void setup()
{
    size(800, 600);
}
void draw()
{
    // Clear Background to prevent shadows
    background(40, 40, 40, 256);

    //First circle
    fill(0, 50, 256);
    circle(mouseX, mouseY, 100);

    // Second circle
    // project mouse pos between 0 and 1. Take inverse to get the point symmetric postion
    PVector fitMouse = new PVector((float) mouseX / (float) width, (float) mouseY / (float) height);
    PVector twinPos = new PVector((1.0f - fitMouse.x) * width, (1.0f - fitMouse.y) * height);

    fill(256, 50, 0);
    circle(twinPos.x, twinPos.y, 100);


    //println(fitMouseX + ", " + inverseMouseX );
}