glDrawElements inside if statement

OpenGL OpenGL: Basic Coding
1

I’m using glDrawElements to draw a flat quad made of lots of triangles. This works fine, but I want to be able to turn the quad on or off, so I put it inside an if statement that draws it if renderTris == true, then I write an event handler for the KeyDown event so I can set renderTris on or off by pressing the T key.

That’s when it gets wierd. Outside the if statement, it works fine. If renderTris is initially false, though, I can’t get the quad to show up no matter how many times I press the T key. If renderTris is initially true, I can press T once to turn the quad off, but I can’t turn it back on again. I’m certain it’s not a problem with the if statement or the event handler, since the same exact code works when I draw the quad with GL_POINTS or GL_LINES rather than GL_TRIANGLES. And it must be registering the key press, since if renderTris is initially true, pressing T to turn the quad off works; I just can’t turn it on again.

My conclusion is that it’s some quirk of OpenGL or glDrawElements or GL_TRIANGLES that I haven’t come across before. Does anyone know what quirk might be causing this? I haven’t found much to explain it with a Google search, though admittedly that might be because I have no idea what to search for.

Here’s the if statement in question:

gl.BindBuffer(OpenGL.GL_ARRAY_BUFFER, vertexBufferObjectIds[0]);
gl.EnableVertexAttribArray(0);
gl.VertexAttribPointer(0, 3, OpenGL.GL_FLOAT, false, 0, new IntPtr(0));

if (renderTris == true)
            {
                gl.Color(0.2f, 0.8f, 0.2f);
                gl.BindBuffer(OpenGL.GL_ELEMENT_ARRAY_BUFFER, indexBufferTriIds[0]);
                gl.DrawElements(OpenGL.GL_TRIANGLES, 486, OpenGL.GL_UNSIGNED_INT, new IntPtr(0));
                gl.BindBuffer(OpenGL.GL_ELEMENT_ARRAY_BUFFER, 0);
            }

gl.BindBuffer(OpenGL.GL_ARRAY_BUFFER, 0);

It’s C# so I apologize if it looks a little wierd.

2

Post the code you’re using to test for key presses. It sounds like that’s the likely culprit.

EDIT - further to the above, it seems like you’re setting a flag on key down, but not clearing it on key up.

3

Here it is:

private void openGLControl_KeyDown(object sender, KeyEventArgs e)
        {
            OpenGL gl = openGLControl.OpenGL;

            if (e.KeyCode == Keys.V)
            {
                if (renderPoints == false)
                {
                    renderPoints = true;
                    openGLControl.Invalidate();
                }
                else if (renderPoints == true)
                {
                    renderPoints = false;
                    openGLControl.Invalidate();
                }
            }
            if (e.KeyCode == Keys.G)
            {
                if (renderGrid == false)
                {
                    renderGrid = true;
                    openGLControl.Invalidate();
                }
                else if (renderGrid == true)
                {
                    renderGrid = false;
                    openGLControl.Invalidate();
                }
            }
            if (e.KeyCode == Keys.T)
            {
                if (renderTris == false)
                {
                    renderTris = true;
                    openGLControl.Invalidate();
                }
                if (renderTris == true)
                {
                    renderTris = false;
                    openGLControl.Invalidate();
                }
            }
        }

As mentioned, it works fine with points and lines (renderGrid), but not with tris. openGLControl.Invalidate() is something from SharpGL that’s kind of like reDraw; it tells the OpenGL window that something’s changed and to redraw everything to take the changes into account.

4

You’ve missed an “else” statement, so it is always executing the second part of code when T is pressed.

5

:doh:
I can’t believe I didn’t notice that. Thanks, Dan. I guess this wasn’t a case of OpenGL being quirky as much as me being an idiot.