GL_LINE_LOOP ordering

anyone know of a quicky way to reorder a list of triangles so that i can put the begin and end statements outside of the loop? the triangles are in no particular order at the moment and this causes a big mess when the line loop is allowed to run through the whole list. <code below>

having a list of length 3 is not very efficient. and i assume that using triangles would be even slower.

any help appreciated


p.s. this is stored in a display list.

t = m_TriangleList.GetNext(pos);
glVertex3d((t->Node1).x, (t->Node1).y, (t->Node1).z); glVertex3d((t->Node2).x, (t->Node2).y, (t->Node2).z);
glVertex3d((t->Node3).x, (t->Node3).y, (t->Node3).z);

[This message has been edited by cunny (edited 08-19-2002).]

If there’s no special reason for doing the the line loops yourself, I would use glPolygonMode(GL_FRONT_AND_BACK, GL_LINE), which should do almost the same.
“Almost” because you still draw triangles and some OpenGL state don’t apply to non-faced primitives like lines.
(edit) Some performance notes:

  • Check if you can use glVertex3dv().
  • Even faster would be to use single precision floats in the node struct, and calling glVertex3fv.

[This message has been edited by Relic (edited 08-22-2002).]

Either way – the line-loops or the GL_POLYGON_MODE hack – has the same problem: edges that are shared between two triangles (almost a certainty) will be drawn twice. If you have a non-manifold surface and can have even more than two triangles per edge. To solve this problem, you need a boundary representation such as a half-edge or winged-edge data structure (czech it out on Flipcode, or Google-it).

If you have an entire mesh (that is, at least a dozen tris or so) with each triangle outlined in it’s own Begin/End all contained in a single display list, I’ll bet that you’re not going to pay for the overhead of having all those “redundant” calls except when you GL_COMPILE them. Watch out for GL_COMPILE_AND_EXECUTE, it doesn’t necessarily do what you think.

But really, you should solve the problem at a representational level, and everyone knows that you should be using indexed vertex arrays, anyway!


the problem here is that i have 100,000 triangles, and nothing special by means of gfx card. i also have to draw the points and the filled triangles simultaneously. sounds wierd but i do!!

i know that having the begin/end within the loop is slow, but triangles for wireframe is also slower than line loops… will try the the other vertex commands though.

any other suggestions?

Maybe you misunderstood my first suggestion. It should move the glBegin(GL_TRIANGLES) out of the loop, of course.
The complete begin-end overhead for line loops is saved like this:

t = m_TriangleList.GetNext(pos);
glVertex3dv((GLdouble *) t->Node1);
glVertex3dv((GLdouble *) t->Node2);
glVertex3dv((GLdouble *) t->Node3);

Don’t know if the (GLdouble *) cast will work for your struct.

If that’s not fast enough you could convert the data into an edge representation only drawing each shared edge once with glBegin(GL_LINES) around the whole loop, like Won suggested.