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:

${\displaystyle b(P)={\vec {n))(P)\cdot {\vec {v))=\cos \varphi }$

where ⁠${\displaystyle {\vec {n))(P)}$⁠ is the unit normal vector of the surface at point P and ⁠${\displaystyle {\vec {v))}$⁠ 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 ⁠${\displaystyle {\vec {v)).}$⁠ 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. ^[1]

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).

• 
  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

For an implicit surface with equation ${\displaystyle f(x,y,z)=0,}$ the isophote condition is $${\displaystyle {\frac {\nabla f\cdot {\vec {v))}{|\nabla f|))=c\ .}$$ That means: points of an isophote with given parameter c are solutions of the nonlinear system $${\displaystyle {\begin{aligned}f(x,y,z)&=0,\\[4pt]\nabla f(x,y,z)\cdot {\vec {v))-c\;|\nabla f(x,y,z)|&=0,\end{aligned))}$$ 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.

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

$${\displaystyle {\frac {({\vec {S))_{s}\times {\vec {S))_{t})\cdot {\vec {v))}{|{\vec {S))_{s}\times {\vec {S))_{t}|))=c\ .}$$

which is equivalent to $${\displaystyle \ ({\vec {S))_{s}\times {\vec {S))_{t})\cdot {\vec {v))-c\;|{\vec {S))_{s}\times {\vec {S))_{t}|=0\ .}$$ 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 ${\displaystyle {\vec {S))(s,t)}$ into surface points.

• Contour line

• 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.

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