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I'm trying to learn how to use VBO (Vertex Buffer Objects) by putting together a simple program that draws a quad to the screen using OpenTK (C# OpenGL bindings).

Unfortunately I'm not seeing anything on screen. If I draw the same quad in immediate mode it shows up fine.

Since it wasn't working I tried adding Color Data too (in case this was the problem) but it didn't help.

Any help would be greatly appreciated!

    private uint indexBufferId;
    private uint vertexBufferId;
    private uint colorBufferId;

    private void InitialiseData()
    {

        // Set-up index buffer:
        ushort[] indices = new ushort[] { 0, 1, 2, 3 };

        GL.GenBuffers(1, out indexBufferId);
        GL.BindBuffer(BufferTarget.ElementArrayBuffer, indexBufferId);
        GL.BufferData(
            BufferTarget.ElementArrayBuffer,
            (IntPtr)(indices.Length * sizeof(ushort)),
            indices,
            BufferUsageHint.StaticDraw);


        // Set-up vertex buffer:
        float[] vertexData = new float[] {
            50.0f, 50.0f,
            100.0f, 50.0f,
            100.0f, 100.0f,
            50.0f, 100.0f };

        GL.GenBuffers(1, out vertexBufferId);
        GL.BindBuffer(BufferTarget.ArrayBuffer, vertexBufferId);
        GL.BufferData(
            BufferTarget.ArrayBuffer,
            (IntPtr)(vertexData.Length * sizeof(float)),
            vertexData,
            BufferUsageHint.StaticDraw);

        // Set-up color buffer:
        float[] colorData = new float[] {
            1.0f, 1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f, 1.0f,
            1.0f, 1.0f, 1.0f, 1.0f
        };

        GL.GenBuffers(1, out colorBufferId);
        GL.BindBuffer(BufferTarget.ArrayBuffer, colorBufferId);
        GL.BufferData(
            BufferTarget.ArrayBuffer,
            (IntPtr)(colorData.Length * sizeof(float)),
            colorData,
            BufferUsageHint.StaticDraw);

    }

    protected override void OnRenderFrame(FrameEventArgs e)
    {

        base.OnRenderFrame(e);

        GL.Clear(ClearBufferMask.ColorBufferBit);

        // Bind vertex buffer:
        GL.BindBuffer(BufferTarget.ArrayBuffer, vertexBufferId);
        GL.EnableClientState(ArrayCap.VertexArray);
        GL.VertexPointer(2, VertexPointerType.Float, 0, IntPtr.Zero);

        // Bind color buffer:
        GL.BindBuffer(BufferTarget.ArrayBuffer, colorBufferId);
        GL.EnableClientState(ArrayCap.ColorArray);
        GL.ColorPointer(4, ColorPointerType.Float, 0, IntPtr.Zero);

        // Bind index buffer:
        GL.BindBuffer(BufferTarget.ElementArrayBuffer, indexBufferId);

        // Draw:            
        GL.DrawElements(
            BeginMode.Quads, 
            1, 
            DrawElementsType.UnsignedShort, 
            IntPtr.Zero);

        // Disable:
        GL.DisableClientState(ArrayCap.VertexArray);
        GL.DisableClientState(ArrayCap.ColorArray);

        SwapBuffers();

    }
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  • \$\begingroup\$ So my call to GL.DrawElements(...) was incorrect. It should be: GL.DrawElements( BeginMode.Quads, 4, DrawElementsType.UnsignedShort, IntPtr.Zero); The second parameter is number of vertices, not number of quads. Looks like my test data is still a bit iffy but at least it's rendering now. \$\endgroup\$
    – Karle
    Commented Jul 31, 2013 at 18:36
  • \$\begingroup\$ You should answer your own question then. \$\endgroup\$
    – user15805
    Commented Jul 31, 2013 at 21:25
  • \$\begingroup\$ Voting to close because this is a debugging question with a very specific programming error. It will be unlikely ot be helpful to future visitors. \$\endgroup\$ Commented Jul 31, 2013 at 23:55
  • \$\begingroup\$ @Alex M: the site doesn't let you answer your own question within the first 8 hours if you are a new user. \$\endgroup\$
    – Karle
    Commented Aug 4, 2013 at 14:59

1 Answer 1

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In the glDrawElements call:

void glDrawElements(GLenum mode,  GLsizei count,  GLenum type,  const GLvoid * indices);

The second parameter count refers to number of vertices, not number of primitives.

Therefore the correct version of the above should be (using the OpenTK API):

GL.DrawElements( BeginMode.Quads, 4, DrawElementsType.UnsignedShort, IntPtr.Zero)
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