CODEWITHSUNDEEP
opengl
Desperado17
Desperado17

Reputation: 1015

Uploading index buffer without changing VAO state

In order to upload an index buffer to the GPU, I have to call glBindBuffer with GL_ELEMENT_ARRAY_BUFFER then call glBufferData etc. on it.

However, for my engine design where everything is condensed into as few functions as possible this comes unhandy as using GL_ELEMENT_ARRAY_BUFFER with glBindBuffer changes VAO state and I would like to deal with the buffers without changing VAO state.

Can someone tell me if it is possible to allocate and upload an index buffer without touching VAO state? Or would I have to restore GL_ELEMENT_ARRAY_BUFFER state manually?

Upvotes: 1

Views: 321

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473537

The following assumes that direct state access functionality is not available to you. If it is, you should just use that, so that you don't have to bind objects just to modify them in the first place.

There is no such thing as an "index buffer" in the OpenGL API. There are just buffer objects, which can be at any point used for any number of purposes (one of which is for vertex indices in a rendering command). No buffer is ever wedded to the notion of being specifically for index data.

Buffer binding points therefore are meaningful only to the degree that you use the functionality associated with those binding points. GL_ARRAY_BUFFER is meaningful only because glVertexAttribPointer calls look at the buffer bound to that binding point. But if you're not actually making a glVertexAttribPointer call, then GL_ARRAY_BUFFER is just a binding point, no different from any other.

For general buffer object management, you therefore can bind any buffer to any binding point. GL_UNIFORM_BUFFER, GL_COPY_READ_BUFFER​, whatever. If you're not using the binding for its special purpose, then no binding is different from another (except GL_ELEMENT_ARRAY_BINDING which is part of VAO state).

So just bind the "index buffer" to some innocuous binding point (the indexed ones like GL_UNIFORM_BUFFER are particularly harmless, since they only get special functions when you use glBindBufferRange) and do your upload from there.

Upvotes: 3

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