Graphics Reference
In-Depth Information
mance. Under perspective projection, most of the viewable frustum is “far” from
the viewer, and small-scale detail is necessarily less visible there. By simplifying
the representation of distant or small objects, we can improve rendering perfor-
mance with minimal impact on image quality. In fact, a simplified representation
may even improve image quality because excluding small details prevents them
from aliasing, especially under animation (see Section 25.4 for a further discus-
sion of this).
14.6.1 Level of Detail
It is common to create geometric representations of a single object with varying
detail and select among them based on the screen-space footprint of the object.
This is called a level of detail (LOD) scheme [HG97, Lue01]. Discrete LOD
schemes contain distinct models. To conceal the transitions, they may blend ren-
dered images of the lower- and higher-detail models when switching levels, or
attempt to morph the geometry. Continuous LOD schemes parameterize the model
in such a way that these morphing transitions are continuous and inherent in the
structure of the model.
To minimize the loss of perceived detail as actual geometric detail is reduced,
structure that is removed from geometry is often approximated in texture maps.
For example, the highest-detail variation of a model may contain only geome-
try, whereas a mid-resolution variation approximates some of the geometry in a
normal or displacement map, and the lowest-resolution version alters the shading
algorithm to approximate the impact of the implicit subpixel geometry.
Heightfields are a special case that offers a simple LOD strategy. Because
the heightfield data is effectively a 2D elevation “image,” image filtering opera-
tions normally applied to rescaling (Chapter 19) can be applied to compute lower-
resolution versions of the heightfield.
14.6.2 Billboards and Impostors
While the location within the viewport of a large, distant, static object changes
with the camera's orientation, the projection of that object is largely unchanged
under small translations or rotations. Thus, a complex three-dimensional shape
in the distance can be approximated by a flat, so-called billboard that bears a
picture of the object rendered from approximately the current viewpoint. Such
billboards are inexpensive to render because they are simply quadrilaterals with
images mapped over them. Billboards may be used as the lowest level of detail in
an LOD scheme, or as the only level of detail if it is known that the viewer will
never approach the object. In some cases, billboards are also used for objects that
are naturally flat, exhibit rotational symmetry, or for which orientation errors are
difficult to notice. For example, a cluster of many leaves on a tree may be mod-
eled as a single billboard, and likewise for a clump of many blades of grass. It is
common to automatically rotate billboards toward the viewer during rendering to
conceal their flat nature, although this is not appropriate in all cases. For example,
distant tree billboards should rotate around their vertical axis to face the viewer,
but should not rotate to face a viewer flying above the forest because doing so
would make it appear that the trees had fallen over.
To increase realism, billboards can be augmented with surface normals or dis-
placement maps [Sch97] that allow dynamic relighting.
 
 
 
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