[PyOpenGL-Users] Using glDrawElements on C-allocated data

[Gordon L. K. <gl...@uc...>]

Hello,

I've recently started using PyOpenGL as the framework for re-writing C/ 
C++ OpenGL wrappers around an large amount of research code in C.  Its  
been a pleasure to have something that allows such literal translation  
from the successful C/C++ OpenGL code to Python code; thanks for  
creating this!

The main hitch I've had is in using glDrawElements.  I've looked  
around for answers to this, and its clear that people people using  
PyOpenGL are using it with geometry that is created on the python  
side.  In my case, all the geometry information is created on the C  
side, and I want to control its display from Python.

The line that's causing problems is:

   glDrawElements(glpt[lpld.type[primIdx]], vertCnt, GL_UNSIGNED_INT,  
lpld.indx + vertIdx)

where lpld.indx is a ctypes-created pointer to a buffer of unsigned  
ints allocated and filled in C, which causes this problem:

   Traceback (most recent call last):
     File "pgltDemo.py", line 118, in display
       pgltDraw(pgltObject, uva)
     File "pgltDemo.py", line 76, in pgltDraw
       GL_UNSIGNED_INT, lpld.indx + vertIdx)
   TypeError: unsupported operand type(s) for +: 'LP_c_ulong' and 'int'

because apparently pointer arithmetic games aren't quite so simply in  
Python.  If you just replace "lpld.indx + vertIdx" with "lpld.indx",  
which removes the pointer arithmetic stuff, it segfaults.  I know that  
others have looked at this issue from the context of having array data  
that's created via numpy:

http://www.mail-archive.com/pyg...@go.../msg01356/t.py

I've tried many different iterations of things like this, and I get  
either python errors or segfaults.

Getting this to work with PyOpenGL would make a huge difference in how  
I can get research work done with python, so I've taken some type to  
put a self-contained example online:

http://people.cs.uchicago.edu/~glk/pglt/

This includes a C library the generates some geometry to render, and a  
ctypeslib-generated wrapper around the library.  pgltDemo.c contains a  
"pgltDraw()" function that shows how I'm currently doing things  
without (for display lists) and with vertex arrays.  pgltDemo.py  
contains the same function, but the vertex array code is broken.

Hopefully someone on this list will be able to spend a little time  
working with these examples, and figure out how to get my  
"pgltDemo.py" to work with vertex arrays.

Thanks very much,
Gordon