Is there an efficient\easy way to draw a concave polygon in Direct3d

Question

I'm trying to draw a polygon using c# and directx

All I get is an ordered list of points from a file and I need to draw the flat polygon in a 3d world.

I can load the points and draw a convex shape using a trianglefan and drawuserprimitives.

This obviously leads to incorrect results when the polygon is very concave (which it may be).

I can't imagine I'm the only person to grapple with this problem (tho I'm a gfx/directx neophyte - my background is in gui\windows application development).

Can anyone point me towards a simple to follow resource\tutorial\algorithm which may assist me?

Answer 1

Direct3D can only draw triangles (well, it can draw lines and points as well, but that's besides the point). So if you want to draw any shape that is more complex than a triangle, you have to draw a bunch of touching triangles that equal to that shape.

In your case, it's a concave polygon triangulation problem. Given a bunch of vertices, you can keep them as is, you just need to compute the "index buffer" (in simplest case, three indices per triangle that say which vertices the triangle uses). Then draw that by putting into vertex/index buffers or using DrawUserPrimitives.

Some algorithms for triangulating simple (convex or concave, but without self-intersections or holes) polygons are at VTerrain site.

I have used Ratcliff's code in the past; very simple and works well. VTerrain has a dead link to it; the code can be found here. It's C++, but porting that over to C# should be straightforward.

Oh, and don't use triangle fans. They are of very limited use, inefficient and are going away soon (e.g. Direct3D 10 does not support them anymore). Just use triangle lists.