Ponerle sombra a un linear

void drawSombreado(float x1, float y1, float x2, float y2, float x3, float y3, float x4, float y4){
    line(x1, y1, x2, y2);
    line(x2, y2, x3, y3);
    line(x3, y3, x4, y4);
    line(x4, y4, x1, y1);
    line(x1, y1, x3, y3);
}