Class Name: Rendering Priority Level
Subclasses
This DRM class is concrete and has no subclasses.
Definition
An instance of this DRM class specifies the method used to describe
the relative priority used to resolve occlusion between two or more
overlapping (or potentially overlapping) objects.
Priority is indicated by the group ID
(rendering_group)
and priority index
(rendering_priority). The
rendering_priority is relative to the
rendering_group,
and is intended to assign priority order (or layer) to coplanar
<Polygon> instances or coplanar
<Feature Representation> instances.
A higher index indicates a higher rendering priority (rendered last).
The scope of the group ID values is the entire transmittal.
Lower numbered priority groups are rendered before
higher numbered priority groups.
<Rendering Priority Level> instance may be components of
<Geometry Representation> instances and/or
<Feature Representation> instances within a
<Union Of Geometry> instance or
<Union Of Features> instance, as appropriate,
to override the ordering specified by the
ordering_reason field.
DRM objects that do not specify a
<Rendering Priority Level> instance are assumed
to have a rendering_priority
value of 0.
Primary Page in DRM Diagram:
Secondary Pages in DRM Diagram:
Example
A particular transmittal may represent a lake as a single
<Polygon> instance located inside of a much larger
<Polygon> instance that represents the surrounding
terrain. The larger terrain polygon and the smaller lake polygon
that it contains may be coplanar, so the modeller assigns a
<Rendering Priority Level> component with
rendering_group = 1,
rendering_priority = 1 to
the terrain <Polygon> instance and a
<Rendering Priority Level> component with
rendering_group = 1,
rendering_priority = 2
to the lake <Polygon> instance,
so that the lake polygon is rendered on top of the terrain
<Polygon> instance.
The side of a <Model> instance, representing the wall of a
building, is a brown <Polygon> instance, and a window in the
wall of the building is a grey <Polygon> instance where the
grey <Polygon> instance is coplanar with the brown
<Polygon> instance. The grey <Polygon> instance is
given a <Rendering Priority Level> component with a higher
rendering_priority value than that
of the brown <Polygon> instance so that the window is
drawn correctly on the side of the building.
A plan view display can render <Point Feature> instances,
<Areal Feature> instances, and <Linear Feature>
instances. Since <Feature Representation> instances are
usually considered to be flat, many features may be coplanar. A
<Rendering Priority Level> component can be used to indicate
the relative drawing order (or layer) within a set of overlapping
<Feature Representation> instances.
It is common for a single <Areal Feature>
instance to cover the entire <Environment Root>
instance area as a default background
<Feature Representation> instance.
This <Areal Feature> instance is assigned
a rendering priority less than all other rendering priorities
so that it is always drawn first and thus is the background.
The following instance diagram shows how
<Rendering Priority Level> instances can be used to override
the layering of a fixed list of DRM objects. The
<Union Of Geometry Hierarchy> instance orders its components
and that order is reflected in the left to right position in
Figure 6.57. The DRM objects are rendered in this order:
Polygon_C, Polygon_B,
Polygon_A, Polygon_E,
Polygon_F, Polygon_D.
Figure 57 — <DRM Rendering Priority Level> example
FAQs
-
Is this the only way to order data for rendering purposes?
No. There are other ways to achieve scope-limited, fixed-ordered
rendering; see ordered <Union Of Geometry>.
-
When should a data provider use a <Rendering Priority Level>
instance?
A <Rendering Priority Level> instance should be used to globally
resolve rendering priority after scope limited occlusion is resolved. After
all <Geometry Representation> instances are rendered that have a
rendering priority <= 0, rendering successive levels of
<Geometry Representation> instances should resolve occlusion.
<Rendering Priority Level> instances can be viewed as a
sorting bin assignment in some occlusion schemes.
-
Are there any 'special' sentinel values for a rendering priority?
Yes. A rendering priority of SE_RENDER_PRIORITY_TOPMOST
means always render 'on top' (last), and a priority of
SE_RENDER_PRIORITY_BOTTOMMOST
means always render 'on bottom' (first)
Constraints
Component of (two-way)
Inherited Field Elements
This class has no inherited field elements.
Notes
Fields Notes
The rendering_group field establishes a
group to which the priorities of DRM objects are relative.
The rendering_priority field
indicates priority between coplanar DRM objects within the same
rendering_group, such that a
higher value indicates a higher priority.
EXAMPLE
A <Polygon> instance with a
<Rendering Priority Level> component having
rendering_group = 1,
rendering_priority = 2 is
on top of a <Polygon> instance with a
<Rendering Priority Level> component with
rendering_group = 1,
rendering_priority = 1.
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