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

[René D. <re...@gm...>]

hi again,

or if you just want to whip up a c module...

the buffer interface is your friend:
    http://docs.python.org/dev/c-api/buffer.html


Check out the docs for this functions:
PyObject* PyBuffer_FromMemory(void *ptr, Py_ssize_t size)
PyObject* PyBuffer_FromReadWriteMemory(void *ptr, Py_ssize_t size)






On Wed, Jun 3, 2009 at 8:44 PM, René Dudfield <re...@gm...> wrote:
>
> hi,
>
> usually you can create the array in python, and then give a pointer to the C/C++/F0oLang
>
> Generally easiest for my mind.  That leaves the memory management in python.
>
> Or create the python buffers from your pointer.
>
>
> Then create the slices in python, rather than using pointer arithmetic.
>
> eg, new_array = array[vertIdx:]
>
>
>
> cu,
>
> On Mon, Jun 1, 2009 at 8:37 AM, Gordon L. Kindlmann <gl...@uc...> 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!
>>
>> 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
>>
>>
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