexture;
        position = newPosition;
        rectangle = newRectangle;
        randY = random.Next(-4, 4);
        randX = random.Next(-4,-1);

        velocity = new Vector2(7, 0);
    }


   public void Update(GraphicsDevice graphics)
    {

        position -= velocity;
        if (position.X < 0 - texture.Width)
            isVisible = false;
    }
   public void Draw(SpriteBatch spriteBatch)
   {

       spriteBatch.Draw(texture, position, rectangle, Color.White);


   }


}
}

and that is my bullet class:

    using System;
   using System.Collections.Generic;
  using System.Linq;
  using Microsoft.Xna.Framework;
  using Microsoft.Xna.Framework.Audio;
  using Microsoft.Xna.Framework.Content;
  using Microsoft.Xna.Framework.GamerServices;
 using Microsoft.Xna.Framework.Input;
  using Microsoft.Xna.Framework.Media;

 namespace Rocket
{
class Bullet
{
    public Texture2D texture;
    public Rectangle rectangle;
    public Vector2 position;
    public Vector2 velocity;

    public bool isVisible = true;

    public Bullet(Texture2D newTexture, Vector2 newPosition, Rectangle newRectangle)
    {

        texture = newTexture;
        position = newPosition;
        rectangle = newRectangle;

        velocity = new Vector2(10, 0);
    }


    public void Update(GraphicsDevice graphics)
    {
       position += velocity;
               if (position.X < 0 - texture.Width)
            isVisible = false;
    }
   public void Draw(SpriteBatch spriteBatch)
   {
       spriteBatch.Draw(texture, position, rectangle, Color.Aqua);

   }
    }

so, I call function Check(): in update method, than in that function i check for collision, but it isn't detecting collision, also meteor and bullet classes, both have rectangle, I don't get any errors and it seems to be right, thanks for any help!

Answer 1

From your code:

for (k = 0; k < bullets.Count; k++)
{
    for (k = 0; k < meteors.Count; k++)
    {
        if (rocket.rectangle.Intersects(meteors[k].rectangle))
        {
            meteors[k].isVisible = false;
        }
    }
}

You use the same iterator in both for loops. It also looks as though you are reusing that (global) iterator from somewhere else in the code; Unless you have a very good reason for doing so, you should likely be using local iterators that are initialized in the for loop.

Ex. for(int i = 0; i < 1000; i++)

A fix for this might be:

for (int iB = 0; iB < bullets.Count; iB++)
{
    for (int iM = 0; iM < meteors.Count; iM++)
    {
        if (rocket.rectangle.Intersects(meteors[iM].rectangle))
        {
            meteors[iM].isVisible = false;
        }
    }
}

But I don't know how you are specifying the location or bounding box of the actual bullets. At the moment it looks like the rocket.rectangle might be at a fixed location because you never update it's position in these loops.

Answer 2

Best thing you could do here, is just convert your Rectangle variables into a read-only property. This way you only need to update your position, and your Rectangle will always be up to date:

class Bullet {
    public Rectangle rectangle { 
        get { return new Rectangle((int)position.X, (int)position.y, 10, 5); }
}


class Meteor {    
    public Rectangle rectangle { 
        get { return new Rectangle((int)position.X, (int)position.y, 50, 50); }
    }    
}

Replace your "public Rectangle rectangle;" in both your Bullet and Meteor classes with the above Rectangle properties, and it should fix your issue.

Answer 3

Looks like you are only updating the position vector. When you are checking for intersection the rectangles haven't moved.

You could change the rectangles x,y values as well but you have to change your spriteBatch.Draw call and omit the position.

You might be tempted to skip the position vector entirely and only use the rectangles position. I don't think that's a good idea since (as far as I know) the rectangle members are of int type. You would get a very choppy movement or if the velocity is too small, it won't move at all.

Answer 4

I'm a VB person, so I don't know the C syntax, isn't there a For Each loop you can use instead of using your k variable? For each will loop all, it doesn't care how many.