glVertexPointer(), glNormalPointer - stride and pointer parameters in PyOpenGL

Question

I am using interleaved VBO to display geometry using Python and PyOpenGL. The VBO consists of vertices, normals and colors:

[vx1, vy1, vz1, nx1, ny1, nz1, R1, G1, B1, vx2...]

Drawing is made with the code:

glEnableClientState(GL_VERTEX_ARRAY)
glEnableClientState(GL_NORMAL_ARRAY)
glEnableClientState(GL_COLOR_ARRAY)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, self.vbo_id)

v_pointer = None
n_pointer = ctypes.c_void_p(12)
c_pointer = ctypes.c_void_p(24)

v_stride = 24
n_stride = 24
c_stride = 24

glVertexPointer(3, 
                GL_FLOAT, 
                v_stride, 
                v_pointer)
glNormalPointer(GL_FLOAT,
                n_stride,
                n_pointer)
glColorPointer(3,
               GL_FLOAT,
               c_stride,
               c_pointer)

glDrawArrays(GL_TRIANGLES,      #POINTS
             0,
             self.len_)
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0) # reset
glDisableClientState(GL_COLOR_ARRAY)
glDisableClientState(GL_NORMAL_ARRAY)
glDisableClientState(GL_VERTEX_ARRAY)

The displayed geometry should be a simple cube but all I get is randomly oriented triangles (see figure). I was reading about the glVertexPointer() problems in PyOpenGL here and found the solution with ctypes.c_void_p() but I am not really sure how to implement it. Can someone point out what am I doing wrong or missing here? Or how can I fix this?

Based on the documentation the stride parameter is offset between 2 consecutive arrays of the same type (vertices, normals, colors...) (in bytes) in VBO array. Or not?

Answer 1

I have solved the problem about normals. The code is:

    v_pointer = ctypes.c_void_p(0)   # or None
    n_pointer = ctypes.c_void_p(12)
    c_pointer = ctypes.c_void_p(24)

    v_stride = 36
    n_stride = 36
    c_stride = 36

But I still have problems with colors as the model is not colorful? :) Any ideas are much appreciated.