Probably some sort of parametric equations, like they show in the example 1, notice it all is derived from x and y

$size = 25;
        $x = (xvals zeroes $size+1,$size+1) / $size;
        $y = (yvals zeroes $size+1,$size+1) / $size;
        $z = 0.5 + 0.5 * (sin($x*6.3) * sin($y*6.3)) ** 3;   # Bumps

        imag3d [$x,$y,$z]; # Draw a shaded surface
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