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

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

On Jun 2, 2009, at 3:24 AM, Mads Ipsen wrote:

> Gordon L. Kindlmann wrote:
>> 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!

...
>> http://people.cs.uchicago.edu/~glk/pglt/
>>
...
>> 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
>>
>>
>>
> Using ctypes, I wrote the following interface to glMultiDrawElements:
>
> # Interface to glMultiDrawElements
> def multiDrawElements(mode, count, gl_type, index):
>   """
>   """
>   # Set up interface for the call
>   n_count = count.shape[0]
>   libGL.glMultiDrawElements.restype  = ctypes.c_void_p
>   libGL.glMultiDrawElements.argtypes = [ctypes.c_int32,
>                                          
> ctypes.POINTER(ctypes.c_int32),
>                                         ctypes.c_int32,
>                                          
> ctypes.POINTER(ctypes.c_int32)*n_count,
>                                         ctypes.c_int32,
>                                         ]
>
>   # Set pointer to data in the count array
>   count_ptr = count.ctypes.data_as(ctypes.POINTER(ctypes.c_int32))
>
>   # Set array of pointers to the index array
>   iptr = ctypes.POINTER(ctypes.c_int32)
>   index_ptr = (iptr*len(index))(*[row.ctypes.data_as(iptr) for row  
> in index])
>
>   # Do the actual call to glMultiDrawElements
>   libGL.glMultiDrawElements(int(mode), count_ptr, int(gl_type),  
> index_ptr, n_count)
>
> Then, using vertex arrays, I call glMultiDrawElements as follows:
>
> multiDrawElements(GL_QUAD_STRIP, count, GL_UNSIGNED_INT, index)

Okay, this is interesting.  However, I'm confused about the types of  
the arguments that you're passing to multiDrawElements().  "count"  
looks like it must be a numpy array, since you're using .shape  
and .ctypes.data_as, right?  "index" is a 2-D numpy array, given your  
use of row.ctypes.data_as for row in index, right?

I agree that this is relevant, but I'm still stumped because my data  
is not coming from numpy.  Its coming from a C-allocated array that is  
being accessed via numpy.  There is no .data_as for me.  I don't quite  
understand the relationship between what .data_as returns, how  
pyopengl's use of ctypes handles that "pointer", and how to  
communicate the analogous information from array allocated in C,  
outside of numpy.

If you have any spare time, I'd be interested if you can tweak the  
call to glDrawElements in

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

so that it works; I've done everything I can to make a self-contained  
but non-trivial demonstration of the problem I'm having.

Thanks,
Gordon

>
>
> You should be able to boil this down to a working example for  
> glDrawElements(), whose arguments are a subset of the arguments to  
> glMultiDrawElements.
>
> Best regards,
>
> Mads
>
>
>