What are Vertex Array Objects?

Question

I am just starting to learn OpenGL today from this tutorial: http://openglbook.com/the-book/
I got to chapter 2, where I draw a triangle, and I understand everything except VAOs (is this acronym OK?). The tutorial has this code:

glGenVertexArrays(1, &VaoId);
glBindVertexArray(VaoId);

While I understand that the code is necessary, I have no clue what it does. Although I never use VaoId past this point (except to destroy it), the code does not function without it. I am assuming this is because it is required to be bound, but I don't know why. Does this exact code just need to be part of every OpenGL program? The tutorial explains VAOs as:

A Vertex Array Object (or VAO) is an object that describes how the vertex attributes are stored in a Vertex Buffer Object (or VBO). This means that the VAO is not the actual object storing the vertex data, but the descriptor of the vertex data. Vertex attributes can be described by the glVertexAttribPointer function and its two sister functions glVertexAttribIPointer and glVertexAttribLPointer, the first of which we’ll explore below.

I don't understand how the VAO describes the vertex attributes. I have not described them in any way. Does it get the information from the glVertexAttribPointer? I guess this must be it. Is the VAO simply a destination for the information from glVertexAttribPointer?

On a side note, is the tutorial I am following acceptable? Is there anything I should watch out for or a better tutorial to follow?

Answer 1

VAO is an object that represents the vertex fetch stage of the OpenGL pipeline and is used to supply input to the vertex shader.

You can create vertex array object like this

GLuint vao;
glCreateVertexArrays(1, &vao);
glBindVertexArray(vao);

First let' do a simple example. Consider such an input parameter in a shader code

layout (location = 0) in vec4 offset; // input vertex attribute

To fill in this attribute we can use

glVertexAttrib4fv(0, attrib); // updates the value of input attribute 0

Although the vertex array object stores these static attribute values for you, it can do a lot more.

After creating vertex array object we can start filling in its state. We will ask OpenGL to fill it automatically using the data stored in a buffer object that we supply. Each vertex attribute gets to fetch data from a buffer bound to one of several vertex buffer bindings. For this end we use glVertexArrayAttribBinding(GLuint vao, GLuint attribindex, GLuint bindingindex). Also we use the glVertexArrayVertexBuffer() function to bind a buffer to one of the vertex buffer bindings. We use the glVertexArrayAttribFormat() function to describe the layout and format of the data, and finally we enable automatic filling of the attribute by calling glEnableVertexAttribArray().

When a vertex attribute is enabled, OpenGL will feed data to the vertex shader based on the format and location information you’ve provided with glVertexArrayVertexBuffer() and glVertexArrayAttribFormat(). When the attribute is disabled, the vertex shader will be provided with the static information you provide with a call to glVertexAttrib*().

// First, bind a vertex buffer to the VAO
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vmath::vec4));

// Now, describe the data to OpenGL, tell it where it is, and turn on automatic
// vertex fetching for the specified attribute
glVertexArrayAttribFormat(vao, 0, 4, GL_FLOAT, GL_FALSE, 0);

glEnableVertexArrayAttrib(vao, 0);

And code in a shader

layout (location = 0) in vec4 position;

After all you need to call to glDeleteVertexArrays(1, &vao).

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You can read OpenGL SuperBible to understand it better.

Answer 2

Vertex Array Objects are like macros in word processing programs and the like. A good description is found here.

Macros just remember the actions you did, such as activate this attribute, bind that buffer, etc. When you call glBindVertexArray( yourVAOId ), it simply replays those attribute pointer bindings and buffer bindings.

So your next call to draw uses whatever was bound by the VAO.

VAO's don't store vertex data. No. The vertex data is stored in a vertex buffer or in an array of client memory.

Answer 3

"Vertex Array Object" is brought to you by the OpenGL ARB Subcommittee for Silly Names.

Think of it as a geometry object. (As an old time SGI Performer programmer, I call them geosets.) The instance variables/members of the object are your vertex pointer, normal pointer, color pointer, attrib N pointer, ...

When a VAO is first bound, you assign these members by calling

glEnableClientState(GL_VERTEX_ARRAY); glVertexPointer...;
glEnableClientState(GL_NORMAL_ARRAY); glNormalPointer...;

and so on. Which attributes are enabled and the pointers you supply are stored in the VAO.

After that when you bind the VAO again, all the those attributes and pointers also become current. So one glBindVertexArray call is equivalent to all the code previously needed to set up all the attributes. It's handy for passing geometry around between functions or methods without having to create your own structs or objects.

(One time setup, multiple use is the easiest way to use VAOs, but you can also change attributes just by binding it and doing more enable/pointer calls. VAOs are not constants.)

More info in response to Patrick's questions:

The default for a newly created VAO is that it's empty (AFAIK). No geometry at all, not even vertexes, so if you try to draw it, you'll get an OpenGL error. This is reasonably sane, as in "initialize everything to False/NULL/zero".

You only need to glEnableClientState when you set things up. The VAO remembers the enable/disable state for each pointer.

Yes the VAO will store glEnableVertexAttribArray and glVertexAttrib. The old vertex, normal, color, ... arrays are the same as attribute arrays, vertex == #0 and so on.