Convert quadriladerals in obj file to triangles?

Question

At first it seemed like it was obvious ... Make 2 triangles on the face where 4 indexes were found, right? Value:

v 1.000000 1.000000 0.000000 v -1.000000 1.000000 -0.000000 v 1.000000 -1.000000 0.000000 v -1.000000 -1.000000 -0.000000 f -4 -3 -2 -1 

..., in turn, needs to be converted to something like:

 v 1.000000 1.000000 0.000000 v -1.000000 1.000000 -0.000000 v 1.000000 -1.000000 0.000000 v -1.000000 -1.000000 -0.000000 f -4 -3 -2 f -2 -3 -1 

This particular example, of course, will display correctly.
However, not all cases are as simple as splitting a face into two faces (where the first face contains the first three vertices of the original face and the second face contains the last 3 vertices, as shown in the example above). Take the following cube for example:

 v 0.000000 1.000000 1.000000 v 0.000000 0.000000 1.000000 v 1.000000 0.000000 1.000000 v 1.000000 1.000000 1.000000 v 0.000000 1.000000 0.000000 v 0.000000 0.000000 0.000000 v 1.000000 0.000000 0.000000 v 1.000000 1.000000 0.000000 f 1 2 3 4 f 8 7 6 5 f 4 3 7 8 f 5 1 4 8 f 5 6 2 1 f 2 6 7 3 

These faces cannot be shared equally in the previous example ... So, I need some way to find out how to split a quadrangle into two faces of a triangle using correct indexes for the second person ...

How can this be achieved? Please note that I am NOT using a fixed function pipeline, and therefore using GL_QUADS is NOT an option. My rendering engine is pretty much stuck only when using GL_TRIANGLES.

Answer 1

If you have 4 indexes, for example:

 0 1 2 3 

The division into two triangles will be one with the first three indices, and one with the first, third and fourth. In this example:

 0 1 2 0 2 3 

Try using some ASCII art to illustrate this:

 3-------2 | /| | / | | / | |/ | 0-------1 

Here you see 0 1 2 3 as a square, 0 1 2 as the first triangle (bottom right) and 0 2 3 as the second triangle (top left).

In the general case, for faces with vertices n you create triangles:

 0 (i) (i + 1) [for i in 1..(n - 2)] 

If you do not insist on individual triangles, you can also use the GL_TRIANGLE_FAN primitives, which are still mostly OpenGL. Thus, you can draw any convex polygon with a triangle fan using the original index sequence. Thus, a triangle with a vertex sequence of 0 1 2 3 describes a quad in this case and is very easy to generalize to a face with more than four vertices.

Edit: since you still have problems, let's see how this applies to the example in your post. I will list the initial quad index sequence for each face and the index sequence for two triangles after splitting the quadrant.

 f 1 2 3 4 --> (1 2 3) (1 3 4) f 8 7 6 5 --> (8 7 6) (8 6 5) f 4 3 7 8 --> (4 3 7) (4 7 8) f 5 1 4 8 --> (5 1 4) (5 4 8) f 5 6 2 1 --> (5 6 2) (5 2 1) f 2 6 7 3 --> (2 6 7) (2 7 3) 

It looks right to me when I draw a cube. Remember to subtract 1 from the indexes for your use, as these are 1-based indexes, and you will almost certainly need 0-based indexes.

Answer 2

Writing your own obj loader and reading the specification is very careful, the details in the "f" parameter are very vague, especially if some files contain the "f" line with 4 arguments to them.

It turns out that this is actually a triangular strip in an odd order. The correct conversion to triangles is as follows (psuedo code):

 n = 0; triangles[n++] = [values[0], values[1], values[2]]; for(i = 3; i < count(values); ++i) triangles[n++] = [ values[i - 3], values[i - 1], values[i] ]; 

Example:

 f: ABCDEFG 

There would be the following 5 triangles:

 ABC ACD BDE CEF DFG