In mathematics, an implicit surface is a surface in Euclidean space defined by an equation

${\displaystyle F(x,y,z)=0.}$

An implicit surface is the set of zeros of a function of three variables. Implicit means that the equation is not solved for x or y or z.

The graph of a function is usually described by an equation ${\displaystyle z=f(x,y)}$ and is called an explicit representation. The third essential description of a surface is the parametric one: ${\displaystyle (x(s,t),y(s,t),z(s,t))}$, where the x-, y- and z-coordinates of surface points are represented by three functions ${\displaystyle x(s,t)\,,y(s,t)\,,z(s,t)}$ depending on common parameters ${\displaystyle s,t}$. Generally the change of representations is simple only when the explicit representation ${\displaystyle z=f(x,y)}$ is given: ${\displaystyle z-f(x,y)=0}$ (implicit), ${\displaystyle (s,t,f(s,t))}$ (parametric).

Examples:

1. The plane ${\displaystyle x+2y-3z+1=0.}$
2. The sphere ${\displaystyle x^{2}+y^{2}+z^{2}-4=0.}$
3. The torus ${\displaystyle (x^{2}+y^{2}+z^{2}+R^{2}-a^{2})^{2}-4R^{2}(x^{2}+y^{2})=0.}$
4. A surface of genus 2: ${\displaystyle 2y(y^{2}-3x^{2})(1-z^{2})+(x^{2}+y^{2})^{2}-(9z^{2}-1)(1-z^{2})=0}$ (see diagram).
5. The surface of revolution ${\displaystyle x^{2}+y^{2}-(\ln(z+3.2))^{2}-0.02=0}$ (see diagram wineglass).

For a plane, a sphere, and a torus there exist simple parametric representations. This is not true for the fourth example.

The implicit function theorem describes conditions under which an equation ${\displaystyle F(x,y,z)=0}$ can be solved (at least implicitly) for x, y or z. But in general the solution may not be made explicit. This theorem is the key to the computation of essential geometric features of a surface: tangent planes, surface normals, curvatures (see below). But they have an essential drawback: their visualization is difficult.

If ${\displaystyle F(x,y,z)}$ is polynomial in x, y and z, the surface is called algebraic. Example 5 is non-algebraic.

Despite difficulty of visualization, implicit surfaces provide relatively simple techniques to generate theoretically (e.g. Steiner surface) and practically (see below) interesting surfaces.

Throughout the following considerations the implicit surface is represented by an equation ${\displaystyle F(x,y,z)=0}$ where function ${\displaystyle F}$ meets the necessary conditions of differentiability. The partial derivatives of ${\displaystyle F}$ are ${\displaystyle F_{x},F_{y},F_{z},F_{xx},\ldots }$.

A surface point ${\displaystyle (x_{0},y_{0},z_{0})}$ is called regular if and only if the gradient of ${\displaystyle F}$ at ${\displaystyle (x_{0},y_{0},z_{0})}$ is not the zero vector ${\displaystyle (0,0,0)}$, meaning

${\displaystyle (F_{x}(x_{0},y_{0},z_{0}),F_{y}(x_{0},y_{0},z_{0}),F_{z}(x_{0},y_{0},z_{0}))\neq (0,0,0)}$.

If the surface point ${\displaystyle (x_{0},y_{0},z_{0})}$ is not regular, it is called singular.

The equation of the tangent plane at a regular point ${\displaystyle (x_{0},y_{0},z_{0})}$ is

${\displaystyle F_{x}(x_{0},y_{0},z_{0})(x-x_{0})+F_{y}(x_{0},y_{0},z_{0})(y-y_{0})+F_{z}(x_{0},y_{0},z_{0})(z-z_{0})=0,}$

and a normal vector is

${\displaystyle \mathbf {n} (x_{0},y_{0},z_{0})=(F_{x}(x_{0},y_{0},z_{0}),F_{y}(x_{0},y_{0},z_{0}),F_{z}(x_{0},y_{0},z_{0}))^{T}.}$

In order to keep the formula simple the arguments ${\displaystyle (x_{0},y_{0},z_{0})}$ are omitted:

${\displaystyle \kappa _{n}={\frac {\mathbf {v} ^{\top }H_{F}\mathbf {v} }{\|\operatorname {grad} F\|))}$

is the normal curvature of the surface at a regular point for the unit tangent direction ${\displaystyle \mathbf {v} }$. ${\displaystyle H_{F))$ is the Hessian matrix of ${\displaystyle F}$ (matrix of the second derivatives).

The proof of this formula relies (as in the case of an implicit curve) on the implicit function theorem and the formula for the normal curvature of a parametric surface.

As in the case of implicit curves it is an easy task to generate implicit surfaces with desired shapes by applying algebraic operations (addition, multiplication) on simple primitives.

