pyopengl-users

[PyOpenGL-Users] VBOs and gl*Pointer offset parameter

[renaud b. <rnd...@gm...>]

hello,

i start to experiment with VBOs to prepare some code to the future of opengl :)
i try to use a single array buffer to store a bunch of vertices and
then the associated colors.
as far as i understand, i should then use the last parameter of the
usual gl[Vertex|Color]Pointer to pass the offset of the first element
inside the array.
for the vertices, it's 0, so i can pass None (the c functions expect a
void* parameter).
the problem is for non-zero offsets, e.g. for the colors in this case.
it seems that the python wrapper interprets the last parameter as an
array, and i have not found a way to pass an offset without
"dewrapping" the function, i.e.

offset = 12
glColorPointer.wrappedOperation(3, GL_FLOAT, 0, offset) # works
glColorPointer(3, GL_FLOAT, 0, any_thing_i_could_think_of(offset)) #
doesn't work

i've tried several any_thing_i_could_think_of functions (casting to
c_void_p, etc.) with no luck.
any advice ?

i've started looking into pyopengl code to see if the  wrapper could
be made more "clever" by distinguishing array-like and offset-like use
of the last parameter but i need some time to fully understand the
wrapping of the pointer functions.

thanks for any advice,

renaud

Re: [PyOpenGL-Users] VBOs and gl*Pointer offset parameter

[Mike C. F. <mcf...@vr...>]

renaud blanch wrote:
> hello,
>
> i start to experiment with VBOs to prepare some code to the future of opengl :)
> i try to use a single array buffer to store a bunch of vertices and
> then the associated colors.
> as far as i understand, i should then use the last parameter of the
> usual gl[Vertex|Color]Pointer to pass the offset of the first element
> inside the array.
> for the vertices, it's 0, so i can pass None (the c functions expect a
> void* parameter).
> the problem is for non-zero offsets, e.g. for the colors in this case.
> it seems that the python wrapper interprets the last parameter as an
> array, and i have not found a way to pass an offset without
> "dewrapping" the function, i.e.
>
> offset = 12
> glColorPointer.wrappedOperation(3, GL_FLOAT, 0, offset) # works
> glColorPointer(3, GL_FLOAT, 0, any_thing_i_could_think_of(offset)) #
> doesn't work
>
> i've tried several any_thing_i_could_think_of functions (casting to
> c_void_p, etc.) with no luck.
> any advice ?
>   
I'm afraid I'm so accustomed to using the OpenGL.arrays.vbo.VBO wrapper
that I haven't documented the base operation:

    ctypes.c_void_p( 12 )

should work as an array offset of 12 bytes, if it isn't, let me know and
I'll dig into why it's not working for you (failing code would be
helpful).  The wrapper includes a VBOOffset type which basically is
interpreted as an offset when using the wrapper (they're created with
myVBO + byteOffset).

HTH,
Mike

-- 
________________________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://www.vrplumber.com
  http://blog.vrplumber.com

Re: [PyOpenGL-Users] VBOs and gl*Pointer offset parameter

[Mike C. F. <mcf...@vr...>]

renaud blanch wrote:
> On Thu, Jul 16, 2009 at 8:32 PM, Mike C. Fletcher<mcf...@vr...> wrote:
>   
...
>>    ctypes.c_void_p( 12 )
>>
>> should work as an array offset of 12 bytes,
...
> no, it does not work (at least for me).
> that's what i was also doing.
>   
Argh.  ctypes c_void_p is a parameter type, not a pointer type (I knew
this, but I mis-coded for it).  I've checked a 2-line patch into the
OpenGL.arrays.ctypesparameters file, it should show up in the next
release (which should happen reasonably soon, if I ever get enough time
to test).  In the meantime, the following patch should work, or you
could use bzr head of PyOpenGL.

HTH,
Mike

=== modified file 'OpenGL/arrays/ctypesparameters.py'
--- OpenGL/arrays/ctypesparameters.py   2009-04-09 19:06:55 +0000
+++ OpenGL/arrays/ctypesparameters.py   2009-07-17 18:21:38 +0000
@@ -30,7 +30,7 @@
                        return ctypes.byref( value )
        from_param = voidDataPointer = classmethod( from_param )
        def dataPointer( cls, value ):
-               if isinstance( value, ParamaterType ):
+               if isinstance( value, DIRECT_RETURN_TYPES ):
                        return value
                else:
                        return ctypes.addressof( value )
@@ -95,6 +95,8 @@
                                yield length
        def asArray( self, value, typeCode=None ):
                """Convert given value to an array value of given
typeCode"""
+               if isinstance( value, DIRECT_RETURN_TYPES ):
+                       return value
                if isinstance( value, ParamaterType ):
                        value = value._obj
                return ctypes.byref( value )

-- 
________________________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://www.vrplumber.com
  http://blog.vrplumber.com

Re: [PyOpenGL-Users] VBOs and gl*Pointer offset parameter

[Mike C. F. <mcf...@vr...>]

renaud blanch wrote:
> On Fri, Jul 17, 2009 at 8:34 PM, Mike C. Fletcher<mcf...@vr...> wrote:
>   
...
> thanks, the patch didn't work against my installed version of
> pyopengl, but 3.0.1a1 did it.
>
> the only thing is that i had to change the order of platform plugin
> declarations so that 'darwin' comes before 'posix'.
> on the mac os.name is 'posix', and thus the GLX platform was found
> first and failed to load.
>   
I've changed the plugin to prefer sys.platform over os.name when doing
resolution.  That should mean that on Linux it is the linux2 plugin
preferred.  I'm presuming that os-x has sys.platform == 'darwin', so the
fix should work there too.

Thanks for the error report(s),
Mike

-- 
________________________________________________
  Mike C. Fletcher
  Designer, VR Plumber, Coder
  http://www.vrplumber.com
  http://blog.vrplumber.com