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Table 6-23: XML Tags for Center of Mass
CDB:Point name = Center_Of_Mass
6.7 Model Conforming
Historically, the integration of models onto the terrain has been performed during the database compilation process. These offline approaches varied considerably from vendor to vendor
because there were no standardized approaches related to terrain meshing structures, varying visual priority and hidden-surface removal mechanisms, runtime LOD mechanisms, number of
LODs, etc.
This section describes a series of model conformal modes that instruct client-devices on how they should conform models to the underlying terrain.
All of the conformal modes rely on the conforming of the model origin andor model vertices onto the terrain mesh directly beneath the model. Note that the Z-component of the model’s
vertices is with respect to model’s XY plane as shown in Figure 6- 36: Conforming Vertices to Terrain.
Figure 6- 36: Conforming Vertices to Terrain
Terrain profile Conform vertices to terrain
V
2
Elev
v2
V
1
V
n
Elev
v1
Elev
vn
z z
2
z
n
z
1
z
2
z
n
z
3
V
3
z
3
Elev
v3
z y
x
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Note that by definition, all portions of the model below its XY plane represent some form of under ground basement. The conforming of models on steep or rough terrain may yield unusual
results because portions of the basement may be visible. This may require the modelers to level the terrain in the immediate vicinity or adjust the model.
A modeler can specify a model’s conformal mode by adding the following XML tags to the zone representing the model.
CDB:Zone Conformal mode=...
CDB:Zone
The conformal modes are listed in
Table 6-24
below.
Table 6-24: Conformal Modes Conformal Mode
Absolute Point
Vertex Line
Plane Surface
6.7.1 Non Conformal Absolute Mode
When attributed as a Non Conformal model, none of the model vertices are conformed to the underlying terrain. Instead the model’s Z-values are used as-is, as elevation values. As a result
the model is absolutely positioned and behaves independently of the terrain. The shape and orientation of the model is preserved. This conformal mode is typically used for the modeled
representation of point-features. Typical use-cases include buildings, trees, and poles.
6.7.2 Point Conformal Mode
The Point Conformal mode conforms a single point of the model its origin onto the underlying terrain. All of the other model vertices are translated along the Z-axis; as a result, the shape of
the model is preserved by this conformal operation. In effect, the Point Conformal mode dynamically positions a model on the underlying terrain so as to preserve the model’s relative
altitude over the terrain. Point-conforming is the default conformal mode for the modeled representation of point-features. Typical use-cases include buildings, trees, and poles.
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Figure 6- 37: Origon Conformal Mode
6.7.3 Vertex Conformal Mode
The Vertex Conformal mode conforms each of the vertices of a model on the underlying terrain. The shape of the model is not preserved by this conformal operation. The model’s XY plane
defines a reference plane used by client-devices to adjust the elevation of each of the model’s vertices. This conformal mode is used for 3D models that represent typically long 3D lineal
features or large 3D areal features that need to follow the terrain profile. Typical uses include fences, walls, trenches, and forest canopies.
Terrain profile
Model origin
H
O
mod
E
x y
z z
y
x
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Figure 6- 38: Vertex Conformal Mode Example
6.7.4 Line Conformal Mode
The Line Conformal mode conforms each of the two reference vertices of the “linear” model on the underlying terrain. All of the other model vertices are sheared along this axis; as a result, the
shape of the model is not preserved by this conformal operation. The model’s XY plane defines a reference plane used by client-devices to adjust the elevation of the two reference vertices.
This conformal mode is used for models that represent lineal features such as powerlines and monorails.
Terrain profile Conform vertices to terrain
H
v
E
t
E
v
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Figure 6- 39: Line Conformal Mode
The line that is used to specify the conforming is defined by a Face node with the following XML tags:
CDB:Face Conformal_Line
CDB:Face
This Face node defines a single line with two vertices, the first one, V
s
, being the start and the second, V
e
, the end of the line.
6.7.5 Plane Conformal Mode
The Plane Conformal mode conforms each of the three reference vertices of the “planar” model on the underlying terrain. The resulting three vertices define a model
transformation matrix that can then be applied to the vertices of the model. As a
result, the shape of the model is preserved by this conformal operation, but the model
Terrain profile
Elev
vs
Elev
ve
x y
z
Into Terrain coordinates
z y
x
V
s
V
e
z
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undergoes a change in pitch and roll angles. Given this property, there are relatively few cases where this conformal mode can be used
20
. However, as shown in
Figure 6- 41: Application of Line and Plane Conformal Modes on 3d Roads
, this conformal mode is required when conforming the curved segments of 3D raised profiled modeled road features.
Figure 6- 40: Plane Conformal Mode
20
Man-made structures and tree vegetation do not tilt regardless of the terrain they are on.
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Figure 6- 41: Application of Line and Plane Conformal Modes on 3d Roads
The plane that is used to specify the conforming is defined by a Face node with the following XML tags:
CDB:Face Conformal_Face
CDB:Face
The Face node has exactly 3 vertices defining the plane used for the conforming. The only restriction on these 3 vertices is that they must not be collinear.
6.7.6 Surface Conformal Mode
This conformal mode is used for models whose points, edges and surfaces must all conform exactly to the underlying terrain. The Surface Conformal mode requires that the model’s edges
and surfaces be clipped to the underlying terrain. The original vertices and the added vertices resulting from the clipping operation are conformed to the underlying terrain. As a result, the
shape of the model is not preserved by this conformal operation. This conformal mode is primarily used for the modeled representation of 2D surface-feature such as paint markings and
other terrain overlays. In addition, it can be used for 3D models that represent typically long or large 3D lineal and 3D areal features that need to perfectly follow the terrain profile. Note that
Line Conformal T2DModels
Plane Conformal T2DModels
V
p1
V
p2
= V
s
V
s
V
e
= V
p2
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in most cases, the vertex conformal mode provides an adequate solution for 3D models and is more economical to use than the surface conformal mode.
Figure 6- 42: Surface Conformal Mode
6.8 Model Levels-of-Detail Requirements Class – Model Levels of Detail
