Reality Engine

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Reality Engine

• Anselmo Lastra
• COMP290-052

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Project Teams

• Reports through semester
• First overall design presentation/discussion
• Then detailed designs/progress reports

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Reality Engine

• First OpenGL machine
• Change from proprietary IrisGL
• Fastest commercial machine of time
• Generations
• Akeleys definitions
• Begins at flat-shaded polygons

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Goals

• ½ million textured tris/sec
• Mip-mapped textures
• Antialising
• w/o sorting
• High fill rate
• Work as well as VGX on 2nd gen work

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Three types of Boards

• Geometry board
• 6, 8, 12 geometry processors
• Raster memory board
• 1, 2, 4 boards to increase fill rate
• Or antialiasing
• Display/video generator

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Command Processor

• Controls work sent to geometry engines
• Broadcasts some state info
• Send some to particular GE
• Round-robin assignment
• Static load balancing
• Sizes of primitives (t-strips) sent to GEs
  important
• Primitive ordering

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Geometry Engines

• Intel i860
• RISC, pipelined FP
• All polygons converted to tris
• Single precision computation
• Typical

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Fragment Generators

• Custom ASICs
• Each a portion of framebuffer
• Interleaved
• Talk about fragment generation later

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Subpixel mask

• Fixed 8x8 grid
• Select 4, 8, or 16 samples on grid
• Computer coverage of samples
• Only one depth and texture coord chosen
• Depths expanded later from dX, dY
• Color at center also

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Texturing

• Texture replicated at each fragment processor
• 5-20 times
• Eight DRAM chips
• One for each mipmap sample

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Image Engine

• 16 per fragment generator
• One DRAM each
• Each computes depth at subpixel covered by
  fragment
• Bits/pixel depends on of boards and display
  resolution
• 12 bits / color component

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Display Board

• Display color computed by image engines every
  fragment
• OpenGL has no explicit end-of-frame
• 50MHz single bit paths to board from each image
  engine
• Color maps, etc.

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Antialiasing

• Alpha
• Coverage on 8x8 grid computed
• Ordering must be observed

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Multisample antialiasing

• Point sampling
• Including accurate edges

• Area sampling
• Can produce artifacts
• Screen-door transparency
• Alpha to coverage

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Texturing

• Default is 16-bits / texel
• Because of bandwidth issues
• Can increase to 32 or 48 bits

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Clipping

• Point out that MIMD better for clipping
• SIMD must execute wasted cycles to compute both
  if and else
• Also less clipping because rasterizers scissor
  the primitives

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Antialiasing

• Single pass
• Multisample
• vs. A-Buffer

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Triangle Bus

• Argues that doing sort before fragment engines
  better than after
• Compares to ES
• Notes PixelFlow frame latency

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Commodity DRAM

• Many other machines used specialized video RAM
• RE stole memory cycles to refresh display
• Commodity parts lowered coast of RE

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Next Time

• Graphics pipeline
• Read
• Mark Segal and Kurt Akeley, The design of the
  OpenGL graphics interface

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Homework

• Compute bandwidths (at dots) required to render a
  million 50-pixel triangles
• Make reasonable assumptions, such as all visible
  (or think about z-buffering)