Isophote

Short description: Curve on an illuminated surface through points of equal brightness

ellipsoid with isophotes (red)

In geometry, an isophote is a curve on an illuminated surface that connects points of equal brightness. One supposes that the illumination is done by parallel light and the brightness b is measured by the following scalar product:

$b(P)=\overrightarrow{n}(P)\cdot \overrightarrow{v}=\cos \phi$

where $\overrightarrow{n}(P)$ is the unit normal vector of the surface at point P and $\overrightarrow{v}$ the unit vector of the light's direction. If b(P) = 0, i.e. the light is perpendicular to the surface normal, then point P is a point of the surface silhouette observed in direction $\overrightarrow{v}.$ Brightness 1 means that the light vector is perpendicular to the surface. A plane has no isophotes, because every point has the same brightness.

In astronomy, an isophote is a curve on a photo connecting points of equal brightness.

Application and example

In computer-aided design, isophotes are used for checking optically the smoothness of surface connections. For a surface (implicit or parametric), which is differentiable enough, the normal vector depends on the first derivatives. Hence, the differentiability of the isophotes and their geometric continuity is 1 less than that of the surface. If at a surface point only the tangent planes are continuous (i.e. G1-continuous), the isophotes have there a kink (i.e. is only G0-continuous).

In the following example (s. diagram), two intersecting Bezier surfaces are blended by a third surface patch. For the left picture, the blending surface has only G1-contact to the Bezier surfaces and for the right picture the surfaces have G2-contact. This difference can not be recognized from the picture. But the geometric continuity of the isophotes show: on the left side, they have kinks (i.e. G0-continuity), and on the right side, they are smooth (i.e. G1-continuity).

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  Isophotes on two Bezier surfaces and a G1-continuous (left) and G2-continuous (right) blending surface: On the left the isophotes have kinks and are smooth on the right

Determining points of an isophote

On an implicit surface

For an implicit surface with equation $f(x,y,z)=0,$ the isophote condition is $$\frac{\mathrm{\nabla }f\cdot \overrightarrow{v}}{|\mathrm{\nabla }f|}=c.$$ That means: points of an isophote with given parameter c are solutions of the nonlinear system $$\begin{array}{rl}f(x,y,z)& =0,\\ \mathrm{\nabla }f(x,y,z)\cdot \overrightarrow{v}-c|\mathrm{\nabla }f(x,y,z)|& =0,\end{array}$$ which can be considered as the intersection curve of two implicit surfaces. Using the tracing algorithm of Bajaj et al. (see references) one can calculate a polygon of points.

On a parametric surface

In case of a parametric surface $\overrightarrow{x}=\overrightarrow{S}(s,t)$ the isophote condition is

$$\frac{({\overrightarrow{S}}_s\times {\overrightarrow{S}}_t)\cdot \overrightarrow{v}}{|{\overrightarrow{S}}_s\times {\overrightarrow{S}}_t|}=c.$$

which is equivalent to $$({\overrightarrow{S}}_s\times {\overrightarrow{S}}_t)\cdot \overrightarrow{v}-c|{\overrightarrow{S}}_s\times {\overrightarrow{S}}_t|=0.$$ This equation describes an implicit curve in the s-t-plane, which can be traced by a suitable algorithm (see implicit curve) and transformed by $\overrightarrow{S}(s,t)$ into surface points.

See also

• Contour line

References

• J. Hoschek, D. Lasser: Grundlagen der geometrischen Datenverarbeitung, Teubner-Verlag, Stuttgart, 1989, ISBN:3-519-02962-6, p. 31.
• Z. Sun, S. Shan, H. Sang et al.: Biometric Recognition, Springer, 2014, ISBN:978-3-319-12483-4, p. 158.
• C.L. Bajaj, C.M. Hoffmann, R.E. Lynch, J.E.H. Hopcroft: Tracing Surface Intersections, (1988) Comp. Aided Geom. Design 5, pp. 285–307.
• C. T. Leondes: Computer Aided and Integrated Manufacturing Systems: Optimization methods, Vol. 3, World Scientific, 2003, ISBN:981-238-981-4, p. 209.

External links

• Patrikalakis-Maekawa-Cho: Isophotes (engl.)
• A. Diatta, P. Giblin: Geometry of Isophote Curves
• Jin Kim: Computing Isophotes of Surface of Revolution and Canal Surface