A scene graph is a general data structure commonly used by
vector-based graphics editing applications and modern computer games, which
arranges the logical and often spatial representation of a graphical scene.
Scene graphs are useful for modern games using 3D graphics
and increasingly large worlds or levels. In such applications, nodes in a scene
graph (generally) represent entities or objects in the scene.
For instance, a game might define a logical relationship
between a knight and a horse so that the knight is considered an extension to
the horse. The scene graph would have a 'horse' node with a 'knight' node
attached to it.
As well as describing the logical relationship, the scene
graph may also describe the spatial relationship of the various entities: the
knight moves through 3D space as the horse moves.
In these large applications, memory requirements are major
considerations when designing a scene graph. For this reason, many large scene
graph systems use geometry instancing to reduce memory costs and increase
speed. In our example above, each knight is a separate scene node, but the
graphical representation of the knight (made up of a 3D mesh, textures,
materials and shaders) is instanced. This means that only a single copy of the
data is kept, which is then referenced by any 'knight' nodes in the scene
graph. This allows a reduced memory budget and increased speed, since when a
new knight node is created, the appearance data does not need to be duplicated.
The following are primary components of scene graph on games
:
· - Visibility
· - Level
of Detail
VISIBILITY
Visibility is a measure of the distance at which an object or
light can be clearly discerned. Visibility optimization is the most effective
way to gain performace in games. There are two basic ways to do visibility
optimization i.e art and level design, technology. The games use a mix of both.
Artists design game worlds so that performance is acceptable. Many technologies
have been used in games for example 2.5D technology. Two and a half dimensional
(shortened to 2.5D, known alternatively as three-quarter perspective and
Pseudo-3D) is a term used to describe either 2D graphical projections and
similar techniques used to cause images or scenes to simulate the appearance of
being three-dimensional (3D) when in fact they are not, or gameplay in an
otherwise three-dimensional video game that is restricted to a two-dimensional
plane or has a virtual camera with a fixed angle. By contrast, games using 3D
computer graphics without such restrictions are said to use true 3D.
Common in video games, these projections have also been
useful in geographic visualization (GVIS) to help understand visual-cognitive
spatial representations or 3D visualization.
LEVEL OF DETAIL
Level of Detail (or LOD for short) is a rather simple but
efficient way of optimizing rendering for large scenes. The basic idea is that
objects that are far away don’t have to be rendered as detailed as close
objects. The following are Primary LOD
selection criteria :
·
Distance
or Size
·
Velocity
·
Eccentricity
·
Depth
of Field
Distance or Size
Select resolution based upon the
distance between an element and the
viewpoint, i.e. coarser resolution for
distant geometry.
·
Simple
to calculate (3-D Euclidean distance)
·
Scale
dependent
·
Resolution
dependent
·
Field
of View dependent
Size LOD
Select resolution based upon the projected screen size (or area) of an element. Objects appear smaller as they move further away.
·
Requires
3-D ® 2-D projection
·
Scale
invariant
·
Resolution
invariant
·
Field
of View invariant
Bounding spheres or ellipsoids
normally used instead of boxes as more
efficient to calculate projected extent
Eccentricity LOD
- Resolution is selected based
upon the degree to which an element
exists in the visual periphery,
i.e. display elements that the user
is looking at in high resolution.
- Humans can resolve less detail in their peripheral field due to:
- more retinal photoreceptors (rods/cones) towards fovea
- retinal and cortical cell receptive field sizes increases linearly with eccentricity
- 80% of cortical cells devoted to central 10 degrees of vision
- Use eye tracking system to track user’s gaze or assume user looking towards center of display
Velocity LOD
- Resolution based upon the angular velocity of an element across the visual field, i.e. faster moving objects appear in lower resolution
- Humans can resolve less spatial detail in objects moving across the retina, causing objects to blur as they move/ rotate, or the user’s gaze moves
- It is believed visual information for small features are destroyed by the process of integrating stimulus energy over time
- Without eye tracking technology, assume angular velocity across display device
Depth of Field LOD
- Resolution
of element dependent upon the depth of field focus of the user’s eyes, i.e. objects
out with the fusional area appear
in lower detail
- Under
binocular vision, both eyes converge on object at certain distance in order to focus
retinal image
- Objects
in front or behind this fusional area are unfocused, suffering from double images
- Must
track both eyes accurately to evaluate convergence distance
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