[PyOpenGL-Users] Using Vertex and index buffer objects in glDrawElements(...)

[Richard G. <rgu...@gm...>]

Hi,
I’d like to know what I am doing wrong.
I want to transfer an array with vertices and another one with index data into buffers on the graphics card and use them in a drawElements(..) call.
As a beginner with OpenGL (coming from Java3d), I am looking for an example that works with PyOpenGL. The C code that I have found on the Internet does not really help in this context because the problem is possibly my lack of understanding the PyOpenGL wrapping.
Here is the code I tried:

class Widget3D:
…
    def On_init( … ):
	…
        # initialize data: just a small coordinates frame for use with GL_LINES
        verts=array([[ 0,0,0 ], [ 9,0,0 ], [ 0,9,0 ], [ 0,0,9 ]], dtype=float32)
        idxs = array([ 0,1, 0,2, 0,3],dtype=ubyte)

        # initialize buffers
        bf=glGenBuffers(1)
        glEnableClientState(GL_VERTEX_ARRAY)
        usage = GL_DYNAMIC_DRAW
        glBindBuffer(GL_ARRAY_BUFFER, bf)
        glBufferData(GL_ARRAY_BUFFER, verts, usage)
        glBindBuffer(GL_ARRAY_BUFFER, 0)
        # I know I should store the size and type of the data as well; for the test I just insert the values in the rendering call because I know them 
        self.bf=bf

        ibf = glGenBuffers(1)
        usage = GL_DYNAMIC_DRAW 
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibf)
        glBufferData(GL_ELEMENT_ARRAY_BUFFER, idxs, usage)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, 0)
        self.ibf=ibf

    # Method called for each redraw
    def drawGL(…):
        glBindBuffer(GL_ARRAY_BUFFER, self.bf)
        glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, self.ibf)
        glEnableClientState(GL_VERTEX_ARRAY)
        # This line is needed to make drawArrays(…) work but may be harmful in context of Index buffer
        glVertexPointer(3,GL_FLOAT,0,None)
        # perhaps the last argument should not be 0; I tried None but does not work either
        # possibly I should use something connected with c_void_ptr but I dont know the syntax
        glDrawElements(GL_LINES,6, GL_UNSIGNED_BYTE,0)
        # the following line works fine (if I do not bind the index buffer) which makes me suspect it has something to do with the index buffer
        # a reading mapBuffer(…) call gives the right content for the index buffer, so the data transfer to the card worked 
        # glDrawElementsui(GL_LINES, [0,1,0,2,0,3])

I am using the OpenGL code in a PyQt QGLWidget subclass; perhaps the issue comes from there.
Usually, the application just crashes if I try to execute the code. 
Please help me to find the right way of convincing PyOpenGL to draw my arrays.
Thanks
Richard

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