CS430 Computer Graphics

1. CS430 Computer Graphics Introduction Chi-Cheng Lin, Winona State University

2. Topics • Introduction • Applications • Elementary Output Primitives • Graphics Output Devices • Graphics Input Devices • Graphics Software Standards • Interactive Computer Graphics

3. 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)

4. 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?

5. Applications • Art, entertainment, publishing • Image processing • Monitoring manufacturing processes • Displaying simulation • CAD • Scientific analysis and visualization

6. Elementary Output Primitives • Polyline • Curves • Text • Filled region • Raster image • Each primitive is associated with a set of attributes

7. Polyline • Connected sequence of straight lines • Figs. 1.8 and 1.9 • Point (vertex, or dot) • Degenerated case of polyline • E.g., drawDot(x, y) • Line • Polyline of one line segment • E.g., drawLine(x1, y1, x2, y2)

8. 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 cross • Convex polygon • Concave polygon • Fig 1.11

9. Attributes of Polyline • Thickness • Color • Style • Solid, dotted, dashed, … • Joint (Fig 1.13) • Butt-end • round end • mitered • trimmed mitered

10. Curves • Circle • Ellipse • Parabola • Hyperbola • B-spline • NURBS • Attributes?

11. Text • Character string • Shape defined by (Fig. 1.16) • Bit map • Polyline or curve • E.g., drawString(x, y, string) • Attributes • Font or “type face” (Fig. 1.15) • Size • Spacing • orientation

12. Filled Region (Area) • Shape filled with color or pattern • Figs. 17 and 18 • Boundary is usually a polygon • E.g., fillPolygon(poly, pattern) • Attributes • Attributes of enclosing border • Pattern of filling • Color of filling

13. 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 pixel • Pixel map: >1 bits per pixel • Formats • pbm • ppm

14. Gray-Scale Raster Image • Pixel depth: • #bits needed to represent the gray level of a pixel • 1 bit: bi-level, black-and-white, monochrome (Fig 1.26) 2 bits: 4 gray levels 4 bits: 16 gray levels : 8 bits: 256 gray levels

15. Color Raster Image • Color value • Ordered triple (r, g, b) representing intensities of red, green, blue, respectively • Color depth: • #bits used to represent the color of a pixel • E.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?

16. Color Raster Image • High-quality true color image • Color depth = 24, 8 bits for each color component • Q: how many colors can be represented? • Q: how many bytes are required to store a 480 x 640 image • Q: Is the image quality improved with a larger color depth?

17. Graphics Output Devices • Vector Raster • Display Hard-copy

18. 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

19. 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)

20. 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

21. 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 lines • Scan line: a row of individual pixels • Raster is stored as a matrix of pixels representing entire screen area • Memory issue • Sequential scan and refresh

22. Raster Display (Monitor)

23. 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

24. Comparison of Vector and Raster • Raster Displays • Advantages • Lower cost • Ability to display filled area • Disadvantages • Discrete nature of pixel presentation • Aliasing: Jaggies or staircasing caused by approximation

25. Comparison of Vector and Raster

26. True-color Display System • Operation (Fig. 1.38)

27. Indexed-Color Display System • Pseudo-color display system • Color lookup table (LUT) • Pixel value used as an index into LUT • Programmable • Palette: set of possible colors can be displayed • If color depth=b and LUT width=w • 2w colors can be displayed, any 2b at one time • Example (Fig1.40): 6 bpp, 15-bit color • 32K colors can be displayed, 64 at one time • The display has a palette of 32K colors

28. Comparisons of Color Systems • Example (Fig 1.41): 1024x1280-pixel-display • True 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!!

29. Hard-Copy Output Devices • Plotter • Flatbed plotter • Drum plotter • Dot matrix printer • Laser printer • Inkjet printer • Film recorder • Which are vector and which are raster?

30. Graphics Input Devices • Logical input devices (primitives) • String • Choice • Valuator • Locator • Pick

31. Graphics Input Devices • Physical input devices • Keyboard • Buttons • Mouse • Tablet • Joystick and trackball • Knobs • Space ball and data glove • Digitizing 3D objects and capturing motion • Which physical input devices are suitable for a particular logical input device?

32. Graphics Software Standards • Core • 3D Core graphics System • Produced by an ACM SIGGRAPH committee • GKS • Graphics Kernel System • 2D • GKS-3D, 1988 • Groups of primitives arranged as segments, but not nested

33. Graphics Software Standards • PHIGS • 1988 (Programmers Hierarchical Interactive Graphics System) • Nested structures • Structure databases • PHIGS PLUS: includes shading and lighting • Complicated, large • de facto Standards: • PEX (PHIGS Extension to X) • PostScript • OpenGL

34. Interactive Computer Graphics • Framework

35. Interactive Computer Graphics • Interactive handling schema Generate initial display do { 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)