Draw wireframe overlay of model in GLSL

Question

I have a 3D model that has a GLSL script, and I want to draw a line on each border of its polygons in the fragment shader.

What should I do? Note: I am using Ogre3D, so I can't use OpenGL's functions.

So far I have this (just assigns a texture to it):

void main(void)
{
  gl_FragColor = texture2D(texture1, gl_TexCoord[0].st); 
}

This means if I have a cube, I want to render 12 lines, on each border of the cube

Answer 1

You may be able to do it in a shader using the linked comment above.

But if that doesn't work, you can still do it by rendering a separate batch of lines after drawing your shape.

If this is just a debug mode thing, there is no need to do this particularly efficiently: just walk the triangles of the mesh and draw a line along each edge of each triangle.

If this is going to be a runtime thing, you need to eliminate the duplicate edges that will result from adjacent triangles.

Answer 2

If you're using Ogre3D, then why don't you do add another pass to the material technique that is assigned to the model, as in (suppose "ball" is an Ogre::Entity):

Ogre::MaterialPtr ptr = ball->getSubEntity(0)->getMaterial();
Ogre::Pass * pass = ptr->getTechnique(0)->createPass();
pass->setDiffuse(1,0,0,0); 
pass->setAmbient(1,0,0);  
pass->setPolygonModeOverrideable(true);
pass->setPolygonMode(Ogre::PM_WIREFRAME);

This will add a red wireframe that gets drawn over the existing material. And if you just want a wireframe, not a wireframe on top of the current material, just substitute "getPass(0)" for "createPass()"

Or, if you would rather do it in the .material file:

material general/wireframe
{
  technique
  {
    // first pass
    pass
    {
      polygon_mode wireframe
      ...

See:

http://www.ogre3d.org/docs/manual/manual_16.html#polygon_005fmode http://www.ogre3d.org/docs/manual/manual_14.html#Material-Scripts