Re: [PyOpenGL-Users] Problems with simple pyopengl program

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

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