How are tripled sequence in IBO working? - opengl

Question

I'm analyzing an obfuscated OpenGL application. I want to generate a .obj file that describes the multi-polygon model which is displayed in the application.
So I froze the app and dig out the values set in VBO and IBO. But the values set in IBO was far more mysterious than what I've expected. The value was
0, 0, 1, 2, 3, 4, 5, 6, 7, 7, 5, 8, 3, 3, 9, 9, 10, 11, 12, 12, 10, 13, 14, 14, 10, 15, 16, 16, 17, 17, 7, 8, 8, 18, 18, 19, 20, 21, 21, 22, 22, 23, 24, 25, 25, 26, 26, 27, 28, 29, 29, 30, 30, 31, 32, 32, 33, 33, 34, 35, 36, 37, 38, 38, 36, 39, 34, 34, 40, 40, 40, 41, 42, 43, 44, 44, 45, 45, 46, 47, 48, 49, 49, 50, 50, 51, 52, 52, 53, 53, 54, 55, 55, 56, 56, 57, 58, 58, 59, 59, 60, 61, 62, 62, 63, 63, 63, 64, 65, 66, 67, 64, 68, 68, 69, 69, 70, 71, 72, 73, 74, 75, 76, 76, 77, 77, 78, 79, 80, 81, 82, 82, 80, 83, 83, 84, 84, 85, 86, 87, 88, 88, 89, 89, 90, 91, 91, 92, 92, 92, 93, 94, 95, 96, 96, 97, 97, 97, 98, 99, 100, 101, 102, 102, 100, 103, 103, 104, 104, 105, 106, 107, 107, 108, 108, 108, 109, 110, 111, 112, 112, 100, 100, 101, 113, 114, 114, ... (length=10495)
As you can see indices like 40, 63, 92 and 108 are tripled, so setting neither GL_TRIANGLES, GL_TRIANGLE_STRIP, GL_TRIANGLE_FAN, GL_QUADS, GL_QUAD_STRIP nor GL_POLYGON to glDrawElements won't work correctly.
Are there some kind of advanced techniques to use triple sequenced indices in IBO? What does it mean? For what reason is it used for?

Answer 1

Repeated indices like that are indicative of aggressive optimization of triangle strips. A repeated index creates degenerate triangles: triangles with zero area. Since they have no visible area, they are not rendered. They exist so that you can jump from one triangle strip to the next without having to issue another draw command.
So a double-index is often used to stitch two strips together. The two triangles it generates will not be rendered.
However, because of the way strips work with the winding order, the facing for the triangles can work out incorrectly. That is, if you stitched two strips together with a double-index, the second strip would start out with the reverse winding order than it desires.
That's where triple indices come in. The third index fixes the winding order for the triangles in the destination strip. The three extra triangles it generates will not be rendered.
The more modern way to handle multiple strips in the same draw call is to use primitive restart indices. But the index list as it currently stands is adequate for use with GL_TRIANGLE_STRIP.
You can read this strip list and process it into a series of separate triangles (as appropriate for GL_TRIANGLES) easily enough. Simply look at each sequence of 3 vertices, and output that to your triangle buffer, so long as it is not a degenerate triangle. And you'll have to reverse the order of two of the indices for every odd-numbered triangle. The code would look something like this:
const int num_faces = indices.size() - 2;
faces.reserve(num_faces);
for(auto i = 0; i < num_faces; ++i)
{
Face f(indices[i], indices[i + 1], indices[i + 2]);
//Don't add any degenerate faces.
if(!(f[0] == f[1] || f[0] == f[2] || f[1] == f[2]))
{
if(i % 2 == 1) //Every odd-numbered face.
std::swap(f[1], f[2]);
faces.push_back(f);
}
}