glVertexAttributePointer scope?

Question

I'm currently programming an OpenGL ES 2.0 application on both iOS and Android platforms.

In this application I render multiple meshes that all use VBOs. In the process of optimizing the rendering, I realized that the meshes I render share two vertex formats. So I wanted to do the following:

First, setup all vertex attribute pointer offsets, and then simply bind each VBO that uses this vertex format and render it without calling the glVertexAttribPointer function again.

But it gives me strange results.

My question is: Do we have to do the calls glVertexAttribPointer each time we bind a new VBO?

Answer 1

First of all, like every OpenGL state, the state set with glVertexAttribPointer keeps unchanged until someone else calls glVertexAttribPointer again (for the same attribute index). But the important thing here is, that the internal state changed with glVertexAttribPointer doesn't just store the buffer offset to be used for rendering, offsetting into the VBO bound when calling glDraw.... It also stores the actual buffer object bound when calling glVertexAttribPointer.

So yes, whenever you want your vertex data sourced from another VBO, you need to bind this VBO and do the appropriate glVertexAttribPointer calls while this VBO is bound. While this may seem cumbersome in your case, this is in fact a good thing. This way you don't need to worry about the currently bound buffer when rendering something, but only about the things set up with glVertexAttribPointer. And even more important it let's bind a different VBO before rendering, thus you can source different vertex attributes from different VBOs in a single render call (how else would you do that?).

EDIT: You can however use Vertex Array Objects to ease the process of setting up your vertex data. They encapsulate all the state neccessary for rendering from a bunch of arrays (and thus all the things changed by glVertexAttribPointer, gl(En/Dis)ableVertexAttribArray and the buffer bound to GL_ELEMENT_ARRAY_BUFFER, but like said, not the buffer bound to GL_ARRAY_BUFFER). You still have to properly bind the buffer before calling glVertexAttribPointer of course. But using a VAO you only need this code in some set up routine and all you need to do for rendering is calling glBindVertexArray. Though I don't know if your particular ES device supports them.

Answer 2

Some cool resource I found on drawing with VAO / VBO

http://www.arcsynthesis.org/gltut/Positioning/Tutorial%2005.html#d0e4720

It shows how you can drive several objects, with several VAOs and a single VBO for example (each VAO holding pointers with different offsets to the same VBO)

Definitively worth a look, not mentioning the part where you learn that

• glBindBuffer(GL_ARRAY_BUFFER...doesn't bind any data, it's just a global pointer for the following glVertexAttribPointer calls which do the actual data binding

BUT

• glBindBuffer(GL_ELEMENT_ARRAY_BUFFER...DOES bind/save the element array into the current VAO