CG Basics - An Introduction to Computer Graphics for Artists - page 11

Fig. 2.3 Positive and negative axes in a Cartesian coordinate system

Cartesian space, global positioning systems, modern cartography, and computer
graphics would not function as they do now (Featherstone and Vanicek 1999 ).
The coordinate system is the fi rst thing CG software needs to have before it can
do anything with your data. CG artists don't have to know this to make objects
in 3D because the software has this function built in. However, not knowing how
spatial data is referenced within the application can cause a great deal of frustration
later, because it affects how tools behave. The coordinate system is how your
application sees the world, and it must be understood to smoothly interact with a
3D application.
Numbers can be added with a calculator by pushing buttons, but if the concept of
adding isn't understood, the calculator is of no use because you won't know what
the results represent. In the same way, a 3D application will perform many functions
invisibly, but if you don't know what it is doing, you won't know what to expect, or
how to work out a problem. The fi rst thing you need to remember is that the global
origin is a station point against which all dimensions are measured and that those
dimensions are measured in Cartesian space.
The world origin is usually represented on screen with a 3D icon of some kind,
to show the three major axes. They are known as the X, Y, and Z axes and represent
the three perpendicular directions used to defi ne space within your program.
Pointing in the opposite direction are the negative X, Y, Z axes, for a total of six
primary directions.
These are very much like the X, Y grids you may have drawn in Math class to
plot the location of a point. They are used in much the same way in 3D applications
as well, but with an additional axis to represent depth. This is the Z-axis, also known
as the depth axis (Fig. 2.3 ).