# html5-canvas Collisions and Intersections Are 2 convex polygons colliding?

## Example

Use the Separating Axis Theorem to determine if 2 convex polygons are intersecting

THE POLYGONS MUST BE CONVEX

Attribution: Markus Jarderot @ How to check intersection between 2 rotated rectangles?

``````// polygon objects are an array of vertices forming the polygon
//     var polygon1=[{x:100,y:100},{x:150,y:150},{x:50,y:150},...];
// THE POLYGONS MUST BE CONVEX
// return true if the 2 polygons are colliding

function convexPolygonsCollide(a, b){
var polygons = [a, b];
var minA, maxA, projected, i, i1, j, minB, maxB;

for (i = 0; i < polygons.length; i++) {

// for each polygon, look at each edge of the polygon, and determine if it separates
// the two shapes
var polygon = polygons[i];
for (i1 = 0; i1 < polygon.length; i1++) {

// grab 2 vertices to create an edge
var i2 = (i1 + 1) % polygon.length;
var p1 = polygon[i1];
var p2 = polygon[i2];

// find the line perpendicular to this edge
var normal = { x: p2.y - p1.y, y: p1.x - p2.x };

minA = maxA = undefined;
// for each vertex in the first shape, project it onto the line perpendicular to the edge
// and keep track of the min and max of these values
for (j = 0; j < a.length; j++) {
projected = normal.x * a[j].x + normal.y * a[j].y;
if (minA==undefined || projected < minA) {
minA = projected;
}
if (maxA==undefined || projected > maxA) {
maxA = projected;
}
}

// for each vertex in the second shape, project it onto the line perpendicular to the edge
// and keep track of the min and max of these values
minB = maxB = undefined;
for (j = 0; j < b.length; j++) {
projected = normal.x * b[j].x + normal.y * b[j].y;
if (minB==undefined || projected < minB) {
minB = projected;
}
if (maxB==undefined || projected > maxB) {
maxB = projected;
}
}

// if there is no overlap between the projects, the edge we are looking at separates the two
// polygons, and we know there is no overlap
if (maxA < minB || maxB < minA) {
return false;
}
}
}
return true;
};
``````