load(draw)$
R:15; r:5; eps:0.05;
x:(R + r * cos(phi)) * cos(theta);
y:(R + r * cos(phi)) * sin(theta);
z:r * sin(phi);
draw3d( terminal      = svg,
        file_name     = "Torus2",
        xtics         = none,
        ytics         = none,
        ztics         = none,
        axis_3d       = false,
        view          = [90,330],
        xu_grid       = 50,
        yv_grid       = 100,
        surface_hide  = true,
        color         = black,
        enhanced3d    = false,
        wired_surface = true,
        color         = gray,
        line_width    = 0.5,
        parametric_surface(z,y,x,phi,-%pi,%pi,theta,-%pi,%pi),
        color         = red,
        line_width    = 1,
        parametric((r+eps)*sin(t), 0, R + (r+eps)*cos(t), t,-%pi,%pi),
        color         = magenta,
        parametric(-r, R*sin(t), R*cos(t), t,-%pi, %pi),
        xrange=[-20,20]
       );