# General local and global coordinate system question

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I know its kind of a simple and general question, but i want to be sure i understand this completely before moving on with my projects. If i understand correctly, local coordinates hold no value if we are talking about particles, right? since it pertains only to rotation?! is this true? If it is, does the coordinate system rotate around with my object? So if i rotate around the x axis for a given PI angle, the next time i want to rotate for PI around x, will it have been rotated for PI od 2*PI?

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Quote:
 Original post by avion85If i understand correctly, local coordinates hold no value if we are talking about particles, right? since it pertains only to rotation?! is this true?
Your english is a little confusing here. If I'm understanding you correctly, you say that because particles have no rotation, you cannot create a local coordinate system based on the transformation of a particular particle. This is partially true, but you can decide on a particular orientation for your local coordinate system, such as defining that the orientation is the same as the world's orientation -- That is, that the rotation part of the transformation is the identity matrix.

Quote:
 If it is, does the coordinate system rotate around with my object?
I don't know what you mean by that. I thought your object was a particle.

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I was asking if i need to concern myself with the "local coordinate system" at all if i am working with particles, in general?

Besides that, i am working on a billiards project where i am using rotation, so i definitely need a local coordinate system.
My question is:

Basically the question is: does my local coordinate system turn around with the ball or is it stationary with its origin in the center of my object. If it is stationary, that would me its always aligned with my global coordinate system, only translated somewhere else.

To put it another way, if i rotate the ball by PI around x, do my y and z axes move along with the object?
thanks

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 Original post by avion85I was asking if i need to concern myself with the "local coordinate system" at all if i am working with particles, in general?
You have to concern yourself with the local coordinate system when working with particles to the extent that if you want to draw an object (say, a tiny teapot) with the transformation of the particle-as-mathematical-object, you'll need to build a transformation matrix for that and transform the tiny teapot by that matrix. In the case of particles, again, the rotation component of the transformation will be the identity matrix.

On the other hand, if you are drawing particles as vertices, there's no real need for that stuff.... the positions of the particles simply become the positions of the vertices, in world space.

Quote:
 Basically the question is: does my local coordinate system turn around with the ball or is it stationary with its origin in the center of my object.
Viewed in terms of the local coordinates, the object from which those coordinates derived never moves.

Imagine drilling a hole in one of the billiard balls, and putting a tiny video camera inside, looking outwards. Now roll that ball down the table, while watching the video. From the point of view of the camera, the table will roll around and around and around, while the ball itself is motionless. This is how the local coordinate system is: its axes rotate as the object rotates, such that each local axis always points along the same direction on the object. Where they point in world-space changes as the object's rotation changes.

OK, i got it!
Thanks Sneftel!

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