Translate/Rotate "World" or "Camera" in OpenGL

Question

Before I ask the question: Yes I know that there doesn't exist a camera in OpenGL - but the setLookAt-Method is nearly the same for me ;)

What I was wondering about: If I have the task, to look at a specific point with a specific distance in my scene I basically have two options:

I could change the eyeX,eyeY,eyeZ and the centerX, centerY, centerZ values of my lookAt-Method to achieve this or I could translate my model itself.

Let's say I'm translating/rotating my model. How would I ever know where to put my center/eye-coords of my setLookAt to look at a specific point? Because the world is rotated, the point (x,y,z) is also translated and rotated. So basically when I want to look at the point x,y,z the values are changing after the rotation/translation and it's impossible for me, to look at this point.

When I only transform my eye and center-values of my lookAt I can easily look at the point again - am I missing something? Seems not like a good way to move the model instead of the camera...

Answer 1

It helps to understand your vector spaces.

Model Space: The intrinsic coordinate system of an object. Basically how it lines up with XYZ axes in your 3D modeling software.

World Space: Where everything is in your universe. When you move your camera in a scene layout program, the XYZ axes don't change. This is the coordinate system you're used to interacting with and thinking about.

Camera Space: This is where everything is with respect to your camera. The camera's position in camera space is the by definition the origin, and your XYZ axes are your orthonormalized right, up, and look vectors. When you move or rotate your camera, all the positions and orientations of your objects change with it in camera space. This isn't intuitive - when you walk around, you see think of everything "staying the same way" - it didn't actually move. That's because you're thinking in world space. In camera space, the position and orientation of everything is relative to your eye. If a chair's position is 5 units in front of you (ie (0,0,-5) in camera space) and you want 2 units towards it, the chair's position is now (0,0,-3).

How do I set a lookat?

What does a lookat function do, exactly? It's a convenient way to set up your view matrix without you having to understand what it's doing.

Your eye variables are the camera's position in world space. IE they're what you think they are. The same goes for the center variables - they're the position of your object in world space. From here you get the transformation from world space to camera space that you give to OpenGL.