var('s,t')
def x(t) :
    return cos(t)*(1-cos(t))

def y(t) :
    return sin(t)*(1-cos(t))

def cf(s,t):
    return sin(s-t)

Surf = parametric_plot3d([(x(t)+x(s))/2,
                          (y(t)+y(s))/2,
                          1/2*sqrt((x(t)-x(s))^2 + (y(t)-y(s))^2)],
                         (t,0,2*pi),(s,0,2*pi),
                         adaptive = True,
                         color=(cf,colormaps.ocean) ,
                         opacity=0.5,
                         mesh = 1)

show(Surf, frame=False)