Object Transformations and VBOs
So I've written a program that renders a mesh using a Vertex Buffer Object, and lets me move the camera around. I now want to make the object move independently of the camera/view.
However, I'm not sure how to go about moving my meshes through space. Googling tends to find sources either telling me to rotate the objects with glRotatef(), etc., or that using glRotatef() and its siblings is a bad idea because they are deprecated. Perhaps I'm not using the right search terms, but I'm not finding all that much that seems like a good starting point. I see vague references to matrix math, but I don't know how to use that and what approach to take. Other sources say I should apply a vertex shader to transform the objects.
I suppose I could manually reconstruct my mesh each frame, but that seems like a horrible idea (the meshes frequently have upwards of 50k triangles, and I'd like to have dozens of them at least), and I don't really need to have the vertices constantly in use in the rest of my memory if they are already stored in a VBO... right?
So how do I go about manipulating meshes that are stored in VBOs independently of the global space? What resources should I use in learning to do so?
You should be using your ModelView matrix to apply transformations to your vertices. To apply a transformation to a particular object/mesh and not to the entire screen, push a copy of your ModelView matrix onto the stack, apply your transformation, draw your object, then pop that matrix off to go back to your old ModelView matrix.
No need to recompute your vertex positions! That's exactly what these matrices are designed to help you avoid. And the fact that they're stored in a VBO won't matter to you - vertices passed to OpenGL manually are treated exactly the same.
And you might want to check out this question, and the transformation article its accepted answer links to - they'll be useful if you're still getting a hang of transformations and the matrix stack.
Hope that helps!
Edit: A quick example of why the stack is useful. Say you're drawing a simple scene: a guy on a raft (with a sail) in the ocean.
First, you'll want to set up your camera angle, so do whatever transformations you need to set that up. You don't need - and in fact don't want - to push and pop matrices here, because these transformations apply to everything in your scene (In OpenGL, moving the camera = moving the entire world. Weird to think about, but you get used to it.).
Then you draw your ocean. No need to transform it, 'cause it's a static object, and doesn't move.
Then you draw your raft. But your raft has moved! It's drifted along the X axis. Now, since the raft is an independent object and transformations that apply to the raft shouldn't apply to the larger world, you push a matrix onto the stack. This copies the existing ModelView matrix. All those camera transformations are already applied; Your "drifting" transformation on the raft is in addition to the transformations you did at lower levels of the stack.
Draw the raft. Then, before you pop that matrix off the stack, draw the things that are on the raft - the guy and the sail. Since they move with the raft, all the transformations that apply to the raft should be applied to them, to.
Say you draw your castaway first. But he's moved too - he's jumping into the air. So you push another matrix onto the stack, apply a "jumping" transformation, and then render your person. If there's anything that should move with the person - if he were holding anything, say - you'd draw it here, too. But he's not. So pop the "jumping" matrix off the stack.
Now you're back in the "raft" context. Since you applied the "jumping" transformation to a copy, the "drifting" transformation was left untouched a stack level down. Draw the sail now, and it'll be on top of the raft, right where it should be.
And then you're done with raft, so you can pop that matrix off the stack too. You're back down to your plain camera transform. Draw some more static geometry - islands or something.
And that's why the matrix stack is useful. It's also why people build more complicated scenes scenes as "scene graphs" - so they can keep track of the nesting of transformations. It's also useful in skeletal animation, where the position of the wrist depends on the position of the elbow, which depends on the position of the shoulder, and so forth.
And that was way longer than I expected - but hopefully useful. Cheers!
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