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View Full Version : Local versus world coords?



02-21-2001, 11:05 PM
I don't understand something, I was hoping someone can explain what the difference is between local & world coords?

I mean when people say world coords, are they talking about where the camera is looking at?

If I move cube to -10,10,10, that is always world coords right?

So then what are local coords of an object? Wouldn't it be the same as -10,10,10 for my cube?

holocaust
02-22-2001, 12:14 AM
In your example, if world coordinates are -10, 10, 10 for the cube, in its local coords they become 0, 0, 0 because in local coord the object is the reference coord.

So if there's another object at, world coords, 5, -5, -5, in the first object local coord system they become (5 - (-10)), (-5 - 10), (-5 - 10) == 15, -15, -15

This may help for things like animation with hierarchy (pivot, bones, IK, etc).

Michael Steinberg
02-22-2001, 04:41 AM
You can define an own R3 (actually R4 in opengl) vector room for any object you want. The best is usually to set the origin to the center of the object. Then, you can apply a transformation from one to another vector room.
The vector room for your object is the local
room. The vector room for the world is the world room. The vector room for the eye is is view room. These 3 vector rooms are 3 dimensional in terms of coord data.

The transformations usually done:
local -> world -> eye

The last vector room is the screen. It is obviously only 2 dimensional.

eye -.-> screen

-.-> can be any projection matrix from 3d to 2d, which are for example: perspective and orthogonal projection.