Chi-Cheng Lin, Winona State University

CS430 Computer Graphics

Introduction

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Topics

Introduction Applications Elementary Output Primitives Graphics Output Devices Graphics Input Devices Graphics Software Standards Interactive Computer Graphics

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Introduction What is Computer Graphics? History

Whirlwind Computer 1950, MIT Computer-driven CRT (Cathode Ray Tube) displays

Sketchpad drawing system 1963, Ivan Sutherland Interactive graphics: keyboard and light pen Data structures

DAC system 1964, GM Automobile design (CAD/CAM)

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Introduction

History (cont’d)Bitmap graphics

Apple and PC Bitmap:

– A ones and zeros representation of the rectangular array of points on the screen.

– The points are called pixels or pels (picture elements).

GUIs SIGGRAPH: www.siggraph.org Why computer graphics?

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Applications

Art, entertainment, publishing Image processing Monitoring manufacturing

processes Displaying simulation CAD Scientific analysis and visualization

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Elementary Output Primitives

Polyline Curves Text Filled region Raster image

Each primitive is associated with a set of attributes

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Polyline

Connected sequence of straight linesFigs. 1.8 and 1.9

Point (vertex, or dot)Degenerated case of polylineE.g., drawDot(x, y)

LinePolyline of one line segmentE.g., drawLine(x1, y1, x2, y2)

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Polyline Polyline

E.g., drawPolyline(poly), where poly is a list containing all endpoints

Fig. 1.10 Polygon

Formed by connecting the first and last points of a polyline

Simple polygon: no two edges crossConvex polygonConcave polygonFig 1.11

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Attributes of Polyline

Thickness Color Style

Solid, dotted, dashed, … Joint (Fig 1.13)

Butt-endround end miteredtrimmed mitered

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Curves

Circle Ellipse Parabola Hyperbola B-spline NURBS

Attributes?

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Text Character string Shape defined by (Fig. 1.16)

Bit mapPolyline or curve

E.g., drawString(x, y, string) Attributes

Font or “type face” (Fig. 1.15) SizeSpacingorientation

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Filled Region (Area)

Shape filled with color or patternFigs. 17 and 18

Boundary is usually a polygon E.g., fillPolygon(poly, pattern) Attributes

Attributes of enclosing borderPattern of fillingColor of filling

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Raster Image

Image made up of pixels (Fig. 1.19)

Stored as a 2D array of numerical values (Fig. 1.20)Bit map: 1 bit per pixelPixel map: >1 bits per pixel

Formatspbmppm

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Gray-Scale Raster Image

Pixel depth:#bits needed to represent the gray

level of a pixel1 bit: bi-level, black-and-white,

monochrome (Fig 1.26)2 bits: 4 gray levels4 bits: 16 gray levels

:8 bits: 256 gray levels

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Color Raster Image

Color valueOrdered triple (r, g, b) representing

intensities of red, green, blue, respectively Color depth:

#bits used to represent the color of a pixelE.g., color depth = 3

(0,0,0) = black, (1,1,1) = white(0,0,1) = blue, (1,0,0) = red, (1,0,1) = ?

Q: how many colors can be represented?

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Color Raster Image

High-quality true color imageColor depth = 24, 8 bits for each color

componentQ: how many colors can be

represented?Q: how many bytes are required to

store a 480 x 640 imageQ: Is the image quality improved with

a larger color depth?

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Graphics Output Devices

Vector Raster

Display Hard-copy

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Vector Display

Electron beam deflected from endpoint to endpoint

One line at a time, similar to pen plotter Dictated by display commands Random Scan, refresh required Display processor must cycle through display

list to refresh CRT's phosphor at least 60 times per second (60Hz) to avoid flicker.

Image changed by changing display commands in the list

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Raster Output Device 2D display surface Built-in 2D coordinate system, either

upside-down or not (Fig. 1.35) A position on the display surface is

associated with an image pixel Frame buffer

A region of memory to hold all of the pixel values for the display surface

Where is it in a computer? Block diagram (Fig. 1.36)

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Raster Output Device Scanning process (Fig. 1.37)

Sequential scan One line at a time, from left to right From top to bottom Back to top (refresh, required for CRT

monitor)– Refresh rate must be >60 times per second to

prevent flicker

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Raster Display (Monitor) Display primitives stored in a frame

buffer in terms of primitives' component pixels

Images formed from the raster, a set of horizontal scan linesScan line: a row of individual pixels

Raster is stored as a matrix of pixels representing entire screen area

Memory issue Sequential scan and refresh

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Raster Display (Monitor)

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Raster Display (Monitor) Beam's intensity set to reflect

pixel's intensity at each pixel Color Systems

Three beams, RED, GREEN, and BLUE, are controlled

Two types True color display system Indexed color display system

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Comparison of Vector and Raster

Raster DisplaysAdvantages

Lower cost Ability to display filled area

Disadvantages Discrete nature of pixel presentation Aliasing: Jaggies or staircasing caused by

approximation

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Comparison of Vector and Raster

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True-color Display System

Operation (Fig. 1.38)

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Indexed-Color Display System Pseudo-color display system Color lookup table (LUT)

Pixel value used as an index into LUTProgrammablePalette: set of possible colors can be displayed

If color depth=b and LUT width=w2w colors can be displayed, any 2b at one time

Example (Fig1.40): 6 bpp, 15-bit color32K colors can be displayed, 64 at one timeThe display has a palette of 32K colors

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Comparisons of Color Systems Example (Fig 1.41):

1024x1280-pixel-displayTrue color system, color depth = 24

Memory requirement:1024 x 1280 x 24 4MB

Indexed color system, color depth = 8, width of LUT = 24 bits Memory requirement:

1024 x 1280 x 8 + 256 x 3 1MB Indexed color displays with LUT is much

cheaper than true color systems!!

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Hard-Copy Output Devices

PlotterFlatbed plotterDrum plotter

Dot matrix printer Laser printer Inkjet printer Film recorder

Which are vector and which are raster?

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Graphics Input Devices

Logical input devices (primitives)StringChoiceValuatorLocatorPick

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Graphics Input Devices Physical input devices

KeyboardButtonsMouseTabletJoystick and trackballKnobsSpace ball and data gloveDigitizing 3D objects and capturing motion

Which physical input devices are suitable for a particular logical input device?

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Graphics Software Standards

Core3D Core graphics SystemProduced by an ACM SIGGRAPH

committee GKS

Graphics Kernel System2DGKS-3D, 1988Groups of primitives arranged as

segments, but not nested

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Graphics Software Standards PHIGS

1988 (Programmers Hierarchical Interactive Graphics System)

Nested structuresStructure databasesPHIGS PLUS: includes shading and lightingComplicated, large

de facto Standards:PEX (PHIGS Extension to X)PostScriptOpenGL

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Interactive Computer Graphics

Framework

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Interactive Computer Graphics Interactive handling schema

Generate initial displaydo { enable selection of commands or objects wait for user selection switch (on selection) { process selection to complete command

or process completed command updating model and screen as needed }} while (!quit)