Newbie problem with glDrawElements/Arrays

OpenGL OpenGL: Basic Coding
1

The following code works and I get a triangle

  
  static GLfloat vtx[] =
  {
       -10.0f, 0.0f, -50.0f,
        10.0f, 0.0f, -50.0f,
         0.0f, 10.0f, -50.0f
  };

  glBegin(GL_TRIANGLES);
  glVertex3f(vtx[0], vtx[1], vtx[2]);
  glVertex3f(vtx[3], vtx[4], vtx[5]);
  glVertex3f(vtx[6], vtx[7], vtx[8]);
  glEnd();

However when I try to use glDrawElements

  static GLuint idx[] = { 0, 1, 2 };

  glEnableClientState(GL_VERTEX_ARRAY);
  glVertexPointer(3, GL_FLOAT, 0, &vtx[0]);
  glDrawElements(GL_TRIANGLES, 3, GL_UNSIGNED_INT, &idx[0]);
  glDisableClientState(GL_VERTEX_ARRAY);

Or replcae the glDrawElements with glDrawArrays

  glDrawArrays(GL_TRIANGLES, 0, 3);

I get nothing…Given that the first code segment works what is it I’m doing wrong? (or even just suggestions).

I’ve tried clockwise/aniti-clockwise indices, but since the first code segment works I didn’t think that could be the problem.

Thing is I’ve got another part of the code all working using glDrawArrays and glDrawElements using VBOs so I’m at a loss as to why this simpler code doesn’t work.

Cheers,

STU!

2

When a VBO is bound, the pointer parameter of the gl*Pointer functions acts as an offset into the buffer object. If you pass a pointer to a vertex array while a VBO is bound it will use the pointer’s value as the offset.

3

Excellent! An addition of

  glBindBuffer(GL_ARRAY_BUFFER, 0);

and everything works.

Thanks,
STU!

4

If you add : glBindBuffer(GL_ARRAY_BUFFER, 0);
you won’t use VBOs…maybe it is what you want…

If you want to use VBO you have to create it with glBufferData and then doing:

glBindBuffer(GL_ARRAY_BUFFER, yourVBObuffer);
glVertexPointer(3, GL_FLOAT, 0, NULL);

But it is not worth to do this if you want to draw only a triangle.

5

I am using both (or maybe). The triangle was just an example because I couldn’t get it to work. I already had glBindBuffer to my VBO buffer IDs when using them. What I didn’t know was to bind to ID 0 when using non-VBOs (obvious now of course).

–Stuart.