Graphics Hardware

1. Graphics Hardware Chapter 4 Chapter 4 -- Graphics Hardware

2. In this chapter, we describe how the important hardware elements of a computer graphics display system work. • Hardware technologies: • printers, pen plotters, laser printers, ink-jet plotters, • Display technologies • monochrome and shadow-mask CRT’s, LCD’s, … • Raster Display Systems • Overview, … • Input Devices • film scanners,… Chapter 4 -- Graphics Hardware

3. Here is the typical relation of the devices to one another: • The key elements are the CPU and the display processor. Chapter 4 -- Graphics Hardware

4. 1. Hardcopy Technologies • In this section, we discuss various hardcopy technologies, then summarize their characteristics. • However, several important terms must be defined first. Chapter 4 -- Graphics Hardware

5. Dot size (also called spot size) is the diameter of a single dot created by a device. • Addressability is the number of individual dots per inch that can be created • This may differ in the horizontal and vertical directions. • Addressability in x is just the reciprocal of the distance between the centers of dots at addresses (x,y) and (x+1,y) • Addressability in y is defined similarly. Chapter 4 -- Graphics Hardware

6. Interdot distance is the reciprocal of addressability. • It is usually desirable that dot size be somewhat greater than the interdot distance, so that smooth shapes can be created • There are tradeoffs: • Dot size several times the interdot distance allows very smooth shapes to be printed. • whereas a smaller dot size allows finer detail. Chapter 4 -- Graphics Hardware

7. Resolution, which is related to the dot size and can be no greater than addressability, is the number of distinguishable lines per inch that a device can create. • If 40 black lines interleaved with 40 white lines can be distinguished across one inch, the resolution is 80 lines per inch • Resolution also depends on the cross-sectional intensity distribution of a spot. A spot with sharply delineated edges yields higher resolution than does one with edges that trail off. Chapter 4 -- Graphics Hardware

8. Color Output: • Many devices to be discussed can create only a few colors at any one point. Additional colors can be obtained with dither patterns (discussed in Chapter 11). This results in a decreased spatial resolution for the resulting image. • With Color Ribbons a Dot-Matrix printer can produce color images in two ways: • Multiple print heads (each with a different color) • Single print head with a multi-colored ribbon • More colors than are actually on the ribbon can be created by overstriking two different colors at the same dot on the paper. Chapter 4 -- Graphics Hardware

9. Hard Copy Devices: • Dot Matrix Printers • use a print head from 7 to 24 pins (thin, stiff pieces of wire) each of which can be individually fired to strike a ribbon against the paper. • The print head moves across the paper one step at a time, the paper is advanced one line, and the print head makes another pass across the paper. • Hence, these printers are raster output devices, requiring scan conversion of vector images prior to printing. Chapter 4 -- Graphics Hardware

10. Pen Plotters: • moves the pen over the paper in a random, vector-drawing style. • In drawing a line, • the pen is positioned at the start of the line, • lowered to the paper, • moved in a straight path to the end of the line, • raised, and moved to the start of the next line. • There are two basic types: • flatbed plotters and • drum plotters. Chapter 4 -- Graphics Hardware

11. flatbed plotter • Pen is moved in x and y on a sheet of paper spread out on a table. • The paper is held by static charge, by vacuum, or stretched tightly. • A carriage moves in one axis over the paper. • The carriage holds a pen mount which can move the length of the carriage. • The pen is raised and lowered and the pen mount and carriage move to draw the line. • flatbed plotters are available in sizes from 12 by 18 inches to 6 by 10 feet and larger. Chapter 4 -- Graphics Hardware

12. drum plotter • The paper is moved along one axis (as opposed to the carriage moving) • The pen-mount moves across the drum • Pins on the drum engage pre-punched holes in the paper to prevent slipping. • desktop plotter • is a special type of drum plotter, • but the paper is moved between pinch rollers instead of using pins and holes. Chapter 4 -- Graphics Hardware

13. Laser Printers • A laser beam is scanned across a positively charged rotating drum (coated with selenium) • The beam changes the charge, and the area left positive will become black. • A negatively charged powder adheres to the positively charged areas of the drum, and is then transferred to the paper. Chapter 4 -- Graphics Hardware

14. Laser Printers (cont.) • In color xerography, this process is repeated three times • once for each primary color • Laser printers have a microprocessor to do scan conversion and to control the printer. • An increasing number also accept the PostScript document and image description language. • Most laser printers work with 8.5x11 inch paper or 8.5x14 inch paper. Chapter 4 -- Graphics Hardware

15. Ink-Jet printers • Spray cyan, magenta, yellow, and sometimes black ink onto paper. • In most cases they are mounted on a head in a printer-like mechanism. • The print head • moves across the page to draw one scan line, and • returns while the paper advances by one inter-scan-line spacing, and • draws the next line. • All the colors are deposited simultaneously • unlike the multi-pass lasers, plotters, and dot matrix printers • Very few have variable dot-size capabilities. Chapter 4 -- Graphics Hardware

