why the second call to glBindBuffer?

Question

I'm reading along in this tutorial and i get down towards the end in how to use Vertex Buffers and i see that the vertex buffer which I generated and called glBindBuffer on once already i have to bind a second time:

glBindBuffer(GL_ARRAY_BUFFER, vbo_triangle);

I'm still very new to openGL (like 3 days) so I'm trying to wrap my mind around how a lot of these things work. I spend most of my time on khronos or opengl.org reading about the commands, but I couldn't figure out why this one gets called twice. any hints? Thanks.

Answer 1

Does that particular example strictly need the second bind? No. OpenGL retains state, so if a buffer object is bound to a target, then it will remain bound until you bind something else to that target.

However, what happens if you insert code after the creation of the buffer that creates a second buffer? After all, you might want to have two objects. Or 10. Or however many you want; they don't have to share buffer objects.

Once you do that, your code breaks because the buffer that your code expects to be bound isn't actually bound. Therefore, unless you're good at managing state and really know what you're doing (and if you're still following tutorials, the answer is "no"), you should set whatever state you need to do what you intend.

Therefore, if you intend to draw from a particular buffer, you should bind it and set the appropriate state (the gl*Pointer calls).

Answer 2

You have to bind and unbind the buffer to copy to it from 'C' and draw with it from openGL. Think of it as locking/unlocking between the program and the graphics.

So the sequence is
create
bind
stuff data
unbind

bind
display
unbind