OpenGL tesselation 2

You can log, what geometry-description is used from glu in your glBegin-callback passed to:

  gluTessCallback(tess, GLU_TESS_BEGIN, ...

With the code from your last question, that would be:

void tcbBegin(GLenum which)
{    
    const char *whichStr;
    switch(which)
    {
        case GL_TRIANGLE_FAN:
            whichStr = "GL_TRIANGLE_FAN";
            break;
        case GL_TRIANGLE_STRIP:
            whichStr = "GL_TRIANGLE_STRIP";        
            break;
        case GL_TRIANGLES:
            whichStr = "GL_TRIANGLES";        
            break;
        case GL_LINE_LOOP:
            whichStr = "GL_LINE_LOOP";        
            break:
        default:
            whichStr = "<Unknown>";
            break;            
   }
   printf("tcbBegin(%s)\n", whichStr); 
  
   glBegin(which);
}

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Thanks. So how do I change my code so I can get the triangles?

It's actually a triangle fan btw

>> My task was to use the OpenGL tesselation functions so I could get the list of triangles out for any given polygon (with holes, later).

That will work anyway, you just have to store the points and the trianglulation type for a set of points(GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, GL_TRIANGLES), thats it :)

I just want to get the triangles out. Obviously, it's easy to do manually with quads but I will have polygons with any number of vertices / holes.

I've done a similar thing in the past with more detailed code and that worked ok, I'm just trying to do the same thing on a simple example.

I'm using the triangles to create a mesh in Ogre3D and am storing the points from the callback function as well as the triangulation type.

However, if I add a hole then there are two calls to the begin callback function. One is type 0 (triangle fan) and the other is type 1 (triangle strip).

How do I know which tessellation type to create the mesh with? Ogre has triangle list, strip and fan but how do I choose which one is correct to use? I'm talking about all possible cases.

Also, surely there are combinations of polygons and holes that cannot be represented by a triangle fan or strip? But a list of triangles will always work. Consider the case where the hole splits the polyogn into two pieces - that's why I always wanted to get a triangle list.

The other code seems to always produce triangle lists so I'm wondering if it is telling glu to always use triangle lists even though they may be less efficient?

>> How do I know which tessellation type to create the mesh with?

Store it with the points, like you said here:

"and am storing the points from the callback function as well as the triangulation type."

Yes, but there are two calls to the begin callback, one is a triangle strip and one is a triangle fan so which one do I know will be correct?

>> Also, surely there are combinations of polygons and holes that cannot be represented by a triangle fan or strip?

Right, you would have to use more than 1 lists for a mesh, but thats normal.


>> The other code seems to always produce triangle lists

What other code?

The other code is a massive project, too large to post here. It was there that I first learned I could intercept the triangles during the tessellation process.

So does the begin callback function get called for every seperate list / strip / fan or once for every contour I put in?

If there's some way to just tell glu to use triangle lists then it would be so much easier.

>> So does the begin callback function get called for every seperate list / strip / fan or once for every contour I put in?

Yes.

>> If there's some way to just tell glu to use triangle lists then it would be so much easier.

No.

Yes? Which one? Every seperate list / strip / fan or every contour?

Taking it one step back. It would be much easier, if you use any 3D-Modeling Tool (Blender, 3Ds Max, Maya) and export it in a format that can be read by Ogre, that'll safe you a lot of trouble.

I'm writing a modelling tool so exporting isn't an option unfortunately.

How do I know how many vertices are for the strip and how many for the fan?

Ahhhh, gotcha (I think)

I mean something like this:

#include <vector>

struct point 
{
    // convenience ctor
    point(GLdouble _x, GLdouble _y, GLdouble _z) : x(_x), y(_y), z(_z) {}
    GLdouble x, y, z; 
};

// represents a vertex list and its geometry type
struct VertexList
{
    // GL_TRIANGLE_FAN, GL_TRIANGLE_STRIP, GL_TRIANGLES or GL_LINE_LOOP
    GLenum geometryType;
    // dynamic array of points
    std::vector<point> points;
};

// array from vertex-lists
std::vector<VertexList> vertexLists;

void tcbBegin(GLenum which)
{
    glBegin(which);

    // add a new vertex list
    vertexLists.resize(vertexLists.size() + 1);

    // and set the geometry type of the new vertex list
    int currentList = vertexLists.size() - 1;
    vertexLists[currentList].geometryType = which;
}

void tcbEnd(void)
{
    glEnd();
}

void tcbVertex(GLvoid *vertex)
{
    const GLdouble *pointer;
    pointer = (GLdouble *) vertex;
    glVertex3dv(pointer);

    int currentList = vertexLists.size() - 1;
    // add this point to the current vertex list
    vertexLists[currentList].points.push_back(point(pointer[0], pointer[1], pointer[2]));
}

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That's great, cheers

Do you need any more assistance here?

Not sure, working on it at the moment...

Ok that seems to be working, when accepting a solution does it affect your points if I just pick one of your replies? Really it's the whole discussion that's helped!

Great to hear it helped

Its best to choose the posts that helped you. it it was more than one post, pick them too. That will lead other readers searching the database to the solution, so its better for the community too.

Points are split between those answers, but since its all my answers, I get the points anyway.

Good luck for your project :)

ike

Thanks, this all seems to be working properly now.

Ike is a legend