html5-canvas

Collisions and Intersections

Are 2 circles colliding?

// circle objects: { x:, y:, radius: }
// return true if the 2 circles are colliding
// c1 and c2 are circles as defined above

function CirclesColliding(c1,c2){
    var dx=c2.x-c1.x;
    var dy=c2.y-c1.y;
    var rSum=c1.radius+c2.radius;
    return(dx*dx+dy*dy<=rSum*rSum);
}

Are 2 rectangles colliding?

// rectangle objects { x:, y:, width:, height: }
// return true if the 2 rectangles are colliding
// r1 and r2 are rectangles as defined above

function RectsColliding(r1,r2){
    return !(
        r1.x>r2.x+r2.width || 
        r1.x+r1.width<r2.x || 
        r1.y>r2.y+r2.height || 
        r1.y+r1.height<r2.y
    );
}

Are a circle and rectangle colliding?

// rectangle object: { x:, y:, width:, height: }
// circle object: { x:, y:, radius: }
// return true if the rectangle and circle are colliding

function RectCircleColliding(rect,circle){
    var dx=Math.abs(circle.x-(rect.x+rect.width/2));
    var dy=Math.abs(circle.y-(rect.y+rect.height/2));

    if( dx > circle.radius+rect.width/2 ){ return(false); }
    if( dy > circle.radius+rect.height/2 ){ return(false); }

    if( dx <= rect.width ){ return(true); }
    if( dy <= rect.height ){ return(true); }

    var dx=dx-rect.width;
    var dy=dy-rect.height
    return(dx*dx+dy*dy<=circle.radius*circle.radius);
}

Are 2 line segments intercepting?

The function in this example returns true if two line segments are intersecting and false if not.

The example is designed for performance and uses closure to hold working variables

    // point object: {x:, y:}
    // p0 & p1 form one segment, p2 & p3 form the second segment
    // Returns true if lines segments are intercepting
    var lineSegmentsIntercept = (function(){ // function as singleton so that closure can be used
        
        var v1, v2, v3, cross, u1, u2;  // working variable are closed over so they do not need creation 
                                   // each time the function is called. This gives a significant performance boost.
        v1 = {x : null, y : null}; // line p0, p1 as vector
        v2 = {x : null, y : null}; // line p2, p3 as vector
        v3 = {x : null, y : null}; // the line from p0 to p2 as vector
        
        function lineSegmentsIntercept (p0, p1, p2, p3) {
            v1.x = p1.x - p0.x; // line p0, p1 as vector
            v1.y = p1.y - p0.y; 
            v2.x = p3.x - p2.x; // line p2, p3 as vector
            v2.y = p3.y - p2.y; 
            if((cross = v1.x * v2.y - v1.y * v2.x) === 0){  // cross prod 0 if lines parallel
                return false; // no intercept
            }
            v3 = {x : p0.x - p2.x, y : p0.y - p2.y};  // the line from p0 to p2 as vector
            u2 = (v1.x * v3.y - v1.y * v3.x) / cross; // get unit distance along line p2 p3 
            // code point B
            if (u2 >= 0 && u2 <= 1){                   // is intercept on line p2, p3
                u1 = (v2.x * v3.y - v2.y * v3.x) / cross; // get unit distance on line p0, p1;
                // code point A
                return (u1 >= 0 && u1 <= 1);           // return true if on line else false.
                // code point A end
            }
            return false; // no intercept;
            // code point B end
        }
        return lineSegmentsIntercept;  // return function with closure for optimisation.
    })();

Usage example

var p1 = {x: 100, y: 0};   // line 1
var p2 = {x: 120, y: 200};
var p3 = {x: 0,   y: 100}; // line 2
var p4 = {x: 100, y: 120};
var areIntersepting = lineSegmentsIntercept (p1, p2, p3, p4); // true

The example is easily modified to return the point of intercept. Replace the code between code point A and A end with

if(u1 >= 0 && u1 <= 1){
    return {
        x : p0.x + v1.x * u1,
        y : p0.y + v1.y * u1,
    };
}

Or if you want to get the intercept point on the lines, ignoring the line segments start and ends replace the code between code point B and B end with

return {
    x : p2.x + v2.x * u2,
    y : p2.y + v2.y * u2,
};

Both modifications will return false if there is no intercept or return the point of intercept as {x : xCoord, y : yCoord}

Are a line segment and circle colliding?

