Real-Time Rendering

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Real-Time Rendering

• SPEEDING UP RENDERING
• Lecture 04
• Marina Gavrilova

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Brief Outline

• Culling
• Hierarchical z-buffering
• Hierarchical Occlusion Map (HOM)
• Impostors and Nailboards
• Hierarchical image caching
• Level of Detail (LOD)
• Triangle Stripping

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Culling

• To cut-off non-visible objects at an early stage
  of the rendering pipeline
• Backface culling Most frequently used culling
• Dot product of surface normal and view vector
• Clustered Culling Determine using 1 test whether
  a group of polygon is visible or not

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Hierarchical View Frustum Culling

• Bounding Volume (BV) Volume that encloses a
  group of objects
• Compute BV Hierarchy and store as Directed
  Acyclic Graph (DAG) or tree
• Leaf holds actual geometry, parent BV contains
  several child BV
• Most common BV Sphere, Axis Aligned Bounding Box
  (AABB), Oriented Bounding Box (OBB)

Hierarchy of BV is often called a Scene Graph
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Rendering Scene with Hierarchical BV

• Child is explored for visibility only if parent
  is completely or partially visible

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BSP trees for static scene

• Excellent for static scene
• Takes a long time to compute
• Efficient culling test
• AABB based BSP
• Take one plane of the box
• Divide objects into two sets
• Subsequent division
• Polygon Aligned BSP
• Choose one polygon as divider
• Use the chosen polygons plane to divide into two
  partition
• Recursively divide the rest

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Rendering scene using BSP

• Polygon aligned BSP trees can be traversed to
  find back to front order of objects
• Render back to front no z-buffer needed
• Render front to back no need to redraw pixels
• Pixel span span of pixels in a row.
• No need to draw pixel spans
• BSP is also known as k-d trees in computational
  geometry

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Portal culling

• Used in architectural design
• Used in Computer games inside buildings with
  doors
• Occluders are large walls
• Compute Potentially visible set (PVS) from one
  point of view
• Divide the entire scene into cells (i.e. room)
• Doors and windows are called portals
• Method 1 impose a BSP on cells (align the
  partitioning planes with the walls)
• Method 2 Subdivide cells and identify portals
  and construct a cell-to-cell visibility data
  structure
• Dramatic Speedup (up to 100 times) when scene is
  very complex with many walls

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Portal Culling (contd)

• It is a refinement process (diminishing view
  frustum)
• Reflection can be seen as another portal (with
  associated PVS)
• Used in Nintendo 64

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Detail Culling

• Object to Screen Pixel Ratio The area of the
  projected BV of an object (in number of pixels)
• Omit objects whos Screen pixel ratio is too
  small
• Replace object with simpler model when screen
  pixel ratio is low

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Occlusion Culling

• Z-buffer is not sufficient for densely packed
  objects (i.e. a forest)
• Occlusion culling saves rendering time by
  preventing occluded objects from being rendered
• General algorithm

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Hierarchical Z-buffering

• Hierarchical Visibility Algorithm (HV)
• Maintain scene model in an Octree
• Maintain a z-pyramid for occlusion test
• Recursively subdivide scene into octree boxes
  (until each leaf containsltk primitives)
• In 2D ? Quadtree representation
• Takes too much time to be generated in runtime
  (only suitable for static scene)

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HV algorithm

• To maintain z-pyramid recurse the furthest value
• Upto 100 times less depth test than standard
  z-buffer

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Other HV based techniques

• Hierarchical polygon tiling ? Very Efficient
• Without hardware implementation HV is too slow
  for real-time graphics
• Take advantage of frame-to-frame coherence
• Rendering antialiased scenes hierarchically
  without error bounds (Greene and Kass)
• Visibility skeleton method (Durand)

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The Hierarchical Occlusion Map (HOM) algorithm

• Another efficient algorithm
• Occlusion is tested using two mechanism
• Depth Test
• Overlap Test
• Identify potentially good occluders
  (preprocessing)

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The HOM Algorithm

• Occlusion map

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The HOM algorithm

• Hierarchy can be generated by hardware texture
  MIP maps
• Test of occlusion is based on a threshold opacity

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Shadow Culling

• Find small number of large occluders
• Make use of separating planes and supporting
  planes

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Impostors

• Impostor image of a complex object that is
  texture mapped onto a rectangle
• Renders faster than the object
• Exploits frame-to-frame coherence
• Objects rendered to a texture from a particular
  viewpoint
• Rendered object is reused until the view point
  changes significantly
• Great for particle systems and complex objects
• Similar to billboards (dynamic/adaptive
  billboard)

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Nailboards

• Nailboard an Imposter frame with an attached
  z-buffer (RGB?, ?z WRT impostor quad)
• Avoids visibility problems of impostors

Real Object
Impostors (without ?)
Nailboard (2 bit ?)
Nailboard (8 bit ?)
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Level Of Detail (LOD)

• Simpler objects as the object moves further away
  from the camera
• LOD is often equals to of triangles
• A more general measure for error metric is screen
  pixel ratio
• Problem When LOD changes popping effects occur

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Correcting LOD popping effect

• Alpha LOD increase transparency of object as
  distance increases
• Object fades away
• More continuous
• Object becomes invisible
• Geomorph LOD
• Use different geometry and perform smooth
  geometric interpolation between LOD
• Smooth continuous transition of LOD
• Hard to implement

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LOD management

• Define an error metric for the object LOD
• Optimally refine object to improve accuracy
• Restrict refinement operations to a constant
  amount per frame

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Triangle Stripping

• Define triangles of an objects as continuous
  strip
• Improves rendering performance
• SGI algorithm (greedy)
• Stripe algorithm

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End of Lecture 04

• Questions?