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
}

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

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