Collision Detection
Table of Contents


The last example of this section combines the circle and rectangle code together. We have a circle with the position (cx,cy) with a radius r and a square at (rx,ry) with a width/height (rw,rh).

Our code will first test which edge of the rectangle is closest to the circle, then see if there is a collision using the Pythagorean Theorem. Let's create a temporary variable for the square's closest X/Y edges. We'll set them as the circle's position to start:

var testX = cx;
var testY = cy;

Then we do the following tests:

If the circle is to the RIGHT of the square, check against the RIGHT edge.

If the circle is to the LEFT of the square, check against the LEFT edge.

If the circle is ABOVE the square, check against the TOP edge.

If the circle is to the BELOW the square, check against the BOTTOM edge.

Here's how that works as an if statement:

if (cx < rx)         testX = rx;      // left edge
else if (cx > rx+rw) testX = rx+rw;   // right edge
if (cy < ry)         testY = ry;      // top edge
else if (cy > ry+rh) testY = ry+rh;   // bottom edge

Notice the slightly different if/else layout. You don't need the curly brackets if your statement is all on one line. This can tidy things up, but be careful you're not saving space at the expense of readable code!

Now that we know which edges to check, we run the Pythagorean Theorem code using the circle's center and the two edges we found above:

var distX = cx-testX;
var distY = cy-testY;
var distance = sqrt( (distX*distX) + (distY*distY) );

Finally, we compare this distance to the circle's radius:

if (distance <= radius) {
    return true;
return false;

Here's a full example:

var cx = 0; // circle position (set with mouse)
var cy = 0;
var r = 30; // circle radius
var sx = 200; // square position
var sy = 100;
var sw = 200; // and dimensions
var sh = 200;
function setup() {
    var canvas = createCanvas(600,400);
function draw() {
    // update square to mouse coordinates
    cx = mouseX;
    cy = mouseY;
    // check for collision
    // if hit, change rectangle color
    var hit = circleRect(cx, cy, r, sx, sy, sw, sh);
    if (hit) {
        fill(255, 150, 0);
    } else {
        fill(0, 150, 255);
    rect(sx, sy, sw, sh);
    // draw the circle
    fill(0, 150);
    ellipse(cx, cy, r * 2, r * 2);
function circleRect(cx, cy, radius, rx, ry, rw, rh) {
    // temporary variables to set edges for testing
    var testX = cx;
    var testY = cy;
    // which edge is closest?
    if (cx < rx)         testX = rx;      // test left edge
    else if (cx > rx+rw) testX = rx+rw;   // right edge
    if (cy < ry)         testY = ry;      // top edge
    else if (cy > ry+rh) testY = ry+rh;   // bottom edge
    // get distance from closest edges
    var distX = cx - testX;
    var distY = cy - testY;
    var distance = sqrt(distX * distX + distY * distY);
    // if the distance is less than the radius, collision!
    if (distance <= radius) {
        return true;
    return false;

This example is built on code by Matt Worden (thanks!).