Wavefront Obj file format, opengl vertex arrays format, and uv texture coords

Loading Wavefront files using Python

Loading .obj files into numpy arrays, then using OpenGL to draw them

Note: for information on parsing wavefront .obj format click here

Our task here is to load wavefront .obj files into numpy arrays and use them in OpenGL, or pyOpenGL of course.

I am here to discuss something that has always bothered me in openGL. The fact that the vertex arrays and glDrawElements uses only 1 pointer to draw the elements, when in reality, it should use 2 (at least). This could be solved by writing special GLSL vertex shaders and using vertex textures, but that is overly complicated.

Understanding the problem:

The problem with indexing a vertex is the texture coordinate. If you are going to have UV islands, you will see that each vertex at that point, will have more than 1 texture coordinate. This means that there is no 1:1 mapping between a vertex, and a texture coordinate, and this breaks the 1 array indexing multiple verticies in an array, and also breaks the glDrawElements method.

The solution?

We basically unravel the arrays, reusing as many vertex:uv mappings as we can, but create new indexes when they differ.


Visual Examples:

 
v1  v2   v3
 --- ---
|      |      | - 2 quads connected
 --- ---
v4  v5   v6

v1 v2  v2  v3
 ---     ---     
|      |   |      |   - UV coordinates NOT connected
 ---     ---
v4  v5 v5   v6

In this example may you see that v2 and v5 don't share UV coordinates, so they will need to have 4 indexies into our index array (because the index array indexes both UV coordinates and Vertex coordinates)

How will we take care of this?

What we are going to do is write a python parser for OBJ files, parse it into the traditional array structure that is laid out in the file. Then we are going to unravel it and create secondary arrays that will work with the glDrawElements.

Code:

from numpy import *
import random


#util to unravel
indexies = dict()
counter = -1
def get_index(key) :
 global indexies, counter
 if key not in indexies :
  counter += 1
  indexies[key] = counter
  return [False, counter]
 else :
  return [True, indexies[key]]

#do the loading of the obj file
def load_obj(filename) :
 V = [] #vertex
 T = [] #texcoords
 N = [] #normals
 F = [] #face indexies

 fh = open(filename)
 for line in fh :
  if line[0] == '#' : continue
 
  line = line.strip().split(' ')
  if line[0] == 'v' : #vertex
   V.append(line[1:])
  elif line[0] == 'vt' : #tex-coord
   T.append(line[1:])
  elif line[0] == 'vn' : #normal vector
   N.append(line[1:])
  elif line[0] == 'f' : #face
   face = line[1:]
   if len(face) != 4 : 
    print line
    #raise Exception('not a quad!')
    continue
   for i in range(0, len(face)) :
    face[i] = face[i].split('/')
    # OBJ indexies are 1 based not 0 based hence the -1
    # convert indexies to integer
    for j in range(0, len(face[i])) : face[i][j] = int(face[i][j]) - 1
   F.append(face)
  
 #Now we lay out all the vertex/texcoord/normal data into a flat array
 #and try to reuse as much as possible using a hash key
  
 V2 = []
 T2 = []
 N2 = []
 C2 = []
 F2 = []

 for face in F :
  for index in face :
   #print V[index[0]], T[index[1]], N[index[2]]
   key = '%s%s%s%s%s' % (V[index[0]][0], V[index[0]][1], V[index[0]][2], T[index[1]][0], T[index[1]][1])
   idx = get_index(key)
  
   if not idx[0] :
    V2.append([float(V[index[0]][0]), float(V[index[0]][1]), float(V[index[0]][2])])
    T2.append([float(T[index[1]][0]), float(T[index[1]][1])])
    N2.append([float(N[index[2]][0]), float(N[index[2]][1]), float(N[index[2]][2])])
    C2.append([random.random(), random.random(), random.random()])

   F2.append(idx[1])
 
 print len(V) * 3 * 4, 'bytes compared to', len(V2) * 3 * 4, 'bytes'
 
 #return numpy arrays
 return [
  array(V2, dtype=float32), 
  array(T2, dtype=float32), 
  array(N2, dtype=float32), 
  array(C2, dtype=float32), 
  array(F2, dtype=uint32)
 ]