Curve on an illuminated surface through points of equal brightness
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:
where is the unit
normal vector of the surface at point P and 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 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.
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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).
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
That means: points of an isophote with given parameter c are solutions of the
nonlinear system
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.
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 into surface points.