// [x0,y0] to [x1,y1] define a line segment
// [cx,cy] is circle centerpoint, cr is circle radius 
function isCircleSegmentColliding(x0,y0,x1,y1,cx,cy,cr){

    // calc delta distance: source point to line start
    var dx=cx-x0;
    var dy=cy-y0;

    // calc delta distance: line start to end
    var dxx=x1-x0;
    var dyy=y1-y0;

    // Calc position on line normalized between 0.00 & 1.00
    // == dot product divided by delta line distances squared
    var t=(dx*dxx+dy*dyy)/(dxx*dxx+dyy*dyy);

    // calc nearest pt on line
    var x=x0+dxx*t;
    var y=y0+dyy*t;
    
    // clamp results to being on the segment
    if(t<0){x=x0;y=y0;}
    if(t>1){x=x1;y=y1;}

    return( (cx-x)*(cx-x)+(cy-y)*(cy-y) < cr*cr );
}

Are line segment and rectangle colliding?

// var rect={x:,y:,width:,height:};
// var line={x1:,y1:,x2:,y2:};
// Get interseting point of line segment & rectangle (if any)
function lineRectCollide(line,rect){

    // p=line startpoint, p2=line endpoint
    var p={x:line.x1,y:line.y1};
    var p2={x:line.x2,y:line.y2};

    // top rect line
    var q={x:rect.x,y:rect.y};
    var q2={x:rect.x+rect.width,y:rect.y};
    if(lineSegmentsCollide(p,p2,q,q2)){ return true; }
    // right rect line
    var q=q2;
    var q2={x:rect.x+rect.width,y:rect.y+rect.height};
    if(lineSegmentsCollide(p,p2,q,q2)){ return true; }
    // bottom rect line
    var q=q2;
    var q2={x:rect.x,y:rect.y+rect.height};
    if(lineSegmentsCollide(p,p2,q,q2)){ return true; }
    // left rect line
    var q=q2;
    var q2={x:rect.x,y:rect.y};
    if(lineSegmentsCollide(p,p2,q,q2)){ return true; }

    // not intersecting with any of the 4 rect sides
    return(false);
}

// point object: {x:, y:}
// p0 & p1 form one segment, p2 & p3 form the second segment
// Get interseting point of 2 line segments (if any)
// Attribution: https://paulbourke.net/geometry/pointlineplane/
function lineSegmentsCollide(p0,p1,p2,p3) {

    var unknownA = (p3.x-p2.x) * (p0.y-p2.y) - (p3.y-p2.y) * (p0.x-p2.x);
    var unknownB = (p1.x-p0.x) * (p0.y-p2.y) - (p1.y-p0.y) * (p0.x-p2.x);
    var denominator  = (p3.y-p2.y) * (p1.x-p0.x) - (p3.x-p2.x) * (p1.y-p0.y);        

    // Test if Coincident
    // If the denominator and numerator for the ua and ub are 0
    //    then the two lines are coincident.    
    if(unknownA==0 && unknownB==0 && denominator==0){return(null);}

    // Test if Parallel 
    // If the denominator for the equations for ua and ub is 0
    //     then the two lines are parallel. 
    if (denominator == 0) return null;

    // test if line segments are colliding
    unknownA /= denominator;
    unknownB /= denominator;
    var isIntersecting=(unknownA>=0 && unknownA<=1 && unknownB>=0 && unknownB<=1)

    return(isIntersecting);
}

Are 2 convex polygons colliding?

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

THE POLYGONS MUST BE CONVEX

Attribution: Markus Jarderot @ https://stackoverflow.com/questions/10962379/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;
};

Are 2 polygons colliding? (both concave and convex polys are allowed)

Tests all polygon sides for intersections to determine if 2 polygons are colliding.

// 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 can be both concave and convex
// return true if the 2 polygons are colliding 