16. Thermal-Transfer printers • another raster hardcopy device. • Use finely spaced (200 per inch) heating nibs to transfer pigments from colored wax paper to plain paper. • The wax paper (made up of strips of cyan, magenta, yellow and black) and the plain paper are drawn together over the strip of heating nibs • The nibs are selectively heated to cause the pigment to transfer. • Because the nibs heat and cool very rapidly, a single color image can be created in less than one minute. • Some of these printers accept video signals and digital bitmap input, making them convenient for creating hardcopy of video images. Chapter 4 -- Graphics Hardware

17. Thermal sublimation dye transfer printers • work similarly to the thermal transfer printers. • Except this process permits 256 intensities of each color to be transferred • cyan, magenta,and yellow • This creates a high-quality full-color image with a spatial resolution of 200 dots per inch. • The process is slower than wax transfer, but the quality is near-photographic • therefore it is the clear choice for full-color pre-press proofs. Chapter 4 -- Graphics Hardware

18. Color Film Recorders: • There are two basic techniques: • The camera records the color image directly from the color CRT. • image resolution is limited • A black and white CRT is photographed through color filters, and the different color components of the image are displayed in sequence • This technique yields very high-quality images. • Colors are mixed by double exposing parts of the image through two or more filters (usually with different intensities) • The input can be a raster video signal, a bitmap, or vector-style instructions. • Video signal must stay constant during the entire recording cycle (which can take up to one minute with slow film) Chapter 4 -- Graphics Hardware

19. 2. Display Technologies • Interactive Computer Graphics demands display devices whose images can be changed quickly. • The CRT is by far the most common display device, however, solid-state technologies are being developed that may, in the long term, substantially reduce the dominance of the CRT. Chapter 4 -- Graphics Hardware

20. Monochrome CRT’s • Essentially the same as those used in black-and-white home television sets. Chapter 4 -- Graphics Hardware

21. Color Raster Displays: • Shadow Mask CRT. Chapter 4 -- Graphics Hardware

22. Liquid-Crystal Display (LCD) • It is made up of six layers • The crystal layers polarize the light (or not) and allow it to pass through and reach the back reflective layer and come back. • The electric current disables the polarization of some of the crystals, so it can pass through one layer, but be blocked by the next. Chapter 4 -- Graphics Hardware

23. 3. Raster-Scan Display Systems • The basic concepts of raster graphics systems were presented in Chapter1, and Chapter 2 provided further insight into the types of operations possible with a raster display. • This section discusses the various elements of a raster display. Chapter 4 -- Graphics Hardware

24. 4. The Video Controller • The most important task for the video controller is the constant refresh of the display. • There are two fundamental types of refresh: • interlaced (used in TV’s) • non-interlaced. (most video controllers) Chapter 4 -- Graphics Hardware

25. Output: • RGB • separate cables carry red, green, and blue signals to the three electron guns of the shadow-mask CRT. another cable carries the sync for vertical and horizontal retraces. • Monochrome • NTSC • National Television System Committee -- North American commercial television • 525 scan lines (only 480 are visible) • Bandwidth limits the quality to an effective resolution of about 350x350. Chapter 4 -- Graphics Hardware

26. 4.1 Video Mixing • Another useful video-controller function is video mixing. • Two images • one defined in the frame buffer. • another defined by video signal coming from some other source (camera, …) • merged into a composite image. Chapter 4 -- Graphics Hardware

27. Examples of this merging are seen regularly on television news, sports, and weather shows Chapter 4 -- Graphics Hardware

28. 5. Input Devices for Operator Interaction • There are five basic logical devices: • the locator - indicates position or orientation. • the pick - to select a displayed entity • the valuator - to input a single real number. • the keyboard - to input a character string • the choice - to select from a set of possible actions or choices. Chapter 4 -- Graphics Hardware

29. 5.1 Locator Devices • Data Tablet • Mouse • Trackball • Joystick • Touch Panel Chapter 4 -- Graphics Hardware

30. 5.2 Keyboard Devices • The alphanumeric keyboard is the prototypical text input device. • The important functional characteristic of this device is that it creates a code uniquely corresponding to a pressed key. • Cording (pressing several keys at once) is generally not possible -- unless the additional keys were shift, control, or other special keys) Chapter 4 -- Graphics Hardware

31. 5.3 Valuator Devices • Most valuator devices that provide scalar values are based on potentiometers • like the volume and tone controls on a stereo set. • They are usually rotary (dials) with either a limited range, or a free-turning (unbounded range) • linear ones are used infrequently in graphics systems Chapter 4 -- Graphics Hardware

32. 5.4 Choice Devices • Function keys are the most common choice device. • Other choice devices are the buttons on many tablet pucks, or mice. • Choice devices are used to enter commands or menu options in a graphics program Chapter 4 -- Graphics Hardware

33. 6. Image Scanners • Although data tablets can be used to digitize existing line drawing manually, • this is a slow tedious process. • unsuitable for mare than a few simple drawings. • it does not work at all for half-tone images • Image scanners provide an efficient solution Chapter 4 -- Graphics Hardware

34. There are may types: • A Television Camera with a digital frame grabber • moderate resolution (1000x1000) • Slow Scan CCD television cameras • 2000x2000 image in about 30 seconds • Scan head • grid of light sensing cells • mounted on the print head of a printer • resolution of 80 units per inch Chapter 4 -- Graphics Hardware

35. Photo Scanner • highest resolution ones use laser light sources and have resolutions greater than 2000 units per inch. Chapter 4 -- Graphics Hardware