What can make glDrawArrays with a VBO not draw anything?

Question

I'm trying to figure out how to work with VBOs, using an OpenGL 2.0 rendering context. I've got a 2D (ortho) rendering context set up, and I can draw a simple rectangle like this:

glBegin(GL_QUADS);
   glColor4f(1, 1, 1, 1);
   glVertex2f(0, 0);
   glVertex2f(0, 10);
   glVertex2f(100, 10);
   glVertex2f(100, 0);
glEnd;

But when I try to do it with a VBO, it fails. I set up the VBO like this, with the same data as before:

procedure initialize;
const
   VERTICES: array[1..8] of single =
   (
   0, 0,
   0, 10,
   100, 10,
   100, 0
   );
begin
   glEnable(GL_VERTEX_ARRAY);
   glGenBuffers(1, @VBO);
   glBindBuffer(GL_ARRAY_BUFFER, VBO);
   glBufferData(GL_ARRAY_BUFFER, sizeof(VERTICES), @VERTICES[1], GL_DYNAMIC_DRAW);
   glBindBuffer(GL_ARRAY_BUFFER, 0);
end;

and I try to draw like this:

begin
   glColor4f(1, 1, 1, 1);
   glEnableClientState(GL_VERTEX_ARRAY);
   glBindBuffer(GL_ARRAY_BUFFER, VBO);
   glVertexPointer(2, GL_FLOAT, 0, 0);
   glDrawArrays(GL_QUADS, 0, 1);
   glBindBuffer(GL_ARRAY_BUFFER, 0);
end;

From everything I've read, that ought to work. I run it through gDEBugger and there are no GL errors, and the data in the VBO is getting loaded correctly, but nothing actually appears when I swap the buffers. Changing the data in the vertex array to use normalized coordinates (from 0..1.0) also ends up displaying nothing. Any idea what I'm doing wrong? (Assume the render context itself is set up correctly and the GL function pointers have all been loaded correctly.)

Answer 1

glDrawArrays(GL_QUADS, 0, 1);

Looks like you're trying to draw a quad with a single vertex. You need three more:

glDrawArrays(GL_QUADS, 0, 4);

Or switch to points:

glDrawArrays(GL_POINTS, 0, 1);