Convert Wavefront (OBJ) faces into triangles for drawing with OpenGLES

As we hashed out in the comments, that 700-entry line seems to be the middle face of your tile: a square with something like 174 T-junctions on each side would give you the 700-entry line you're seeing.

All those T-junctions are vertices in one big polygon (an n-gon, or specifically a 700-gon), so we need to break it into triangles for rendering just like we do for quads.

If you have a list of vertices in order around the perimeter of a convex polygon, you can triangulate it into a strip by alternating two steps:

• Take two vertices from the end of the list and one from the start to make a triangle. Remove one vertex from the end of the list.

  MakeTriangle(startIndex, endIndex - 1, endIndex); endIndex--; 

• Take two vertices from the start of the list and one from the end to make a triangle. Remove one vertex from the start of the list.

  MakeTriangle(startIndex, startIndex + 1, endIndex); startIndex++; 

(Note that this reduces to the same thing you're already doing if the input has 3 or 4 vertices. I've expressed the code examples above as shifting indices for the start and end instead of removing a vertex and shifting the rest of the list down, for efficiency.)

To avoid degenerate triangles, you may need to cycle the list (move vertices from the end to the start) so that the first triangle you pick has non-zero area. In the case of the big square face in the image, that means starting in one of the corners, so that the alternating steps above give you zig-zag cuts diagonally across it.

You can also build a fan by using just one of these steps repeatedly, but this will tend to give you more long and skinny triangles, including many with zero area in the example from the image. The strip strategy helps balance out the sizes.

In general though, you want to do most of the hard work of triangulating the mesh in the DCC (Digital Content Creation) software like Blender before importing the content into your game/engine. These tools are designed for this work, and have the structure/adjacency information available to do higher-order operations, handle non-convex polygons, etc. Digesting your meshes into quads and triangles before import lets you keep your game or engine's import code simpler (and faster!) and can give you higher quality results, as artists can control exactly how those complex n-gons get triangulated, rather than leaving it up to one fixed algorithm.

In this case, an artist working in Blender could notice that most of the edges along the straight sides of the tile are unnecessary and remove them, so the top face becomes just a single big quad without T-junctions all along its length. This reduces your polygon and vertex counts for better rendering performance.

The resulting topology would look like this, all quads, no n-gons:

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