The Mathematica graphics language has a Polygon primitive, but (as of Mathematica 7) there is no way to create a Polygon with holes punched out (more specifically, any polygon with two or more contours). However, there is a fairly good way to fake this feature.
- Draw the first polygon contour.
- Draw a line from the ending point of the first contour to the starting point of the second contour.
- Draw the second polygon contour.
- Repeat steps 1-3 for N contours
- Once all contours are drawn, draw a line from the ending point of contour N-1, then N-2, ..., all the way back to contour 1. These are the same ending points we used earlier, except in reverse order.
MultiContourPolygon[polys_List] := Module[
{contours=Append[#, First[#]]&/@polys},
{
Polygon@Join[Join@@contours, Reverse[First/@contours]],
Line[contours]
}
]
{contours=Append[#, First[#]]&/@polys},
{
Polygon@Join[Join@@contours, Reverse[First/@contours]],
Line[contours]
}
]
Graphics@MultiContourPolygon[{
{{0, 0}, {0, 1}, {1, 1}, {1, 0}},
{{.25, .25}, {.75, .25}, {.75, .75}, {.25, .75}},
{{.4, .4}, {.6, .4}, {.6, .6}, {.4, .6}}
}]
{{0, 0}, {0, 1}, {1, 1}, {1, 0}},
{{.25, .25}, {.75, .25}, {.75, .75}, {.25, .75}},
{{.4, .4}, {.6, .4}, {.6, .6}, {.4, .6}}
}]
Mathematica's built-in PDF import feature uses this same algorithm to represent arbitrary multi-contoured paths from PDF files.
Import["ExampleData/mathematica.pdf"]
3 comments:
Thank you for the info, it was really helpful! And it can be extended to 3-D by just setting the points in 3 dimensions!
Good post. Thank you.
For computations that depend on the edges of the polygon, such as intersection calculations, this isn't idea, but it gets the job done in many cases.
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