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dalahimself

Holes and contours...

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How do I make a hole in a surface or cut a contour around a surface? To try to explain more clearly; I start with a simple square, which represents a piece of paper, and the paper has a picture on it. (The picture is put on the square as a texture.) How can I make a hole in the paper? ...and i also want to "contour-cut" the paper, ie. I have a picture texturemapped on a square, and I want to be able cut the paper to different shapes. I can of course just start with the required shape, but then the problem is to get the texture correctly mapped to the irregularly shaped surface. Does anyone understand my question :), and if you do, can you give me any advice on how to solve the problems? Thanks in advance...

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I am not sure what you mean by the mapping problems, but as far as displaying the hole:

If you are just looking for a solution to display the hole as transparent I would use alpha transparency on the mapped texture.

If you are looking to alter the mesh I would suggest the following solution (untested):
1) Detect all of the intersection points on the mesh edges between the paper and the cut's you want to make.
2) Tessalate the mesh in the following way: For each edge detected which intersects the contour split he edge into 2 egdes (and split all triangles who own that edge by passing a new edge to the opposing vertex).
Do not count vertices who are directly on the contour as edge intersections.
3)Repeat 2 until there are no more edges intersecting the contour.
4)Once done with 2+3 look for triangles who have all three vertecies on the contour. Add a vertex in the center of these and split into three different triangles
5) Repeat 4 until no face remains for which all triangles are on the contour.
6) Remove all triangles with a single vertex (or more) inside/outside the contour (depending if you want the outside or inside of the shape.).
Do not count vertices who are directly on the contour as inside/outside.


I think that should do it.

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