Computer Graphics Graphics Hardware

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Computer GraphicsGraphics Hardware

• CO2409 Computer Graphics
• Week 19

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Lecture Contents

• Graphics Architecture
• Processing
• GPU vs CPU
• Memory
• Video Memory vs System Memory
• Graphics Unit Specifications
• Interface
• Custom vs AGP vs PCI Express
• Concurrency

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Graphics Architecture

• The basic graphics architecture for all modern
  PCs and game consoles is similar

• Two key parts
• Main System
• Graphics Unit
• 1 Processor each
• Local RAM for each processor
• Fast access
• Interface between components slow
• Compared to local access

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Graphics Unit

• The graphics unit describes the graphics
  processing components of a system
• PC usually a separate graphics card
• Console built-in custom components

• The core of a graphics unit is the Graphics
  Processing Unit (GPU)
• Properly called a Graphics Adapter
• Equivalent of a CPU for graphics

Xbox 360 GPU
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Graphics Unit

• The graphics unit also contains
• Local RAM (a.k.a. video memory, VRAM, etc.)
• Output sockets for monitor, TV etc.

• Can be integrated into the motherboard
• Consoles, some PCs and laptops
• Or attached via an interface
• Recent PC cards use AGP or PCI Express sockets

NVidia 9600 Graphics Card
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GPU vs CPU

• A GPU is a dedicated graphics processor for a PC
  or game console
• Usually highly parallel
• i.e. Can run many processes simultaneously
• Much faster than CPU for graphics algorithms
• Particularly vector/matrix operations
• All modern GPUs have a programmable core pipeline
  (shaders)
• Power of a GPU is measured by its
• Clock speed
• Parallelism. E.g. number of simultaneous vertex /
  pixel operations (ROPs - Raster Operation Process)

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Graphics Memory

• Many graphics units have local memory
• To store vertices, primitives and textures for
  the GPU
• This memory is similar to standard CPU memory
• Usually more closely coupled to the GPU
• I.e. Bandwidth to the GPU will be greater
• Graphics memory can be measured by
• Clock speed (how fast can it serve data)
• Bandwidth to the GPU (speed of the bus between
  graphics memory and the GPU)
• Some GPUs have no dedicated memory
• Must share memory with the CPU
• This can be a benefit or a penalty

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Graphics vs System Memory

• GPUs can usually access system memory
• i.e. CPU-local memory
• Bandwidth is often lower
• Usually better to have graphics data in GPU memory

• System memory can be used as a back-up
• But expect lag if relied on too heavily
• Some architectures share CPU GPU memory
• Xbox 360 entire console architecture designed
  around this sharing performance is high
• On-board video cards Lack of GPU memory is
  cost-cutting feature memory access is slow

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Graphics Unit - Specifications

• The key specifications for a graphics unit are
• GPU clock speed (MHz ) speed of processor
• Amount of Memory (Mb)
• Memory clock speed (MHz / GHz)
• GPUlt-gt Memory Bandwidth (GB / s)
• Speed of transfers between GPU and local memory
• Other factors to take into account
• Pixel / Texture Fill Rate (Giga-Pixels / s)
• Speed of drawing (textured) pixels
• Amount of parallelism in the pipeline
• E.g. Number of pixel (shader) pipes how many
  pixels can be calculated at the same time

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Specs Comparison (Indicative)
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GPU / System Interface

• Try to keep graphics data local to the GPU
• But the CPU and system memory still need to
  interface with the graphics unit
• To get data into the graphics memory
• To issue instructions to the GPU (DrawPrimitive)
• For dynamic geometry / textures
• So the interface between the graphics unit and
  the rest of the system is critical
• Games consoles have the graphics unit built into
  the motherboard and so are closely coupled
• E.g. Xbox 360, PS3 have fairly symmetrical
  performance between GPU, CPU memory

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PC Graphics Interfaces

• PCs have two key graphics interfaces

• AGP (Accelerated Graphics Port)
• Shares some CPU memory with the GPU (AGP
  Aperture)
• Various speeds (1x to 8x) up to 2 GB/s transfer
• PCI Express (newer)
• Uses serial lanes to transfer streams of data
  in parallel
• Double AGP bandwidth (theoretical)

• Both slow compared to local GPU memory
• PCs must rely on more graphics memory

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Concurrency

• GPU operates in parallel with the CPU
• And the GPU is parallel internally
• This is a concurrent system and we need to
  program it as such
• Try to ensure both CPU and GPU working all the
  time
• Neither should be waiting for the other to
  complete its current task
• Has implications about how we should program
  graphics applications
• Games students will see this in the 3rd year