Graphics Reference
In-Depth Information
Figure 13.5.
The left-handed and right-handed coordinate systems.
and because of the opposite z -axis direction, the system on the right identifies the
position as
(
x 1 ,
y 1 ,
z 1 ) .
In order to uniquely identify a position in 3D space, we must distinguish between
these two options.
Figure 13.5 illustrates that if we let our thumb point in the x -direction, our
index finger point in the y -direction, and our middle finger point in the z -direction,
then the two options of Figure 13.4 can be represented by these three fingers of
our left and right hands. For this reason, we term the two options of describing
positions in three dimensional space the left-handed coordinate system and the
right-handed coordinate system. There is no real difference between these two
coordinate systems except that the z -directions are opposite. One should never
mix the usage of the two coordinate systems. Traditionally, the computer graphics
community has adopted and followed the right-handed coordinate system. In the
rest of this topic, coordinate system will mean the right-handed coordinate system.
One other computer graphics convention is that we typically map the y -axis
to the vertical direction, and the xz -plane to the horizontal plane.
13.2
The Model and the Scene
With the right-handed coordinate system, we can now specify the set-up of Fig-
ure 13.2. Just as in the 2D case, our first task is to identify a convenient coordinate
system. Once again, by convenience we are referring to a coordinate system that
 
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