function polygonsCollide(p1,p2){
    // turn vertices into line points
    var lines1=verticesToLinePoints(p1);
    var lines2=verticesToLinePoints(p2);
    // test each poly1 side vs each poly2 side for intersections
    for(i=0; i<lines1.length; i++){
    for(j=0; j<lines2.length; j++){
        // test if sides intersect
        var p0=lines1[i][0];
        var p1=lines1[i][1];
        var p2=lines2[j][0];
        var p3=lines2[j][1];
        // found an intersection -- polys do collide
        if(lineSegmentsCollide(p0,p1,p2,p3)){return(true);}
    }}
    // none of the sides intersect
    return(false);
}
// helper: turn vertices into line points
function verticesToLinePoints(p){
    // make sure polys are self-closing
    if(!(p[0].x==p[p.length-1].x && p[0].y==p[p.length-1].y)){
        p.push({x:p[0].x,y:p[0].y});
    }
    var lines=[];
    for(var i=1;i<p.length;i++){
        var p1=p[i-1];
        var p2=p[i];
        lines.push([ 
            {x:p1.x, y:p1.y},
            {x:p2.x, y:p2.y}
        ]);
    }
    return(lines);
}
// helper: test line intersections
// point object: {x:, y:}
// p0 & p1 form one segment, p2 & p3 form the second segment
// Get interseting point of 2 line segments (if any)
// Attribution: https://paulbourke.net/geometry/pointlineplane/
function lineSegmentsCollide(p0,p1,p2,p3) {
    var unknownA = (p3.x-p2.x) * (p0.y-p2.y) - (p3.y-p2.y) * (p0.x-p2.x);
    var unknownB = (p1.x-p0.x) * (p0.y-p2.y) - (p1.y-p0.y) * (p0.x-p2.x);
    var denominator  = (p3.y-p2.y) * (p1.x-p0.x) - (p3.x-p2.x) * (p1.y-p0.y);        

    // Test if Coincident
    // If the denominator and numerator for the ua and ub are 0
    //    then the two lines are coincident.    
    if(unknownA==0 && unknownB==0 && denominator==0){return(null);}

    // Test if Parallel 
    // If the denominator for the equations for ua and ub is 0
    //     then the two lines are parallel. 
    if (denominator == 0) return null;

    // test if line segments are colliding
    unknownA /= denominator;
    unknownB /= denominator;
    var isIntersecting=(unknownA>=0 && unknownA<=1 && unknownB>=0 && unknownB<=1)

    return(isIntersecting);
}

Is an X,Y point inside an arc?

Tests if the [x,y] point is inside a closed arc.

enter image description here

var arc={
    cx:150, cy:150,
    innerRadius:75, outerRadius:100,
    startAngle:0, endAngle:Math.PI
}

function isPointInArc(x,y,arc){    
    var dx=x-arc.cx;
    var dy=y-arc.cy;
    var dxy=dx*dx+dy*dy;
    var rrOuter=arc.outerRadius*arc.outerRadius;
    var rrInner=arc.innerRadius*arc.innerRadius;
    if(dxy<rrInner || dxy>rrOuter){return(false);}
    var angle=(Math.atan2(dy,dx)+PI2)%PI2;
    return(angle>=arc.startAngle && angle<=arc.endAngle);
}

Is an X,Y point inside a wedge?

Tests if the [x,y] point is inside a wedge.

enter image description here

// wedge objects: {cx:,cy:,radius:,startAngle:,endAngle:}
// var wedge={
//     cx:150, cy:150,  // centerpoint
//     radius:100,
//     startAngle:0, endAngle:Math.PI
// }
// Return true if the x,y point is inside the closed wedge

function isPointInWedge(x,y,wedge){
    var PI2=Math.PI*2;    
    var dx=x-wedge.cx;
    var dy=y-wedge.cy;
    var rr=wedge.radius*wedge.radius;
    if(dx*dx+dy*dy>rr){return(false);}
    var angle=(Math.atan2(dy,dx)+PI2)%PI2;
    return(angle>=wedge.startAngle && angle<=wedge.endAngle);
}

Is an X,Y point inside a circle?

Tests if an [x,y] point is inside a circle.

// circle objects: {cx:,cy:,radius:,startAngle:,endAngle:}
// var circle={
//     cx:150, cy:150,  // centerpoint
//     radius:100,
// }
// Return true if the x,y point is inside the circle

function isPointInCircle(x,y,circle){
    var dx=x-circle.cx;
    var dy=y-circle.cy;
    return(dx*dx+dy*dy<circle.radius*circle.radius);
}

Is an X,Y point inside a rectangle?

Tests if an [x,y] point is inside a rectangle.

// rectangle objects: {x:, y:, width:, height: }
// var rect={x:10, y:15, width:25, height:20}
// Return true if the x,y point is inside the rectangle

function isPointInRectangle(x,y,rect){
    return(x>rect.x && x<rect.x+rect.width && y>rect.y && y<rect.y+rect.height);
}
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