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CHAPTER 9                                                      Multimedia Features



work. The relationship between camera and artwork can be thought of in two
ways:

• The 3D artwork is in a fixed position and orientation, and the camera moves to
  different positions and orientations.
• The camera is in a fixed location, and the 3D artwork is translated and rotated.
Both approaches can achieve the same visual effects; in practice, 3D systems typi-
cally use a combination of both. Conceptually, there are three distinct coordinate
systems:

• The artwork coordinate system.
• The camera coordinate system, in which the camera is positioned at (0, 0, 0) fac-
  ing out along the positive z axis, with the positive x axis to the right and the
  positive y axis going straight up.
• An intermediate system called the world coordinate system.
Two 3D transformation matrices are used in coordinate conversions:

• The artwork-to-world matrix specifies the position and orientation of the art-
  work in the world coordinate system. This matrix is contained in the 3D
  stream.
• The camera-to-world matrix specifies the position and orientation of the cam-
  era in the world coordinate system. This matrix is specified by either the C2W
  or U3DPath entries of the 3D view dictionary.

When drawing 3D artwork in a 3D annotation’s target coordinate system, the fol-
lowing transformations take place:

1. Artwork coordinates are transformed to world coordinates:

   x w y w z w 1 = x a y a z a 1 × aw

2. World coordinates are transformed to camera coordinates:
                                        –1
   x c y c z c 1 = x w y w z w 1 × ( cw )

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