PyOpenGL

On 12-02-22 07:47 PM, James Gao wrote:
> Hi everyone,
> I'm trying to get a very very simple pass-through shader to work in 
> pyopengl. I compiled and tested a simple C program, available here: 
> http://pastebin.com/FGVxUUXH
> This program works exactly as expected -- a red plane is drawn, 
> completely covering the screen.
>
> I tried to make the *exact* duplicate of this C program here: 
> http://pastebin.com/dYbbXTY1
> This apparently does not work. Only the gray glClear color is 
> displayed, and the two triangles are never drawn.
>
> Any insight into why this isn't working?

You need to modify the calls which are using voidp offsets to explicitly 
create GLvoidp instances, the integer 0 values are being interpreted as 
single-value arrays and the result is a useless offset:

     glVertexAttribPointer( posidx, 2, GL_FLOAT, GL_FALSE, 4*2, GLvoidp(0))

     # Draw the two triangles
     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebuf);
     glDrawElements(
         GL_TRIANGLES,           # mode
         6,           # count
         GL_UNSIGNED_SHORT,      # type
         GLvoidp(0)                       # element array buffer offset
     )

This is a mis-feature of the wrapping in PyOpenGL allowing you to pass 
in an integer where an array is expected.

HTH,
Mike

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

Ahh, that did it, thank you so much! I had written a much larger program
based on a shader tutorial. I spent the last day and a half slowly paring
down the program until I got to that stub. I copied all of it to C, saw
that it worked, and finally had to ask someone...

Perhaps a very large note in the documentation would be good?

-James

On Wed, Feb 22, 2012 at 6:58 PM, Mike C. Fletcher <mcfletch@...:

> On 12-02-22 07:47 PM, James Gao wrote:
> > Hi everyone,
> > I'm trying to get a very very simple pass-through shader to work in
> > pyopengl. I compiled and tested a simple C program, available here:
> > http://pastebin.com/FGVxUUXH
> > This program works exactly as expected -- a red plane is drawn,
> > completely covering the screen.
> >
> > I tried to make the *exact* duplicate of this C program here:
> > http://pastebin.com/dYbbXTY1
> > This apparently does not work. Only the gray glClear color is
> > displayed, and the two triangles are never drawn.
> >
> > Any insight into why this isn't working?
>
> You need to modify the calls which are using voidp offsets to explicitly
> create GLvoidp instances, the integer 0 values are being interpreted as
> single-value arrays and the result is a useless offset:
>
>     glVertexAttribPointer( posidx, 2, GL_FLOAT, GL_FALSE, 4*2, GLvoidp(0))
>
>     # Draw the two triangles
>     glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebuf);
>     glDrawElements(
>         GL_TRIANGLES,           # mode
>         6,           # count
>         GL_UNSIGNED_SHORT,      # type
>         GLvoidp(0)                       # element array buffer offset
>     )
>
> This is a mis-feature of the wrapping in PyOpenGL allowing you to pass
> in an integer where an array is expected.
>
> HTH,
> Mike
>
> --
> ________________________________________________
>   Mike C. Fletcher
>   Designer, VR Plumber, Coder
>   http://www.vrplumber.com
>   http://blog.vrplumber.com
>
>
>
> ------------------------------------------------------------------------------
> Virtualization & Cloud Management Using Capacity Planning
> Cloud computing makes use of virtualization - but cloud computing
> also focuses on allowing computing to be delivered as a service.
> http://www.accelacomm.com/jaw/sfnl/114/51521223/
> _______________________________________________
> PyOpenGL Homepage
> http://pyopengl.sourceforge.net
> _______________________________________________
> PyOpenGL-Users mailing list
> PyOpenGL-Users@...
> https://lists.sourceforge.net/lists/listinfo/pyopengl-users
>

On 12-02-22 10:31 PM, James Gao wrote:
> Ahh, that did it, thank you so much! I had written a much larger 
> program based on a shader tutorial. I spent the last day and a half 
> slowly paring down the program until I got to that stub. I copied all 
> of it to C, saw that it worked, and finally had to ask someone...
>
> Perhaps a very large note in the documentation would be good?
Yeah, though I'm tempted to just disallow that functionality (int/float 
as an array) when the data-type is a GLvoidp (which should cover 
essentially all of the pointer-as-offset types, IIRC)... or maybe just 
disable it globally, as it really isn't all *that* useful most of the time.

Have fun,
Mike
>
>
> On Wed, Feb 22, 2012 at 6:58 PM, Mike C. Fletcher 
> <mcfletch@... <mailto:mcfletch@...>> wrote:
>
>     On 12-02-22 07:47 PM, James Gao wrote:
>     > Hi everyone,
>     > I'm trying to get a very very simple pass-through shader to work in
>     > pyopengl. I compiled and tested a simple C program, available here:
>     > http://pastebin.com/FGVxUUXH
>     > This program works exactly as expected -- a red plane is drawn,
>     > completely covering the screen.
>     >
>     > I tried to make the *exact* duplicate of this C program here:
>     > http://pastebin.com/dYbbXTY1
>     > This apparently does not work. Only the gray glClear color is
>     > displayed, and the two triangles are never drawn.
>     >
>     > Any insight into why this isn't working?
>
>     You need to modify the calls which are using voidp offsets to
>     explicitly
>     create GLvoidp instances, the integer 0 values are being
>     interpreted as
>     single-value arrays and the result is a useless offset:
>
>         glVertexAttribPointer( posidx, 2, GL_FLOAT, GL_FALSE, 4*2,
>     GLvoidp(0))
>
>         # Draw the two triangles
>         glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ebuf);
>         glDrawElements(
>             GL_TRIANGLES,           # mode
>             6,           # count
>             GL_UNSIGNED_SHORT,      # type
>             GLvoidp(0)                       # element array buffer offset
>         )
>
>     This is a mis-feature of the wrapping in PyOpenGL allowing you to pass
>     in an integer where an array is expected.
>
>     HTH,
>     Mike
>
>     --
>     ________________________________________________
>       Mike C. Fletcher
>       Designer, VR Plumber, Coder
>     http://www.vrplumber.com
>     http://blog.vrplumber.com
>
>
>     ------------------------------------------------------------------------------
>     Virtualization & Cloud Management Using Capacity Planning
>     Cloud computing makes use of virtualization - but cloud computing
>     also focuses on allowing computing to be delivered as a service.
>     http://www.accelacomm.com/jaw/sfnl/114/51521223/
>     _______________________________________________
>     PyOpenGL Homepage
>     http://pyopengl.sourceforge.net
>     _______________________________________________
>     PyOpenGL-Users mailing list
>     PyOpenGL-Users@...
>     <mailto:PyOpenGL-Users@...>
>     https://lists.sourceforge.net/lists/listinfo/pyopengl-users
>
>


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