The electrical potential of a point charge ${\displaystyle q_{i))$ at point ${\displaystyle \mathbf {p} _{i}=(x_{i},y_{i},z_{i})}$ generates at point ${\displaystyle \mathbf {p} =(x,y,z)}$ the potential (omitting physical constants)

${\displaystyle F_{i}(x,y,z)={\frac {q_{i)){\|\mathbf {p} -\mathbf {p} _{i}\|)).}$

The equipotential surface for the potential value ${\displaystyle c}$ is the implicit surface ${\displaystyle F_{i}(x,y,z)-c=0}$ which is a sphere with center at point ${\displaystyle \mathbf {p} _{i))$.

The potential of ${\displaystyle 4}$ point charges is represented by

${\displaystyle F(x,y,z)={\frac {q_{1)){\|\mathbf {p} -\mathbf {p} _{1}\|))+{\frac {q_{2)){\|\mathbf {p} -\mathbf {p} _{2}\|))+{\frac {q_{3)){\|\mathbf {p} -\mathbf {p} _{3}\|))+{\frac {q_{4)){\|\mathbf {p} -\mathbf {p} _{4}\|)).}$

For the picture the four charges equal 1 and are located at the points ${\displaystyle (\pm 1,\pm 1,0)}$. The displayed surface is the equipotential surface (implicit surface) ${\displaystyle F(x,y,z)-2.8=0}$.

A Cassini oval can be defined as the point set for which the product of the distances to two given points is constant (in contrast, for an ellipse the sum is constant). In a similar way implicit surfaces can be defined by a constant distance product to several fixed points.

In the diagram metamorphoses the upper left surface is generated by this rule: With

${\displaystyle {\begin{aligned}F(x,y,z)={}&{\Big (}{\sqrt {(x-1)^{2}+y^{2}+z^{2))}\cdot {\sqrt {(x+1)^{2}+y^{2}+z^{2))}\\&\qquad \cdot {\sqrt {x^{2}+(y-1)^{2}+z^{2))}\cdot {\sqrt {x^{2}+(y+1)^{2}+z^{2))}{\Big )}\end{aligned))}$

the constant distance product surface ${\displaystyle F(x,y,z)-1.1=0}$ is displayed.

A further simple method to generate new implicit surfaces is called metamorphosis of implicit surfaces:

For two implicit surfaces ${\displaystyle F_{1}(x,y,z)=0,F_{2}(x,y,z)=0}$ (in the diagram: a constant distance product surface and a torus) one defines new surfaces using the design parameter ${\displaystyle \mu \in [0,1]}$:

${\displaystyle F(x,y,z)=\mu F_{1}(x,y,z)+(1-\mu )F_{2}(x,y,z)=0}$

In the diagram the design parameter is successively ${\displaystyle \mu =0,\,0.33,\,0.66,\,1}$ .

${\displaystyle \Pi }$-surfaces ^[1] can be used to approximate any given smooth and bounded object in ${\displaystyle R^{3))$ whose surface is defined by a single polynomial as a product of subsidiary polynomials. In other words, we can design any smooth object with a single algebraic surface. Let us denote the defining polynomials as ${\displaystyle f_{i}\in \mathbb {R} [x_{1},\ldots ,x_{n}](i=1,\ldots ,k)}$. Then, the approximating object is defined by the polynomial

${\displaystyle F(x,y,z)=\prod _{i}f_{i}(x,y,z)-r}$^[1]

where ${\displaystyle r\in \mathbb {R} }$ stands for the blending parameter that controls the approximating error.

Analogously to the smooth approximation with implicit curves, the equation

${\displaystyle F(x,y,z)=F_{1}(x,y,z)\cdot F_{2}(x,y,z)\cdot F_{3}(x,y,z)-r=0}$

represents for suitable parameters ${\displaystyle c}$ smooth approximations of three intersecting tori with equations

${\displaystyle {\begin{aligned}F_{1}=(x^{2}+y^{2}+z^{2}+R^{2}-a^{2})^{2}-4R^{2}(x^{2}+y^{2})=0,\\[3pt]F_{2}=(x^{2}+y^{2}+z^{2}+R^{2}-a^{2})^{2}-4R^{2}(x^{2}+z^{2})=0,\\[3pt]F_{3}=(x^{2}+y^{2}+z^{2}+R^{2}-a^{2})^{2}-4R^{2}(y^{2}+z^{2})=0.\end{aligned))}$

(In the diagram the parameters are ${\displaystyle R=1,\,a=0.2,\,r=0.01.}$)

There are various algorithms for rendering implicit surfaces,^[2] including the marching cubes algorithm.^[3] Essentially there are two ideas for visualizing an implicit surface: One generates a net of polygons which is visualized (see surface triangulation) and the second relies on ray tracing which determines intersection points of rays with the surface.^[4] The intersection points can be approximated by sphere tracing, using a signed distance function to find the distance to the surface.^[5]

• Implicit curve

• Gomes, A., Voiculescu, I., Jorge, J., Wyvill, B., Galbraith, C.: Implicit Curves and Surfaces: Mathematics, Data Structures and Algorithms, 2009, Springer-Verlag London, ISBN 978-1-84882-405-8
• Thorpe: Elementary Topics in Differential Geometry, Springer-Verlag, New York, 1979, ISBN 0-387-90357-7

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