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12 Chapter 1 3D Compositing
Relative Coordinates
To better understand the concept of relative coordinates, think of a system of satellites,
like the earth, the moon, and the sun. The moon’s parent is the earth, and the earth’s
parent is the sun. Usually, when considering these three bodies, the moon’s position is
described in terms of its position relative to the earth (the moon’s parent), and the
earth’s position is described relative to the sun (the earth’s parent). In Motion, an
object’s position and orientation are always relative to its parent.
When you add a new group to a Motion project, that group is created at the origin
coordinates of its parent. In the case of a root-level group (a group that is not nested
within any other groups in the Layers list), the parent is the project itself. An object
placed inside of a group has its position described relative to its parent: the group.
In the previous example, a group has been positioned at X, Y, and Z coordinates of 50,
50, and 50, respectively. The group is located 50 units away from its parent’s origin on
all axes (the parent in this case being the project itself). The image inside the group is
positioned at 0, 0, 0. Because the image’s position is relative to its parent, the group, it
shares its parent’s origin and has an apparent position in the world of 50, 50, 50.
Group at X=50, Y=50, Z=50 Group Inspector