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Alpha Values of Points on a Graph

asked 2011-07-14 11:04:29 -0500

anonymous user


updated 2015-01-14 03:13:37 -0500

FrédéricC gravatar image

Hello, I'm working on a project that requires the overlapping of many distinct polygons. I have the polygons set up so that they each have a small alpha value so that the overlaps can be seen visually. However, when alpha is very small, it is hard to tell when there is overlap between twelve polygons or thirteen polygons merely by looking at the graph since the transparency difference is so minimal. Because of this, I would like the program user to be able to specify a point in the plane, and be returned what alpha is equal to. Is this feasible?

Thanks in advance, and I'll be glad to specify any additional details that are necessary.

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answered 2013-04-22 08:21:40 -0500

updated 2013-04-22 08:24:01 -0500


The graphical objects are quite static (you can not use them for computations). If you would like to find intersections between several polyhedra you may use the object Polyhedron for which it is possible to compute intersections:

sage: P = Polyhedron([(0,0),(1,0),(2,2),(3,1)])
sage: Q = Polyhedron([(-1,-3),(1,1),(2,4),(4,-5)])
sage: Z = P.intersection(Q)
sage: Z.vertices_list()
[[18/7, 10/7], [27/10, 17/20], [1, 0], [2, 2], [1/2, 1/2], [2/7, 0]]

As intersection of polyhedra are polyhedra you may plot them as well.

sage: Z.plot()

Note that in sage-5.9 there will be fine options to plot polyhedron (ticket #14175).

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Asked: 2011-07-14 11:04:29 -0500

Seen: 99 times

Last updated: Apr 22 '13