Advanced Light and Shadow Culling Methods Eric Lengyel

Advanced Light and Shadow Culling Methods Eric Lengyel

Fully Dynamic Environment n Anything in the world can move — Can’t precompute any visibility information n Lights completely dynamic — Can’t precompute any lighting information — Shadows also completely dynamic

Problems to Be Solved at Run-time Determine the set of objects visible to the camera n Determine the set of lights that can influence the region of space visible to the camera n For each light, also determine what subset of the visible objects can be illuminated n

Problems to Be Solved at Run-time n Determine the set of objects that could possibly cast shadows into the region of space visible to the camera —A superset of the illuminated objects that are visible to the camera

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

Portal Systems n World divided into ‘zones’ —A zone is the region of space bounded by a convex polyhedron n Zones are connected by ‘portals’ —A portal is a planar convex polygon — From the front side, a portal’s vertices are wound CCW

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

Portal Systems Camera

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

Portal Systems A D C B C D A Camera B

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

Portal Systems n 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 n Plane normals point inward

Portal Systems n 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 region’s planes — This is the visible object set

Light Region Trees n 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

Light Region Trees n Three fundamental light types — Point light — Spot light, special case of point light — Infinite (directional) light

Light Region Trees n Point light — Omnidirectional — Has maximum range — Root illumination region bounded only be zone boundary and light’s bounding sphere

Point Light Tree B B A A D C

Spot Light Tree n Spot light almost same as point 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

Infinite Light Tree n Light rays parallel for infinite 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

Visible Light Determination n 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

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

Visible Light Determination B D Light 1 Light 2 Light 3 A B D C A D C C B A

Visible Light Determination For any given zone, we can walk the linked list of illumination regions and collect unique lights n Repeat process for all zones referenced in the camera’s visibility tree n We now have the set of visible lights n

Illuminated Object Set n 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

Illuminated Object Set Light Camera

Illuminated Object Set n 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

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

Lighting Optimization Max Depth Min Depth Image Plane Camera

Lighting Optimization n 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

Shadow Map Optimization n Same optimization can be applied from light’s perspective when rendering shadow maps — Transform vertices of region intersections into light space — Calculate scissor rectangle and depth bounds — Reduces unnecessary depth fill

Shadow Map Optimization Max Depth Min Depth Image Plane Light

Shadow-Casting Object Set n All objects in the illuminated set are also in the shadow-casting set — But an object doesn’t 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

Shadow-Casting Object Set Need to find objects between visible regions and light source n We already have a structure in place to make this easy n From an illumination region, walk up the light’s tree to the root n

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

Shadow-Casting Object Set n Collect objects in branch of illumination tree containing visible regions — Can use lateral bounding planes of deepest visible regions

Shadow-Casting Object Set Light Camera Culled Caster

Shadow-Casting Object Set n Where illumination regions and camera regions intersect. . . — Some objects inside illumination region, but not between camera region and light source

Shadow-Casting Object Set Extra Caster

Shadow-Casting Object Set n Eliminating shadow-casting objects on wrong side of camera 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

Shadow-Casting Object Set

Demonstrations

Questions? n lengyel@terathon. com n Slides available at http: //www. terathon. com/