Visibility Of Computer Graphics

Appearance of Computer Game Scene Graph

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
• Size LOD
• 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

• 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:
  1. more retinal photoreceptors (rods/cones)  towards fovea
  2. retinal and cortical cell receptive field sizes  increases linearly with eccentricity
  3. 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

• 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

• 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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1. Claudia Lukita. D (51415532) 

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Referensi :
https://en.wikipedia.org/wiki/Scene_graph

https://graphics.pixar.com/library/LOD2002/2-perception.pdf