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Why we need local and world coordinates

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Say an artist creates a model in a 3D modelling program. The vertices of this model are in "model space" (i.e. relative to the center of the model).

If a programmer wants to put 10 copies of this model into the world ("world space") at 10 different locations, then you must transform the vertices from model-space into world-space - usually using a transformation matrix.


Without using two different coordinate systems, the artist would have to create 10 copies of the model, with the vertices manually transformed to the correct positions - For a dynamic object (like a player/car/whatever) moving the vertices manually is just not feasible.

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It also gains value when you try to make queries against a certain model, since you can perform optimization steps on that model in it's model space that don't translate so well if you have to do it in world space [especially if the model is not static]. Things like optimizing lighting or collision detection don't always work too well if you have to do everything in world space, and it's often easier/more efficient to transform something into a known local coordinate system that than to change two things into a static world-coordinate system.

Then there is skeletal animation, which is an extreme use of local coordinate systems that make it possible to draw a figure only once, yet position it however many times [since each vertex will have it's own coordinate system that is derived from the figures skeleton]. Such a thing just isn't reasonable to do strictly in world space.

It's a matter of efficiency and convenience mostly. Technically you *could* do everything in world-space.

The world coordinates though are needed when you need to have a big picture view of a scene, like for rendering. Most other operations are easier/faster to do in optimized local coordinate systems.

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