import three; import palette; size(20cm); currentprojection=orthographic(1,1,1); draw(box((-2,-2,-1),(2,2,1))); draw(shift(-Z)*surface(box((-2,-2),(2,2))),blue); draw(shift(Z)*surface(box((-2,-2),(2,2))),orange+opacity(0.5)); surface s=unitsphere; s.colors(palette(s.map(zpart),Gradient(green+opacity(0.6),white, green+opacity(0.6)))); draw(shift(0.5X+0.5Y)*s); draw(shift(-0.5X-0.5Y)*s);