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

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

Hello,

I'm aware that most users of PyOpenGL are creating their arrays on the  
python side.

In my case, I'd like to make use of a large set of pre-existing C  
libraries, which do what they do very well, and do it rapidly.  This  
includes extracting features from scientific data, and representing  
them as polydata, which I'd like to visualize from python.  Creating  
the buffers in python is not an option for me.  Copying the data into  
a python array, just so I can hand it back to PyOpenGL, which in turn  
goes back through ctypes to give it OpenGL, is going to incur a memory  
and performance cost that should be avoided.

I know that what I'm hoping to do is possible, somehow, because the  
underlying array data is already being used perfectly well to call  
glDrawElements from C.

I remain hopeful that someone on this list has the time and expertise  
to help me make this work.

Gordon

On Jun 3, 2009, at 5:44 AM, René Dudfield 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
>
>
> ------------------------------------------------------------------------------
> Register Now for Creativity and Technology (CaT), June 3rd, NYC. CaT
> is a gathering of tech-side developers & brand creativity  
> professionals. Meet
> the minds behind Google Creative Lab, Visual Complexity, Processing, &
> iPhoneDevCamp as they present alongside digital heavyweights like  
> Barbarian
> Group, R/GA, & Big Spaceship. http://p.sf.net/sfu/creativitycat-com
> _______________________________________________
> PyOpenGL Homepage
> http://pyopengl.sourceforge.net
> _______________________________________________
> PyOpenGL-Users mailing list
> PyO...@li...
> https://lists.sourceforge.net/lists/listinfo/pyopengl-users
>
> ------------------------------------------------------------------------------
> OpenSolaris 2009.06 is a cutting edge operating system for enterprises
> looking to deploy the next generation of Solaris that includes the  
> latest
> innovations from Sun and the OpenSource community. Download a copy and
> enjoy capabilities such as Networking, Storage and Virtualization.
> Go to: http://p.sf.net/sfu/opensolaris-get_______________________________________________
> PyOpenGL Homepage
> http://pyopengl.sourceforge.net
> _______________________________________________
> PyOpenGL-Users mailing list
> PyO...@li...
> https://lists.sourceforge.net/lists/listinfo/pyopengl-users