pyopengl-users

[PyOpenGL-Users] Having a problem with glBufferData

[Gijs <in...@bs...>]

Hey List,

I'm trying to use PBOs for the first time so I'm not completely aware of 
what I'm doing with the code I found on this site 
<http://www.mathematik.uni-dortmund.de/%7Egoeddeke/gpgpu/tutorial3.html>. One 
thing I do know is that the code I can execute in C, I cannot execute in 
Python. This has to do with a piece of code that makes use of 
glBufferData and its 4th argument, namely "size".

The C code:
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer);
glBufferData(GL_PIXEL_PACK_BUFFER_ARB,4*sizeof(float), NULL, 
GL_STREAM_READ);
glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0);
void* mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
assert(mem);
glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0);

Python Code:
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer)
glBufferData(GL_PIXEL_PACK_BUFFER_ARB,None, GL_STREAM_READ)
glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0)
mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY)
glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB)
glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0)

Since I cannot specify 4 arguments for glBufferData, as I can do in C, 
the code I execute in Python is doing something different than the C 
version. This results in an OpenGL error 1282 (invalid operation) when I 
call glReadPixels. I guess when the interface for this function was 
written, it was never expected to be called like this. A normal call 
would supply a variable, and PyOpenGL would be able to determine the 
size, so the size argument wouldn't be needed. Too bad in a case like 
this, it is needed, as far as I can see. Or is there some other way of 
accessing this function?

Regards, Gijs

Re: [PyOpenGL-Users] Having a problem with glBufferData

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

Gijs wrote:
> Hey List,
>
> I'm trying to use PBOs for the first time so I'm not completely aware
> of what I'm doing with the code I found on this site
> <http://www.mathematik.uni-dortmund.de/%7Egoeddeke/gpgpu/tutorial3.html>.
> One thing I do know is that the code I can execute in C, I cannot
> execute in Python. This has to do with a piece of code that makes use
> of glBufferData and its 4th argument, namely "size".
This looks like a real API failing in the wrapper.  Basically I didn't
realize that passing in a NULL with a real size was valid when I did
that wrapper, now even if I didn't I likely wouldn't have wrapped it
quite that way (i.e. I would have made a wrapper that allowed passing
size or not, instead of always taking size from the array).  Will have
to rework the function's wrapper to allow passing in size explicitly.

Thanks for pointing out the problem,
Mike

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

Re: [PyOpenGL-Users] Having a problem with glBufferData

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

Mike C. Fletcher wrote:
> Gijs wrote:
>   
>> Hey List,
>>
>> I'm trying to use PBOs for the first time so I'm not completely aware
>> of what I'm doing with the code I found on this site
>> <http://www.mathematik.uni-dortmund.de/%7Egoeddeke/gpgpu/tutorial3.html>.
>> One thing I do know is that the code I can execute in C, I cannot
>> execute in Python. This has to do with a piece of code that makes use
>> of glBufferData and its 4th argument, namely "size".
>>     
> This looks like a real API failing in the wrapper.  Basically I didn't
> realize that passing in a NULL with a real size was valid when I did
> that wrapper, now even if I didn't I likely wouldn't have wrapped it
> quite that way (i.e. I would have made a wrapper that allowed passing
> size or not, instead of always taking size from the array).  Will have
> to rework the function's wrapper to allow passing in size explicitly.
>
> Thanks for pointing out the problem,
> Mike
>   
I've finally got the fix for this published on PyPI.  (i.e. a new alpha
release of PyOpenGL is up)

Take care,
Mike

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

Re: [PyOpenGL-Users] Having a problem with glBufferData

[Gijs <in...@bs...>]

On 11-9-2009 20:03, Mike C. Fletcher wrote:
> Mike C. Fletcher wrote:
>    
>> Gijs wrote:
>>
>>      
>>> Hey List,
>>>
>>> I'm trying to use PBOs for the first time so I'm not completely aware
>>> of what I'm doing with the code I found on this site
>>> <http://www.mathematik.uni-dortmund.de/%7Egoeddeke/gpgpu/tutorial3.html>.
>>> One thing I do know is that the code I can execute in C, I cannot
>>> execute in Python. This has to do with a piece of code that makes use
>>> of glBufferData and its 4th argument, namely "size".
>>>
>>>        
>> This looks like a real API failing in the wrapper.  Basically I didn't
>> realize that passing in a NULL with a real size was valid when I did
>> that wrapper, now even if I didn't I likely wouldn't have wrapped it
>> quite that way (i.e. I would have made a wrapper that allowed passing
>> size or not, instead of always taking size from the array).  Will have
>> to rework the function's wrapper to allow passing in size explicitly.
>>
>> Thanks for pointing out the problem,
>> Mike
>>
>>      
> I've finally got the fix for this published on PyPI.  (i.e. a new alpha
> release of PyOpenGL is up)
>
> Take care,
> Mike
>
>    
Great, thanks for the fix,
Gijs

Re: [PyOpenGL-Users] Having a problem with glBufferData

[René D. <re...@gm...>]

Hi,

remember that you can use the OpenGL.raw.GL calls... which are more
direct wrappers.  Sometimes easier to use if you have a bug/can't
figure out how to use the normal python wrappers.  Of course then you
need to know ctypes... but that's not too hard.

cheers,


On Sun, Aug 30, 2009 at 9:18 PM, Gijs<in...@bs...> wrote:
> Hey List,
>
> I'm trying to use PBOs for the first time so I'm not completely aware of
> what I'm doing with the code I found on this site. One thing I do know is
> that the code I can execute in C, I cannot execute in Python. This has to do
> with a piece of code that makes use of glBufferData and its 4th argument,
> namely "size".
>
> The C code:
> glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer);
> glBufferData(GL_PIXEL_PACK_BUFFER_ARB,4*sizeof(float), NULL,
> GL_STREAM_READ);
> glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0);
> void* mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY);
> assert(mem);
> glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB);
> glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0);
>
> Python Code:
> glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, thebuffer)
> glBufferData(GL_PIXEL_PACK_BUFFER_ARB,None, GL_STREAM_READ)
> glReadPixels (0, 0, 1, 1, GL_RGBA, GL_FLOAT, 0)
> mem = glMapBuffer(GL_PIXEL_PACK_BUFFER_ARB, GL_READ_ONLY)
> glUnmapBuffer(GL_PIXEL_PACK_BUFFER_ARB)
> glBindBuffer(GL_PIXEL_PACK_BUFFER_ARB, 0)
>
> Since I cannot specify 4 arguments for glBufferData, as I can do in C, the
> code I execute in Python is doing something different than the C version.
> This results in an OpenGL error 1282 (invalid operation) when I call
> glReadPixels. I guess when the interface for this function was written, it
> was never expected to be called like this. A normal call would supply a
> variable, and PyOpenGL would be able to determine the size, so the size
> argument wouldn't be needed. Too bad in a case like this, it is needed, as
> far as I can see. Or is there some other way of accessing this function?
>
> Regards, Gijs
>
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