3.4.3 Infinite Solid Primitives

There are five polynomial primitive shapes that are possibly infinite and do not respond to automatic bounding. They are plane, cubic, poly, quadric and quartic. They do have a well defined inside and may be used in CSG and inside a clipped_by statement. As with all shapes they can be translated, rotated and scaled. Plane

The plane primitive is a simple way to define an infinite flat surface. The plane is not a thin boundary or can be compared to a sheet of paper. A plane is a solid object of infinite size that divides POV-space in two parts, inside and outside the plane. The plane is specified as follows:

        <Normal>, Distance

The <Normal> vector defines the surface normal of the plane. A surface normal is a vector which points up from the surface at a 90 degree angle. This is followed by a float value that gives the distance along the normal that the plane is from the origin (that is only true if the normal vector has unit length; see below). For example:

  plane { <0, 1, 0>, 4 }

This is a plane where straight up is defined in the positive y-direction. The plane is 4 units in that direction away from the origin. Because most planes are defined with surface normals in the direction of an axis you will often see planes defined using the x, y or z built-in vector identifiers. The example above could be specified as:

  plane { y, 4 }

The plane extends infinitely in the x- and z-directions. It effectively divides the world into two pieces. By definition the normal vector points to the outside of the plane while any points away from the vector are defined as inside. This inside/outside distinction is important when using planes in CSG and clipped_by. It is also important when using fog or atmospheric media. If you place a camera on the "inside" half of the world, then the fog or media will not appear. Such issues arise in any solid object but it is more common with planes. Users typically know when they have accidentally placed a camera inside a sphere or box but "inside a plane" is an unusual concept. In general you can reverse the inside/outside properties of an object by adding the object modifier inverse. See "Inverse" and "Empty and Solid Objects" for details.

A plane is called a polynomial shape because it is defined by a first order polynomial equation. Given a plane:

  plane { <A, B, C>, D }

it can be represented by the equation A*x + B*y + C*z - D*sqrt(A^2 + B^2 + C^2) = 0.

Therefore our example plane{y,4} is actually the polynomial equation y=4. You can think of this as a set of all x, y, z points where all have y values equal to 4, regardless of the x or z values.

This equation is a first order polynomial because each term contains only single powers of x, y or z. A second order equation has terms like x^2, y^2, z^2, xy, xz and yz. Another name for a 2nd order equation is a quadric equation. Third order polys are called cubics. A 4th order equation is a quartic. Such shapes are described in the sections below. Poly, Cubic and Quartic

Higher order polynomial surfaces may be defined by the use of a poly shape. The syntax is

        Order, <A1, A2, A3,... An>

Poly default values:

sturm : off

where Order is an integer number from 2 to 15 inclusively that specifies the order of the equation. A1, A2, ... An are float values for the coefficients of the equation. There are n such terms where n = ((Order+1)*(Order+2)*(Order+3))/6.

The cubic object is an alternate way to specify 3rd order polys. Its syntax is:

        <A1, A2, A3,... A20>

Also 4th order equations may be specified with the quartic object. Its syntax is:

        <A1, A2, A3,... A35>

The following table shows which polynomial terms correspond to which x,y,z factors for the orders 2 to 7. Remember cubic is actually a 3rd order polynomial and quartic is 4th order.

2nd3rd4th5th6th7th 5th6th7th 6th7th
A1 x2 x3 x4 x5 x6 x7 A41 y3 xy3 x2y3 A81 z3 xz3
A2 xy x2y x3y x4y x5y x6y A42 y2z3 xy2z3 x2y2z3 A82 z2 xz2
A3 xz x2z x3z x4z x5z x6z A43 y2z2 xy2z2 x2y2z2 A83 z xz
A4 x x2 x3 x4 x5 x6 A44 y2z xy2z x2y2z A84 1 x
A5 y2 xy2 x2y2 x3y2 x4y2 x5y2 A45 y2 xy2 x2y2 A85 y7
A6 yz xyz x2yz x3yz x4yz x5yz A46 yz4 xyz4 x2yz4 A86 y6z
A7 y xy x2y x3y x4y x5y A47 yz3 xyz3 x2yz3 A87 y6
A8 z2 xz2 x2z2 x3z2 x4z2 x5z2 A48 yz2 xyz2 x2yz2 A88 y5z2
A9 z xz x2z x3z x4z x5z A49 yz xyz x2yz A89 y5z
A10 1 x x2 x3 x4 x5 A50 y xy x2y A90 y5
A11 y3 xy3 x2y3 x3y3 x4y3 A51 z5 xz5 x2z5 A91 y4z3
A12 y2z xy2z x2y2z x3y2z x4y2z A52 z4 xz4 x2z4 A92 y4z2
A13 y2 xy2 x2y2 x3y2 x4y2 A53 z3 xz3 x2z3 A93 y4z
A14 yz2 xyz2 x2yz2 x3yz2 x4yz2 A54 z2 xz2 x2z2 A94 y4
A15 yz xyz x2yz x3yz x4yz A55 z xz x2z A95 y3z4
A16 y xy x2y x3y x4y A56 1 x x2 A96 y3z3
A17 z3 xz3 x2z3 x3z3 x4z3 A57   y6 xy6 A97 y3z2
A18 z2 xz2 x2z2 x3z2 x4z2 A58 y5z xy5z A98 y3z
A19 z xz x2z x3z x4z A59 y5 xy5 A99 y3
A20 1 x x2 x3 x4 A60 y4z2 xy4z2 A100 y2z5
A21 y4 xy4 x2y4 x3y4 A61 y4z xy4z A101 y2z4
A22 y3z xy3z x2y3z x3y3z A62 y4 xy4 A102 y2z3
A23 y3 xy3 x2y3 x3y3 A63 y3z3 xy3z3 A103 y2z2
A24 y2z2 xy2z2 x2y2z2 x3y2z2 A64 y3z2 xy3z2 A104 y2z
A25 y2z xy2z x2y2z x3y2z A65 y3z xy3z A105 y2
A26 y2 xy2 x2y2 x3y2 A66 y3 xy3 A106 yz6
A27 yz3 xyz3 x2yz3 x3yz3 A67 y2z4 xy2z4 A107 yz5
A28 yz2 xyz2 x2yz2 x3yz2 A68 y2z3 xy2z3 A108 yz4
A29 yz xyz x2yz x3yz A69 y2z2 xy2z2 A109 yz3
A30 y xy x2y x3y A70 y2z xy2z A110 yz2
A31 z4 xz4 x2z4 x3z4 A71 y2 xy2 A111 yz
A32 z3 xz3 x2z3 x3z3 A72 yz5 xyz5 A112 y
A33 z2 xz2 x2z2 x3z2 A73 yz4 xyz4 A113 z7
A34 z xz x2z x3z A74 yz3 xyz3 A114 z6
A35 1 x x2 x3 A75 yz2 xyz2 A115 z5
A36 y5 xy5 x2y5 A76 yz xyz A116 z4
A37 y4z xy4z x2y4z A77 y xy A117 z3
A38 y4 xy4 x2y4 A78 z6 xz6 A118 z2
A39 y3z2 xy3z2 x2y3z2 A79 z5 xz5 A119 z
A40 y3z xy3z x2y3z A80 z4 xz4 A120 1

