SECTION 9.5
807
3D Artwork
The form XObject specified by the
O
entry is subject to the following restrictions;
failure to abide by them could result in misalignment of the overlay with the ren-
dered 3D graphics:
•
The form XObject is associated with a specific view (not with the camera posi-
tion defined by the 3D view dictionary). It should only be drawn when the user
navigates using the 3D view, not when the user happens to navigate to the same
orientation by manual means.
•
It should only be drawn if the artwork-to-world matrix has not been altered.
•
It may only be specified in 3D view dictionaries in which both a camera-to-
world matrix (
MS
and associated entries) and a projection dictionary (the
P
en-
try) are present.
The
CO
entry specifies the distance from the camera to the
center of orbit
for the
3D view, which is the point around which the camera should rotate when per-
forming an orbit-style navigation. Figure 9.4 illustrates camera positioning when
orbiting around the center of orbit.
Center Of Orbit
FIGURE 9.4
Rotation around the center of orbit
The
LS
entry allows the lighting of the 3D artwork to be changed without chang-
ing the artwork itself. This enables consumers to view a given piece of 3D artwork
with a variety of lighting options without requiring multiple copies of the 3D art-
work stream that differ only in lighting. It also enables artwork with poor lighting