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AoI allows the user to specify null objects. These are objects
that do not show up in the rendered image. In the editing screen
these look like a set of coordinate axes. These serve two
different purposes in Aoi.
AoI does not allow objects to be grouped. But one objected may be
a child of another object. This parent-child relation may be
nested arbitrarily. Also an arbitrary number of objects may be
made children to a given object. This provides a handy way to
achieve "grouping".
Simply add a Null object, and make all the objects to be grouped
as its children.
This is particularly important if you want to rotate all the
objects maintaining their relative positions rigidly. You may be
tempted to select all the objects first (without using the Null
parent), and then applying the rotation. This will cause all the
objects to rotate around their respective centres.
Rotation requires a centre. AoI uses the "centre" of the object
as the default centre. Unfortunately, this default cannot be
changed. If you want to rotate an object around a diferent
centre, then make the object a child of a Null object placed at
that point. Now rotate the Null object only. This will
cause the child object to rotate in the desired way.
We often need to look at a given object from a suitable
viewpoint. Finding that suitable viewpoint is often rather
difficult by using only the Camera View. Here adding a Null
object a the desired centre of the rendered image helps. Simply
make the camera a child of this Null, with the camera pointing at
it. Now you can rotate the Null object suitably. This will change
the viewpoint but will always look at the same centre. Finally
you may zoom in and out the camera using the Camera View.
If you are working with a linkage, then you may need to transmit
the motion of one link to the subsequent links. Here add Null
objects at each joint, and make them children in a recursive
way.