Question

Warning: trying to learn OpenGL …

I have drawn a square with the following code.

    static const GLfloat squareVertices[] = {
        -0.5f, -0.33f, 
         0.5f, -0.33f,
        -0.5f,  0.33f, 
         0.5f,  0.33f,
    };

...

        glVertexAttribPointer(ATTRIB_VERTEX, 2, GL_FLOAT, 0, 0, squareVertices);
        glEnableVertexAttribArray(ATTRIB_VERTEX);
...

        glVertexPointer(2, GL_FLOAT, 0, squareVertices);

So naturally, I thought to add two more vertices and get a hexagon. Instead, I have square that is folded like origami:

    static const GLfloat hexagonVertices[] = {
        -0.5f, -0.33f, 
        +0.0f, -0.66f, 
        +0.5f, -0.33f,
        +0.75f, +0.33f,
        -0.1f, +0.5f, 
        +0.25f, +0.33f,
    };

...

        glVertexAttribPointer(ATTRIB_VERTEX, 3, GL_FLOAT, 0, 0, hexagonVertices);
        glEnableVertexAttribArray(ATTRIB_VERTEX);

...

        glVertexPointer(3, GL_FLOAT, 0, hexagonVertices);

I have even tried changing the vertices to effect the shape. I can morph the square into a irregular, four-sided polygon, and I can skew the “hexagon”, but it is still just a folded piece of paper.

Can I do it this way?

What is the preferred alternate way?

Answer 1

Your problem comes from sending a value of 3 in the second parameter of glVertexAttribPointer

This tells opengl to use 3 coordinates per vertex instead of the two that you want. Since you have 12 floats, this becomes 4 vertices, hence the square.

Edit: oh, and you need the first parameter of glVertexPointer to be 2 also.