Using a Vertex Buffer in two different Vertex Array Objects

Question

This is a theoretical question, so I don't have code until now.

Assuming that I have a VBO with vertex position data and am using it within a VAO to render an indexed (glDrawElements()) figure out of triangles with a special index array.

Now I want to reuse this data buffer within a second VAO to render some other figures consisting out of lines, but with a different index array.

How do I need to bind the buffers, so that I can reuse the vertex data of the first VAO?

Idk why, but to me it makes more sense to me to create multiple element arrays and change them than multiple vao's. — RamblingMad
– RamblingMad, Commented May 8, 2014 at 13:17
It seems to me a bit more modular, but I will think again about it. Thanks. — user3572032
– user3572032, Commented May 8, 2014 at 14:59

ratchet freak · Accepted Answer · 2014-05-08 15:24:13Z

4

no special code is needed to share the buffers between VAOs just bind them as you normally would.

setting up:

glBindVertexArray(VAO[0]);
glBindBuffer(GL_ARRAY_BUFFER, locBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, triangleBuffer);
glVertexAttrPointer(...);

glBindVertexArray(VAO[1]);
glBindBuffer(GL_ARRAY_BUFFER, locBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, lineBuffer);
glVertexAttrPointer(...);

and when drawing:

glBindVertexArray(VAO[0]);
glDrawElements(...);

glBindVertexArray(VAO[1]);
glDrawElements(...);

edited May 8, 2014 at 15:24

answered May 8, 2014 at 13:04

ratchet freak

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4 Comments

Reto Koradi Over a year ago

To streamline it further, you can bind the ELEMENT_ARRAY_BUFFER in the setup as well. That binding is part of the VAO state. Then you only need the glBindVertexArray() call before drawing.

ratchet freak Over a year ago

@RetoKoradi wasn't sure about that, the docs are quite ambiguous about that.

user3572032 Over a year ago

I think it's running now. I will test it further and inform you. Thanks.

Reto Koradi Over a year ago

@ratchetfreak: It's not extensively documented. One conclusive data point is table 6.5 in the OpenGL 3.3 spec. The table is labeled "Vertex Array Object State", and contains ELEMENT_ARRAY_BUFFER_BINDING. This contrasts with table 6.6, labeled "Vertex Array Data (not in Vertex Array objects)", which contains ARRAY_BUFFER_BINDING.

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