Texturing in OpenGL, Should texture coordinates be assigned to vertices in the shader?

Question

I attempting to texture 3D models (a cube for example) using ibo’s, with OpenGL in Java. Currently, my textures are distorted. I believe this is because only a single texture coordinate is being assigned to each vertex, as the meshes are loaded from wave front (.obj) files.

As each vertex should correspond to multiple texture coordinates, should I be assigning texture coordinates to vertices as the data is loaded from file or would it be better to assign texture coordinates to vertices in the shader?

I would appreciate example code.

Answer 1

Your picture is of a cube, so I assume that's what you're talking about.

You can only share vertices between continuous surfaces. That is, on a sphere, you share all/most of the vertices. On a cube, you must you separate vertices for each face.

Aside from texturing, think of the normals you need for lighting. The normal for each side of a cube should point directly out of it. That means that the vertices' that form that side must all have normals pointing in the same direction. That's a problem for the vertices in the corners, as those vertices must have normals that somehow point in three different directions; clearly impossible. The solution is to just make separate vertices for the corners.

On a sphere you can share the normals/vertices because you want the normal to smoothly interpolate along the surface in order to get a rounded look.

For more complex models, whether or not you can share vertices between two surfaces comes down to whether the "edge" between them is something you distinctly want (it's a hard, pointy edge) or whether the edge is just a necessity of polygonal rendering (it's an edge that's simulating a smooth, rolling surface).

Short version: a cube needs a minimum of 24 vertices, 4 for each face. More if you tessellate.