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I create a box with a material and set that material to be wire framed.

Box b1 = new Box(new Vector3f(1.0f, -2.0f, 1.0f), 1.0f, 1.0f, 1.0f);
Geometry geom1 = new Geometry("Box", b1);
Material mat1 = new Material(assetManager, "Common/MatDefs/Misc/Unshaded.j3md");
mat1.getAdditionalRenderState().setWireframe(true);
mat1.setColor("Color", ColorRGBA.Blue);
geom1.setMaterial(mat1);

However, because the box is drawn using triangles there are diagonal wires visible:

Screenshot

How can I get rid of these diagonal wires (not draw them)?

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Why? Wireframe is a debugging tool that makes polygons visible, it's not intended as a graphical effect. If you want to be selective about what wires to draw you probably need to implement your own wireframe renderer. –  eBusiness Jan 28 '12 at 22:27

3 Answers 3

up vote 8 down vote accepted

As said in the comments. Wireframe rendering is used to show what polygons are being drawn, and is generally only used when debugging.

To solve you problem...

What you want is to only draw lines around the edge of the cube. And there are many ways you can acheive this...

  1. Render a texture on to the cube, which has alpha everywhere but the edges.
  2. Use a pixel shader to turn all pixels that are not at the edge of the cube to alpha. <- This is recommended by me, as you will be able to set the line length around the cube.
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I found the following resource that might be helpful in implementing the technique described by Twitchy, although it's written for Direct3D. It's taken from the NVIDIA Direct3D SDK 10 Code Samples page. Quoting the page:

Render the wireframe of a mesh using filled triangles, coloring only the fragments near the edges of the triangle. This technique produce high quality thick antialised lines on the mesh edges, without any z fighting issues. This technique is a DirectX10 implementation of the SIGGRAPH 2006 sketch "Single-Pass Wireframe Rendering". This technique can be easily added in an existing graphics engine as it only requires one pass and all algorithm is based on the position of the vertices.

And you can find a whitepaper describing the technique in detail here. The downside is that it requires a geometry shader.

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I don't know how you are drawing, what libraries you are using...

In DirectX and OpenGL, you usually use vertex and index buffers to hold your geometry data. If the triangles in whatever format you are storing them in are still in the order of the quads they appear in, as is the case with at least some OBJ exporters, it can be relatively easy to figure out which vertices form a quad.

For example, in an obj file, a quad with the vertices 1,2,3 and 4 that was translated to triangles upon export might look like this (pseudo code):

f 1 2 3 f 3 4 1

which would look pretty much the same in your index buffer. A very simple solution would be to keep a second index buffer for your quads, where you could store the indices 1,2,2,3,3,4,4,1 or even 1,2,3,4,1. In directx there are the linelist and linestrip topology types which would render these indices as lines treated with your regular shading.

If you create your own file format for storing models in, it becomes easier as you could store quads where possible and tris when necessary in the file, and only translate them to triangles upon import.

I've done this, and since I usually create my 3D models with quads and only use tris where necessary I find a quad wireframe much more intelligible. It is relatively easy to implement provided you are lucky and your storage format accomodates you, but it does have a number of drawbacks. For one thing it does not produce thick and soft wireframes. It also does not produce a shaded wireframe, but a completely transparent one, so to have your wireframe ontop of your regularly shaded geometry you would have to first draw your object and then draw your wireframe, so there is a performance hit. Still, even drawing the objects twice, once shaded once as wire, this still performs better than just directx' wireframe RasterizerState setting alone. This method is also likely to create z-fighting, which might make it necessary to increase the scale momentarily to avoid that.

Here you can see how it looks in my test implementation, you can see both the z-fighting and the moire patterns created by the aliasing of the lines (there are ways to draw antialised lines though): http://i.stack.imgur.com/N6X3Y.png (sorry I can't embedd images here yet)

All in all nvidia's solution David posted looks very interesting, I've seen it in the samples and I've been meaning to take a closer look at that.

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