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storm33229

OpenGL Using Vertex Buffer Objects in OpenGL 2.1 and C

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(I specified 2.1 because my laptop won't go past that version. I would have probably done this anyway since 3.x and on introduces shaders as mandatory?).
Thanks to Wikipedia: http://en.wikipedia.org/wiki/Vertex_Buffer_Object I am starting to grasp how simple it can be to use VBO's (I am still not positive about IBO's?). What I have understood so far is the primary reason to use them is a performance boost gained due to the fact that data is now stored in video memory.

What I would like to know is how I am supposed to use them in a practical context. For instance, all of what I have seen has been setting up one Vertex Buffer Object and drawing one triangle or one cube, etc. What if I want to draw 2 or more? Do I set up a new VBO for each entity that I want to draw? Or do I magically append to some static VBO that I setup early on?


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Memory usage strategy strongly depends on your setup. Do you need to draw many different objects (large VBO holding objects), or only few (no harm in one VBO per object)? Do you want to draw many objects with the same data (one location (VBO or a part of it) per shared object)? Do you have very large meshes for which you wish to cull some parts? (maybe several VBOs per object). Answers to such (and many other) questions will help you decide what strategy to use.

As for "magical" appending: no one forces you to allocate a VBO that can only hold one object: you can safely create a memory pool out of VBO, that is, a large VBO which you will fill with objects as you need them. But again, this is only useful in case when you're certain you won't waste too much memory (which would indicate you chose a wrong strategy).

And, it's not strictly "video" memory. It's just that the memory is managed by the driver. The exact location can be anywhere, but it's the driver that decides where, depending on buffer usage and some other implementation details. Furthermore, since the driver is in charge, it can freely move the data while you're not looking, but it's free to do so since client code has no way of depending on exact data location anyway (glMapBuffer is not an exception. In fact, it only emphasizes this). Edited by capricorn

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There is an app for that
http://www.opengl.org/wiki/Vertex_Buffer_Object
http://www.opengl.org/wiki/VBO_-_more

and also
http://www.opengl.org/wiki/VBO_-_just_examples

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