GL_TRIANGLE_STRIP vs GL_TRIANGLE_FAN

opengl gl-triangle-strip
tiggares picture tiggares · Dec 5, 2013 · Viewed 51k times · Source

I need an example of a polygon that can be done only by GL_TRIANGLE_STRIP and another polygon that can be done only by GL_TRIANGLE_FAN.

Answer

vallentin picture vallentin · Dec 5, 2013

When knowing the difference between Triangle Strip and Triangle Fan a shape will be easy to make.

Triangle Strip

For instance a Triangle Strip is a set of connected triangles which share vertices.

Example of Triangle Strip

Using Triangle Strip we will be able to get the following output, using those given vertices.

enter image description here

Triangle Fan

Where a Triangle Fan is also a set of connected triangles, though all these triangles have a common vertex, which is the central vertex.

In OpenGL the central vertex is the first given vertex, in the Triangle Fan.

Example of Triangle Fan

Using Triangle Fan and the same vertices as in the other example, we will only be able to get the colored area as output. That is due to the importance of the arranged order of the vertices in Triangle Fan. Basically, all the vertices need to go around the central vertex.

enter image description here

Conclusion

As you can see on our 2 example sets of vertices those "output shapes" are unique to both Triangle Strip and Triangle Fan.

Note: The image examples uses clockwise winding order, while in OpenGL the front side uses counter-clockwise winding order, i.e. the examples are literally facing away from the camera. This is an important detail if face culling is enabled.

Extra

I made a similar answer here, you can read it if you want, I actually used the same images since the questions are closely related.

Related questions

Generate a plane with triangle strips

What would be the best algorithm to generate a list of vertices to draw a plane using triangle strips? I'm looking for a function which receives the plane's width and height and returns a float array containing correctly indexed vertices. …

Read More
What's the de facto standard of triangle winding direction in OpenGL?

We have two options in triangle winding direction, clock-wise counter-clockwise Anyway converting between them could take some cost. I want to avoid conversion as much as possible, and to do that, I need to know de facto standard of winding …

Read More
Cube using single GL_TRIANGLE_STRIP

Is it possible to draw a whole cube using just a single GL_TRIANGLE_STRIP? Obviously it's just the cube combinatorics I'm concerned about here, it might as well be stretched into any kind of box or similar object.

Read More