Graphics

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Graphics
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Introduction

• 1995 began the modern era of gaming graphics
• First truly 3D capable consoles
• Sony Playstation
• Sega Saturn
• First serious 3D consumer 3D card for PCs
• 3Dfx Voodoo

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Introduction

• Before 1995 console games were 2D affairs
• with a few exceptions (Star Fox, SNES)
• There were several 3D PC games
• Ultima Underworld, Doom, Descent, etc.
• relied on heavily optimised software renderers
• content limited to achieve decent frame rate

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Anatomy of a Renderer

• Geometry processing
• vertex transformation
• lighting, fog
• view frustum clipping
• backface culling
• Rasterisation
• texture mapping
• alpha blending
• depth testing
• Gouraud shading

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Geometry Processing

• Performed on
• host CPU (e.g. PC/Voodoo 1)
• graphics sub-system (e.g. PC/G-Force)
• specialised co-processor (Playstation 2)
• Flexibility ranges from fully programmable to
  completely fixed in hardware
• Generally computation-bound
• in some cases, bandwidth-bound

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Rasterisation

• Extremely memory bound
• e.g. 32 bit trilinear, alpha, z-buffered
• 8 texture reads
• 1 frame buffer read
• 1 frame buffer write
• 1 z buffer read
• 1 z buffer write
• 12 memory ops (48 bytes) per pixel
• each 1024x768 screen fill uses 37MB bandwidth

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Comparing Graphics Architectures

• Speed
• fill rate
• triangle rate
• peak vs. sustained in-game rate
• Features
• antialiasing
• multi-texture
• stencil buffer
• Some modern systems are very fast, but relatively
  feature poor

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Comparative Fill Rates
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Comparative Triangle Rates
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Features

• All modern graphics systems offer
• triangles
• Gouraud shading
• texture mapping
• hidden-surface removal
• blending
• These are the basic tools of a real-time graphics
  programmer
• Many offline rendering algorithms can be restated
  using the above constructs

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Gouraud Shading

• Per-pixel values generated via interpolation of
  vertex components
• first-order smoothing algorithm for faceted
  objects
• most people think of colour interpolation
• hardware also interpolates
• texture co-ordinates
• fog
• alpha
• depth

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Gouraud Shading contd

• Texture co-ordinates are generally perspective
  correct
• exception Playstation
• Other components might not be
• especially colour, fog
• Watch for artifacts
• Mach bands
• warping
• rotational variance

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Texture Mapping

• Invented by Ed Catmul in 1974

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Multi-texture
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Artifact Magnification

• Happens when pixel-to-texel ratio 1
• Texture becomes blocky
• Solutions
• higher resolution
• bilinear filtering
• pull camera back

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Artifact Minification

• Happens when pixel-to-texel ratio
• Texture sparkles and swims
• Solutions
• lower frequency textures
• mip-mapping

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Artifact LOD Banding

• Occurs at transitions between mip-map levels
• Noticeable discontinuity between LODs
• Solution
• trilinear filtering
• interpolate between LODs
• usually incurs a performance penalty

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Blending

• Compostion of rendered pixels
• addition, subtraction, multiplication, etc.
• Many useful applications
• transparency
• glows
• decals
• shadows
• caustics
• Blending allows for multi-pass techniques

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OpenGL / D3D Blending Equation

• C CsFs CdFd
• C output colour
• Cs source colour
• generated from rasterisation
• Fs source factor
• zero, one, src, dest, alpha, inverse alpha, etc.
• Cd destination colour
• in the frame buffer
• Fd destination factor
• zero, one, src, dest, alpha, inverse alpha, etc.

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Blending Example

• Additive ? C Cs Cd
• Cs1 Cd1
• glBlendFunc(GL_ONE, GL_ONE)
• Multiplicative ? C Cs Cd
• CsCd Cd0
• glBlendFunc(GL_DST_COLOR, GL_ZERO)
• Transparency ? C Cs? Cd(1-?)
• glBlendFunc(GL_SRC_ALPHA, GL_ONE_MINUS_SRC_
  ALPHA)

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Hidden Surface Removal

• Older systems used the painters algorithm
• Playstation, Saturn, early PC
• per-polygon
• unpleasant sorting artifacts
• Everything now uses a z-buffer
• per-pixel
• suffers accuracy problems
• transparency is difficult to handle properly
• sometimes consumes frame buffer memory

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Stencil

• Used to mask out areas of the frame buffer
• Many uses
• shadows
• mirrors
• screen wipes

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Techniques
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Surface Geometry
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Lighting
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Shadows
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Fog Haze
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Reflection
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Refraction
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Micro-detail
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Transparency
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Volumetric Effects