utils.js

function lerp(a,b,t){
    return a+(b-a)*t;
}

function getIntersection(A,B,C,D){
    const tTop=(D.x-C.x)*(A.y-C.y)-(D.y-C.y)*(A.x-C.x);
    const uTop=(C.y-A.y)*(A.x-B.x)-(C.x-A.x)*(A.y-B.y);
    const bottom=(D.y-C.y)*(B.x-A.x)-(D.x-C.x)*(B.y-A.y);

    if(bottom!=0){
        const t=tTop/bottom;
        const u=uTop/bottom;
        if(t>=0 && t<=1 && u>=0 && u<=1){
            return {
                x:lerp(A.x,B.x,t),
                y:lerp(A.y,B.y,t),
                offset:t
            }
        }
    }

    return null;
}

//polylines
function polysIntersect(poly1, poly2){
    for(let i=0;i<poly1.length-1;i++){
        for(let j=0;j<poly2.length-1;j++){
            const touch=getIntersection(
                poly1[i],
                poly1[i+1],
                poly2[j],
                poly2[j+1]
            );
            if(touch){
                return true;
            }
        }
    }
    return false;
}

function getRGBA(value){
    const alpha=Math.abs(value);
    const R=value<0?0:255;
    const G=R;
    const B=value>0?0:255;
    return "rgba("+R+","+G+","+B+","+alpha+")";
}