GDC 2005

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(No Transcript)
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Advanced Light and Shadow Culling Methods

• Eric Lengyel

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Fully Dynamic Environment

• Anything in the world can move
• Cant precompute any visibility information
• Lights completely dynamic
• Cant precompute any lighting information
• Shadows also completely dynamic

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Problems to Be Solved at Run-time

• Determine the set of objects visible to the
  camera
• Determine the set of lights that can influence
  the region of space visible to the camera
• For each light, also determine what subset of the
  visible objects can be illuminated

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Problems to Be Solved at Run-time

• Determine the set of objects that could possibly
  cast shadows into the region of space visible to
  the camera
• A superset of the set of the illuminated objects
  that are visible to the camera

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Visibility Determination

• Organize the world in some way
• Tree structures (BSP, octree, etc.)
• Hierarchical bounding volumes
• Portal system
• A combination of these can work extremely well

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Portal Systems

• World divided into zones
• A zone is the region of space bounded by a convex
  polyhedron
• Zones are connected by portals
• A portal is a planar convex polygon
• From the front side, a portals vertices are
  wound CCW

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Portal Systems

• During visibility determination, only have to
  worry about zones that can be seen through a
  sequence of portals
• For each reachable zone, there is a convex region
  of space visible to the camera

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Portal Systems
Camera
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Portal Systems

• The visible regions form a tree structure
• The region in the zone containing the camera is
  the root of the tree
• Zones seen through n portalshave regions at the
  n-th level in the tree

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Portal Systems
A
D
C
B
C
D
A
B
Camera
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Portal Systems

• Start with view frustum in zone containing the
  camera
• Clip portals leading out of the zone against the
  view frustum
• For any portals intersecting the view frustum,
  construct a new region by extruding clipped edges
  away from camera position

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Portal Systems

• Every visibility region is bounded by a convex
  polyhedron defined by at least 4 planes
• At least 3 lateral planes
• A back plane
• A front plane in non-root nodes
• Plane normals point inward

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Portal Systems

• After region tree has been built...
• Traverse the tree
• Collect objects in each zone that intersect the
  visible region of space for that zone
• Use any frustum/bounding volume test, but test
  against regions planes
• This is the visible object set

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Light Region Trees

• Portals can be used to construct illumination
  trees
• Similar to the visibility tree constructed for
  the camera
• One tree for each light source
• Only recalculated when light moves
• Each node in the tree corresponds to a convex
  region of space

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Light Region Trees

• Three fundamental light types
• Point light
• Spot light, special case of point light
• Infinite (directional) light

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Light Region Trees

• Point light
• Omnidirectional
• Has maximum range
• Root illumination region bounded only be zone
  boundary and lights bounding sphere

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Point Light Tree
B
B
A
A
D
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C
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Spot Light Tree

• Spot light almost same aspoint light
• Difference is the root node of the illumination
  tree
• Spot light starts with a frustum, just like a
  camera does
• Point light affects entire root zone

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Infinite Light Tree

• Light rays parallel forinfinite light
• The lateral planes of each illumination region
  intersect at parallel lines
• The extrusion of planes from a portal always goes
  in one direction instead of away from a point

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Visible Light Determination

• Each zone keeps a linked list of illumination
  regions
• One or more region nodes for each light that can
  shine into the zone
• Each region knows which light generated it

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Visible Light Determination
A
B
For example, consider zone C
Light 1
Light 2
C
D
Light 3
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Visible Light Determination
Light 1
Light 2
Light 3
D
B
A
C
B
A
D
B
C
A
C
D
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Visible Light Determination

• For any given zone, we can walk the linked list
  of illumination regions and collect unique lights
• Repeat process for all zones referenced in the
  cameras visibility tree
• We now have the set ofvisible lights

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Illuminated Object Set

• Given one visible zone and one visible light
  shining into that zone
• Illuminated objects are those which intersect
  both a camera region and a light region

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Illuminated Object Set
Light
Camera
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Illuminated Object Set

• Objects are often only partially within an
  illumination region
• Lighting the whole object wastes rendering time
  due to extra fill
• Fortunately, hardware provides an opportunity for
  optimization

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Lighting Optimization

• Use hardware scissor rectangle
• Calculate intersections of camera regions and
  illumination regions
• Camera-space bounding box determines scissor
  rectangle
• GL_EXT_depth_bounds_test
• Works like a z axis for scissor box

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Lighting Optimization
Max Depth
Min Depth
Image Plane
Camera
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Lighting Optimization

• Scissor rectangle and depth bounds test
• Limits rendering for a single light to the
  maximal visible extents
• Can also be applied to stencil shadow volumes

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Shadow Map Optimization

• Same optimization can be applied from lights
  perspective when rendering shadow maps
• Transform vertices of region intersections into
  light space
• Calculate scissor rectangle and depth bounds
• Reduces unnecessary depth fill

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Shadow Map Optimization
Max Depth
Min Depth
Image Plane
Light
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Shadow-Casting Object Set

• All objects in the illuminated set are also in
  the shadow-casting set
• But an object doesnt have to be visible to be
  casting a shadow into one of the visible camera
  regions
• The shadow-casting set is a superset of the
  illuminated set

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Shadow-Casting Object Set

• Need to find objects between visible regions and
  light source
• We already have a structure in place to make this
  easy
• From an illumination region, walk up the lights
  tree to the root

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Shadow-Casting Object Set
A
B
C
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E
B
Light
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D
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Camera
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Shadow-Casting Object Set

• Collect objects in branch of illumination tree
  containing visible regions
• Can use lateral bounding planes of deepest
  visible regions

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Shadow-Casting Object Set
Light
Culled Caster
Camera
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Shadow-Casting Object Set

• Where illumination regions and camera regions
  intersect...
• Some objects inside illumination region, but not
  between camera region and light source

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Shadow-Casting Object Set
Extra Caster
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Shadow-Casting Object Set

• Eliminating shadow-casting objects on wrong side
  ofcamera region
• Test objects against planes of camera region
  having a positive dot product with light position
• For infinite light, use planes having a positive
  dot product with direction to light

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Shadow-Casting Object Set
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Demonstrations
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Questions?

• lengyel_at_terathon.com
• Slides available at
• http//www.terathon.com/