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 is measured by the following scalar product: is the unit normal vector of the surface at point and the unitvector of the light's direction. In case of, i.e. the light is perpendicular to the surface normal, point is a point of the surface silhouette looked in direction. Brightness 1 means, the lightvector is perpendicular to the surface. A plane has no isophotes, because any point has the same brightness. In astronomy an isophote is a curve on a photo connecting points of equal brightness.
In computer-aided design isophotes are used for checking optically the smoothness of surface connections. For a surface, 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, the isophotes have there a kink. In the following example 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 and on the right side they are smooth.
For an implicit surface with equation the isophote condition is That means: points of an isophote with given parameter are solutions of the non linear system which can be considered as the intersection curve of two implicit surfaces. Using the tracing algorithm of Bajaj et al. one can calculate a polygon of points.
In case of a parametric surface the isophote condition is which is equivalent to This equation describes an implicit curve in the s-t-plane, which can be traced by a suitable algorithm and transformed by into surface points.
