Graphics Reference
In-Depth Information
Figure 8.14. A number of different triangle properties used for determining the appropriate tessellation
level.
needed for a given triangle. There are many metrics that we could use, so we will consider
what situations would require the highest level of detail.
Clearly, when a triangle face points away from the viewer, we don't want to increase
its complexity at all. In fact, if possible, we want to eliminate it from further processing
completely, since it won't contribute to the rendered output image. At the transition point
from a back-facing triangle to a front-facing triangle, we have silhouette edges. These
edges have very high importance, because the user will be able to fairly clearly differenti-
ate the outline of the object from its background. If there is a low-resolution silhouette,
the user will spot it quite easily. After the silhouette edge, we have the triangles that are
potentially visible to the user as long as they are not occluded by other geometry within the
scene. If a triangle is potentially visible, we want to increase its level of detail to the high-
est degree that will be visible under the current viewing conditions. This particular type of
situation provides a number of potential metrics.
The simplest property is the distance from the camera. If a triangle is very close to
the camera, it should use the highest level of detail possible. If it is very far away from the
camera, it should use a much lower detail. This calculation depends on the average size of
a triangle, as well as the angle the triangle is being viewed with. When combined, these pa-
rameters describe the effective screen-space size of the triangle. Figure 8.14 demonstrates
the various situations we have described up to this point.
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