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Intersection curve : ウィキペディア英語版
Intersection curve
In geometry, an intersection curve is, in the most simple case, the intersection line of two non-parallel planes in Euclidean 3-space. In general, an intersection curve consists of the common points of two ''transversally'' intersecting surfaces, meaning that at any common point the surface normals are not parallel. This restriction excludes cases where the surfaces are touching or have surface parts in common.
The analytic determination of the intersection curve of two surfaces is easy only in simple cases; for example: a) the intersection of two planes, b) plane section of a quadric (sphere, cylinder, cone, etc.), c) intersection of two quadrics in special cases. For the general case, literature provides algorithms, in order to calculate points of the intersection curve of two surfaces.〔(''Geometry and Algorithms for COMPUTER AIDED DESIGN'' ), p. 94〕
== Intersection line of two planes ==
Given: two planes \varepsilon_i: \quad \vec n_i\cdot\vec x=d_i, \quad i=1,2, \quad
\vec n_1,\vec n_2 linearly independent, i.e. the planes are not parallel.
Wanted: A parametric representation \vec x= \vec p + t\vec r of the intersection line.
The direction of the line one gets from the crossproduct of the normal vectors: \vec r=\vec n_1\times\vec n_2.
A point P:\vec p of the intersection line can be determined by intersecting the given planes \varepsilon_1, \varepsilon_2 with the plane \varepsilon_3: \vec x = s_1\vec n_1 + s_2\vec n_2, which is perpendicular to \varepsilon_1 and \varepsilon_2. Inserting the parametric representation of \varepsilon_3 into the equations of \varepsilon_1 und \varepsilon_2 yields the parameters s_1 und s_2.
P: \vec p= \frac
\vec n_1
+ \frac \vec n_2\ .
''Example:''
\varepsilon_1:\ x+2y+z=1, \quad \varepsilon_2:\ 2x-3y+2z=2 \ .
The normal vectors are \vec n_1=(1,2,1)^\top, \ \vec n_2=(2,-3,2)^\top and the direction of the intersection line is \vec r=\vec n_1\times\vec n_2=(7,0,-7)^\top. For point P:\vec p, one gets from the formula above \vec p=\tfrac(1,0,1)^\top \ . Hence
:\vec x=\tfrac(1,0,1)^\top + t (7,0,-7)^\top
is a parametric representation of the line of intersection.
''Remarks:''
# In special cases, the determination of the intersection line by the Gaussian elimination may be faster.
# If one (or both) of the planes is given parametrically by \vec x= \vec p + s\vec v + t \vec w , one gets \vec n = \vec v \times \vec w as normal vector and the equation is: \vec n\cdot \vec x = \vec n\cdot \vec p.

抄文引用元・出典: フリー百科事典『 ウィキペディア(Wikipedia)
ウィキペディアで「Intersection curve」の詳細全文を読む



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