Why no glVertexAttribPointer for index buffer object?

Question

I am here to clear out some confusions regarding opengl from experts. I appreciate your help!

private int vbo;
private int ibo;

vbo = glGenBuffers();
ibo = glGenBuffers();

glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, Util.createFlippedBuffer(vertices), GL_STATIC_DRAW);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, Util.createFlippedBuffer(indices), GL_STATIC_DRAW);


glEnableVertexAttribArray(0);
glEnableVertexAttribArray(1);

glBindBuffer(GL_ARRAY_BUFFER, vbo);

glVertexAttribPointer(0, 3, GL_FLOAT, false, Vertex.SIZE * 4, 0);
glVertexAttribPointer(1, 2, GL_FLOAT, false, Vertex.SIZE * 4, 12);

glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ibo);


glDrawElements(GL_TRIANGLES, size, GL_UNSIGNED_INT, 0);

glDisableVertexAttribArray(0);
glDisableVertexAttribArray(1);

The vertex shader code looks like

#version 330

layout (location = 0) in vec3 position;
layout (location = 1) in vec2 texCoord;

out vec2 texCoord0;

uniform mat4 transform;

void main()
{
    gl_Position = transform * vec4(position, 1.0);
    texCoord0 = texCoord;
}

So, here is my understanding. The purpose of glVertexAttribPointer is to define the format of data in the vertex buffer object. So, in vbo it stores data as follows

buffer.put(vertices[i].getPos().getX());
buffer.put(vertices[i].getPos().getY());
buffer.put(vertices[i].getPos().getZ());
buffer.put(vertices[i].getTexCoord().getX());
buffer.put(vertices[i].getTexCoord().getY());
buffer.put(vertices[i].getNormal().getX());
buffer.put(vertices[i].getNormal().getY());
buffer.put(vertices[i].getNormal().getZ());

So, we have two glVertexAttribPointer lines because we have two variables defined in the vertex shader. So basically we are defining what these two variables point to. So, the first glVertexAttribPointer defines that the first variable "position" is a vertex with three coordinates each being float. The second glVertexAttribPointer defines the second variable "texCoord" being a pair of texture coordinates each being float.

Now what confuses me is that we are using glVertexAttribPointer to make sense of data stored in vbo to opengl. Now, why dont we also use another glVertexAttribPointer or similar code to make sense of data in ibo. Why is this buffer object left alone?

I appreciate any help. Thanks a lot!

BDL BDL · Accepted Answer · 2014-07-24T15:00:15

The reason is, that you can only have one index-buffer active in a draw call. The format of the index-buffer is specified in draw call (see glDrawElements, 3rd parameter).

You can also see the calls to glVertexAttribPointer as a binding of a vbo to a in variable in your shader. Since the index-Buffer is never attached to a uniform, you don't have to call glVertexAttribPointer.

Why no glVertexAttribPointer for index buffer object?

1 Answers