Sun-Jeong Kim

Graphics Systems and Models

2nd Week, 2007

Computer Graphics Applications2

Graphics System

Five major elementsInput device

Processor

Memory

Frame buffer

Output device

A Graphics System

Computer Graphics Applications3

Input Devices

Most graphics systems provide a keyboard and at least one other input device

The mouse, the joystick, and the data tablet

To provide positional information to the system and signals to processor with one or more buttons

Spaceball

Mouse TrackballData Tablet

Joystick Lightpen

Computer Graphics Applications4

Pixels and the Frame Buffer (1)

Almost all graphics systems are raster based

RasterRasterAn array of pixelspixels (picture elements)

Frame bufferFrame bufferA part of memory where the pixels are stored

The core element of a graphics system

Pixels

Computer Graphics Applications5

Pixels and the Frame Buffer (2)

ResolutionResolutionThe number of pixels in the frame buffer

To determine the detail that you can see in the image

DepthDepth or precisionprecision of the frame bufferThe number of bits that are used for each pixel

To determine how many colors can be representedEx) 8 bits per pixel: 28=256 colors, 24 bits (true-color system)

RasterizationRasterization or scan conversionscan conversionConversion of geometric entities (such as lines, circles, and polygons) to pixel colors and locations in the frame buffer

Computer Graphics Applications6

Output Devices (1)

CRT (cathode-ray tube)Refresh rateRefresh rate

The same path is retraced by the beam at a sufficient high rate

Ex) 60 Hz (60 cycles per second)

TriadsTriadsTriangular group of three phosphors (red, green, and blue)

Shadow maskShadow mask

CRT Shadow-Mask CRT

Computer Graphics Applications7

Output Devices (2)

Flat-panel monitorsLight-emitting diodes (LEDs), liquid-crystal displays (LCDs), and plasma panels

Using a two-dimensional grid to address individual light-emitting elements

Projection systemsDigital light projection (DLP)

Hard-copy devicesPrinters and plotters

Generic Flat-Panel Display

Computer Graphics Applications8

Image Formation (1)

There always has been analogous process how image are formed by physical imaging systems

Cameras

Microscopes

Telescopes

Human visual systemRods and cones are light sensors

Rods – monochromatic, night vision

Cones – color sensitiveThree types of cones

Only three values (the tristimulus

values) are sent to the brain The Human Visual System

Computer Graphics Applications9

Image Formation (2)

Elements of image formationObjects

Independent of any viewer and

of any image-formation process

Viewer (camera)To form the image of objects

Light sourcesCamera System

Image Seen by Three Different Viewers

Computer Graphics Applications10

Light and Images

Interaction between light and the surfaces of the object How much light enters the camera

Light = visible spectrumvisible spectrumWavelengths in the range 350~780 nm

Long wavelengths: reds

Short wavelengths: blues

The Electromagnetic Spectrum

Computer Graphics Applications11

Imaging Models

Ray tracingRay tracingOne way to form an image

To follow rays of light from a point source finding which rays enter the lens of the camera

RadiosityRadiosityBased on conservation of energy

Ray Interactions

Computer Graphics Applications12

Imaging Systems

Pinhole camera

ProjectionProjection of point (x, y, z)

Field of viewField of view (angle of viewangle of view)

Pinhole Camera

Side View

Angle of View

dz

xxp −=

dz

yyp −= dzp −=

d

h

2tan2 2−=θ

Computer Graphics Applications13

Synthetic-Camera Model

Conceptual foundation for three-dimensional computer graphics

ProjectorProjectorLine from the center of lens to a point on the object

COPCOP (center of projectioncenter of projection)The center of the lens

Projection planeProjection planeVirtual image plane that are moved in front of the lens

Equivalent views of image formation

Synthetic Camera

Computer Graphics Applications14

Graphics Architectures

Graphics pipelineGeometryGeometry – collection of primitive types and vertices

Vertex processingWorld and view transformations

Projection

Lighting

Clipping and primitive assembly

Rasterization

Fragment processing

Geometric pipeline

Clipping