bool ray_intersect(pixel point, pixel direction, float& dist) {
        pixel vpc = sub(center, point);
        float thc;
        float tca;
        float d;
        tca = cdot(vpc, direction);
        d = cdot(vpc, vpc) - tca * tca;
        thc = sqrtf(radius * radius - d);
        float nvpc = norm(vpc);
        if (tca < 0) {
            if (nvpc > radius) {
                return false;
            }
            else if (nvpc == radius) {
                dist = 0;
                return true;
            }
            else {
                dist = thc - tca;
                return true;
            }
        }
        else {
            if (d > radius * radius) {
                return false;
            }
            else {
                if (nvpc > radius) {
                    dist = tca - thc;
                    return true;
                }
                else {
                    dist = tca + thc;
                    return true;
                }
            }
        }
    }