Axis Aligned Rectangle Collision Detection - Java

Question

I have some problem checking if the 2 rectangles has a collision given with 4 points, so I have a rect1 with p1(x,y) for lower left corner and p2(x,y) for upper right corner, same as rect2.

So this is my graph illustration:

This is my sample code so far:

    int[][] rect1 = {{2,0},{3,1}};
    int[][] rect2 = {{0,0},{1,2}};
    if(rect1[1][0] >= rect2[0][0] && rect1[1][1] >= rect2[0][1] && rect1[0][0] <= rect2[1][0] && rect1[0][1] <= rect2[1][1])
    {
        System.out.println("Collision");
    } else {
        System.out.println("No Collision");
    }

I'm not pretty sure if I got it correctly with this code, can anyone give my some idea? (I'm beginner!)

Kent Shikama Kent Shikama · Accepted Answer · 2015-01-04T06:56:12

Short Answer: Yes

Essentially you have four conditions you have to check to be sure you have colliding rectangles:

The first rectangle's upper right corner must be right of the second's bottom left corner
The first rectangle's upper right corner must be above the second's bottom left corner
The first rectangle's bottom left corner must be left of the second's upper right corner
The first rectangle's bottom left corner must be below the second's upper right corner

Refactor your code like follows and it will become clearer how the above four conditions are met:

public class HelloWorld {

     public static void main(String []args){
        int[][] rect1 = {{1,5},{4,8}};
        int[][] rect2 = {{4,1},{7,4}};
        if( isRect1UpperRightCornerRightOfRect2BottomLeftCorner(rect1, rect2) && 
            isRect1UpperRightCornerAboveRect2BottomLeftCorner(rect1, rect2) && 
            isRect1BottomLeftCornerLeftOfRect2UpperRightCorner(rect1, rect2) && 
            isRect1BottomLeftCornerBelowRect2UpperRightCorner(rect1, rect2) ) {
            System.out.println("Collision");
        } else {
            System.out.println("No Collision");
        }
     }

     private static boolean isRect1UpperRightCornerRightOfRect2BottomLeftCorner(int[][] rect1, int[][] rect2) {
        return rect1[1][0] >= rect2[0][0];
     }

     private static boolean isRect1UpperRightCornerAboveRect2BottomLeftCorner(int[][] rect1, int[][] rect2) {
         return rect1[1][1] >= rect2[0][1];
     }

     private static boolean isRect1BottomLeftCornerLeftOfRect2UpperRightCorner(int[][] rect1, int[][] rect2) {
         return rect1[0][0] <= rect2[1][0];
     }

     private static boolean isRect1BottomLeftCornerBelowRect2UpperRightCorner(int[][] rect1, int[][] rect2) {
         return rect1[0][1] <= rect2[1][1];
     }
}

Also here is a good interactive website showing visually how the above would work. Note the corners are flipped.

Axis Aligned Rectangle Collision Detection - Java

2 Answers

Short Answer: Yes