Polynomial shapes can be used to describe a large class of shapes including the torus, the lemniscate, etc. For example, to declare a quartic surface requires that each of the coefficients (A1 ... A35) be placed in order into a single long vector of 35 terms. As an example let's define a torus the hard way. A Torus can be represented by the equation: x4 + y4 + z4 + 2 x2 y2 + 2 x2 z2 + 2 y2 z2 - 2 (r_02 + r_12) x2 + 2 (r_02 - r_12) y2 - 2 (r_02 + r_12) z2 + (r_02 - r_12)2 = 0

Where r_0 is the major radius of the torus, the distance from the hole of the donut to the middle of the ring of the donut, and r_1 is the minor radius of the torus, the distance from the middle of the ring of the donut to the outer surface. The following object declaration is for a torus having major radius 6.3 minor radius 3.5 (Making the maximum width just under 20).

  // Torus having major radius sqrt(40), minor radius sqrt(12)
  quartic {
    < 1,   0,   0,   0,   2,   0,   0,   2,   0,
   -104,   0,   0,   0,   0,   0,   0,   0,   0,
      0,   0,   1,   0,   0,   2,   0,  56,   0,
      0,   0,   0,   1,   0, -104,  0, 784 >

Poly, cubic and quartics are just like quadrics in that you do not have to understand one to use one. The file shapesq.inc has plenty of pre-defined quartics for you to play with.

Polys use highly complex computations and will not always render perfectly. If the surface is not smooth, has dropouts, or extra random pixels, try using the optional keyword sturm in the definition. This will cause a slower but more accurate calculation method to be used. Usually, but not always, this will solve the problem. If sturm does not work, try rotating or translating the shape by some small amount.

There are really so many different polynomial shapes, we cannot even begin to list or describe them all. We suggest you find a good reference or text book if you want to investigate the subject further. Quadric

The quadric object can produce shapes like paraboloids (dish shapes) and hyperboloids (saddle or hourglass shapes). It can also produce ellipsoids, spheres, cones, and cylinders but you should use the sphere, cone, and cylinder objects built into POV-Ray because they are faster than the quadric version.

Note: do not confuse "quaDRic" with "quaRTic". A quadric is a 2nd order polynomial while a quartic is 4th order.

Quadrics render much faster and are less error-prone but produce less complex objects. The syntax is:


Although the syntax actually will parse 3 vector expressions followed by a float, we traditionally have written the syntax as above where A through J are float expressions. These 10 float that define a surface of x, y, z points which satisfy the equation A x2 + B y2 + C z2 + D xy + E xz + F yz + G x + H y + I z + J = 0

Different values of A, B, C, ... J will give different shapes. If you take any three dimensional point and use its x, y and z coordinates in the above equation the answer will be 0 if the point is on the surface of the object. The answer will be negative if the point is inside the object and positive if the point is outside the object. Here are some examples:

X2 + Y2 + Z2 - 1 = 0 Sphere
X2 + Y2 - 1 = 0 Infinite cylinder along the Z axis
X2 + Y2 - Z2 = 0 Infinite cone along the Z axis

The easiest way to use these shapes is to include the standard file shapes.inc into your program. It contains several pre-defined quadrics and you can transform these pre-defined shapes (using translate, rotate and scale) into the ones you want. For a complete list, see the file shapes.inc.