It's possible to build most, if not all, of the basic POV shapes with
isosurfaces. In practice, you'd probably not want to bother with an
isosurface if there's an equivalent basic shape available, but understanding
these simple shapes gives us insight into how to develop more complex
surfaces.
The syntax used for the isosurfaces used on this page is like
isosurface {
function { x*2 + y*2 + z*2  R*R }
accuracy 0.001
max_gradient 4
contained_by{sphere{0,1.2}}
pigment {rgb .9}
finish {phong 0.5 phong_size 10}
}
sphere {0,1.2 pigment {rgbt <1,0,0,0.9>}}


Sphere
function { x*x + y*y + z*z  R*R }
Maths: Take a look at what happens on the x, y, and z planes.
On the x plane, x=0, the equation reduces to y*y + z*z  R*R = 0
the two dimensional equation of a circle radius R. Similarly the intersection with the y and z planes are
also circles. This probably doesn't come as much of a surprise, but if you
get a feel for how the 2D equations come together to make the 3D equation
you can sometimes get a feel for how to build other 3D surfaces.

 Cylinder
function { y*y + z*z  R*R }
Maths: The cross section of this object on any x plane is clearly always the circle
y*y + z*z  R*R = 0.

 Plane
function { y }
Maths: Well, functions don't come much simpler than that.
The isosurface exists wherever y takes the value zero.
This is the y plane. The x plane and z plane are simply function {x}
and function {z}.
The plane through the point <0,1,0> is given mathematically by y = 1
so the function (with threshold zero) is therefore function { y  1 }.

 Box
function { max((y*y1),(x*x1),(z*z1)) }
Maths: What I've done here is to take the doubleplane function { y*y  1}
and intersect it with similar x and z double planes. max() performs intersections of isosurfaces.
function { y*y  1} is mathematically y^2 1 = 0 which
has the two planar solutions y = +1 and y = 1, so the function describes the two planes.

 Paraboloid
function { x*x + y +z*z  1 }
Maths: Observe that the intersection with the y plane is a circle x^2 + z^2  1 = 0
and the intersections with the x and z planes are parabolas y = x^2 + 1.

 Hyperbolic Paraboloid
function { x*x + y  z*z }
Maths: Observe that the intersection with the y plane is a hyperbola x^2  z^2 = 0
and the intersections with the x and z planes are parabolas y = x^2.

 Octahedron
function { abs(x)+abs(y)+abs(z)  1 }

