Calculate collision between circle and rotated rectangle

circle rotated rectangle collision.js

// Ported to JavaScript from 
// http://www.migapro.com/circle-and-rotated-rectangle-collision-detection/
//
// An example:
// var circle: { x: 20, y: 10, radius: 20 };
// var rect: { x: 30, y: 30, width: 100, height: 100, rotation: Math.PI / 2 };
// collideCircleWithRotatedRectangle( circle, rect );
// // returns true.
// 
//
// Please note:
// This code assumes that rect.x and rect.y are the CENTER coordinates
// of the rectangle. You may want to to change this.
// Also rotation values need to be in RADIANS.
function collideCircleWithRotatedRectangle ( circle, rect ) {
	
	var rectCenterX = rect.x;
	var rectCenterY = rect.y;

	var rectX = rectCenterX - rect.width / 2;
	var rectY = rectCenterY - rect.height / 2;

	var rectReferenceX = rectX;
	var rectReferenceY = rectY;
	
	// Rotate circle's center point back
	var unrotatedCircleX = Math.cos( rect.rotation ) * ( circle.x - rectCenterX ) - Math.sin( rect.rotation ) * ( circle.y - rectCenterY ) + rectCenterX;
	var unrotatedCircleY = Math.sin( rect.rotation ) * ( circle.x - rectCenterX ) + Math.cos( rect.rotation ) * ( circle.y - rectCenterY ) + rectCenterY;

	// Closest point in the rectangle to the center of circle rotated backwards(unrotated)
	var closestX, closestY;

	// Find the unrotated closest x point from center of unrotated circle
	if ( unrotatedCircleX < rectReferenceX ) {
		closestX = rectReferenceX;
	} else if ( unrotatedCircleX > rectReferenceX + rect.width ) {
		closestX = rectReferenceX + rect.width;
	} else {
		closestX = unrotatedCircleX;
	}
 
	// Find the unrotated closest y point from center of unrotated circle
	if ( unrotatedCircleY < rectReferenceY ) {
		closestY = rectReferenceY;
	} else if ( unrotatedCircleY > rectReferenceY + rect.height ) {
		closestY = rectReferenceY + rect.height;
	} else {
		closestY = unrotatedCircleY;
	}
 
	// Determine collision
	var collision = false;
	var distance = getDistance( unrotatedCircleX, unrotatedCircleY, closestX, closestY );
	
	if ( distance < circle.radius ) {
		collision = true;
	}
	else {
		collision = false;
	}

	return collision;
}

function getDistance( fromX, fromY, toX, toY ) {
	var dX = Math.abs( fromX - toX );
	var dY = Math.abs( fromY - toY );

	return Math.sqrt( ( dX * dX ) + ( dY * dY ) );
}