Display of Images

1. Display of Images Prof. K. J. Hintz Department of Electrical and Computer Engineering George Mason University

2. Digital Image Displays • Psychophysiological Optical Impedance Matching • Human visual system characteristics and capabilities • Display device capabilities • Dynamic • Print

3. Visual Receptors • Retina • Cones (photopic or bright-light vision) • 6-7 million cones • one per nerve • high resolution • Located in fovea (central portion) • Sensitive to color • Rods (scotopic or dim-light vision) • 75-150 million rods • several per nerve • low resolution • Blind spot ~17o off axis

4. Human Visual System • Intensity Resolution • Approximately 40 grey levels (~ 5 bits) • Retinal Edge-enhancement Allows Detection of More Levels • More Sensitive to High Spatial Frequencies Than Low • More Sensitive to Low Intensities Off-axis

5. Human Visual System • Acuity • Minimum visible • 1 arc-second • Minimum separable • 1 arc-minute * Geldard, The Human Senses, Wiley, 1972

6. Human Visual System (Color) • Normal color range violet ( =380 nm) to red ( = 760 nm) • 156 total discriminable hue steps • 1 to 6 nm shift required to discriminate, varies with  • Purkinje phenomenon • Colors matched at high intensity do not match at low intensity

7. Human Visual System (Color) • Bezold-Brucke effect • Except for certain invariant colors, all observed colors upon being brightened shift toward either yellow or blue (middle of spectrum). Invariants colors are: • yellow (=572 nm) • green (=503 nm) • blue (=478 nm)

8. Brightness Perception • Not a simple function of intensity • Depends on background and adjacency • Mach band pattern • If rapid change of intensities, see light and dark bands • Simultaneous contrast (Not a good drawing)

9. Simultaneous Contrast Center is same grey level

10. Displays • Photometric resolution • Correct brightness • Correct color • Pantone color scale • Discrete grey levels • Fewer than or equal to 2(number of bits) • Resolution limited by RMS noise level independent of resolution of driving signal • Grey-scale linearity • Human eye is not good photometer

11. Display Calibration • Turn Contrast and Intensity to Lowest Setting • Increase Intensity to Just Below Observable Raster • Increase Contrast to See All Shades of Test Pattern • Color more complex • Printed scale accompanies software along with sequence of steps for color and intensity calibration

12. Display Spot • Gaussian Intensity Distribution About Mean Position • Letting R be the 1/2 Intensity Radius

13. Static Display Criteriafor Flat Field • Desire to Have Uniform Grey Scale Produced by an Array of Dots • Best Flat-field Occurs With Horizontal and Vertical Spacing Between 1.55 R and 1.65 R for Gaussian Intensity Distribution

14. Dynamic Display Criteria • High Frequency Line Pattern ||||||||||||| • Modulation depth falls off as line spacing decreases below 2R • Modulation is 70% at best flat-field of 1.6R • Pixel Checkerboard :::::: • Modulation depth falls off worse than line pattern • Modulation is ~40% at best flat-field of 1.6R

15. Theoretical Best Spot • (Sin x) / (x) Since This Is Transform of Square Sample •  is / •  is sample spacing • Not Realizable but Can Reduce Effect by • Oversampling • effect of each Gaussian pixel reduced • Resampling • increase size by 2 or 3

16. Display Noise • Amplitude • Random: salt & pepper • Periodic: herringbone • Synchronized with H or V: bars • Spot Position • Random pixel movement produces amplitude modulation • Most obvious with flat-fields

17. Permanent Displays • Binary Printers Produce Grey Scale by Dithering or Half-toning • CRT Cameras Photograph Phosphor Display

18. Permanent Displays • Color Printers Use Cyan, Magenta, and Yellow Plus Black • Ink-jet • Colors sprayed onto media • Thermal wax transfer • Colors already on plastic sheet which is passed between the heated print head and paper • Dye Sublimation • Dye on ribbon is heated and turns to gas which is absorbed by polyester coating on print medium...best quality

19. Printing Terminology, HSI • Chromaticity Diagram • Any color characterized by its H, S, and I • Mathematically convertible to other representations, e.g., RGB, CYMK • Hue • Commonly referred to as color • Saturation • How “red” the color red is • Intensity • How “bright” the color is

20. Printing Terminology • Resolution Measured in Dots Per Inch, dpi • 75: minimal • 300: LaserJet • 1440: Professional print quality

21. Creating Color • Two Methods for Creating a Particular Hue • Multiple layers require transparent inks • Color also depends on medium which it is applied to • Adjacent • Different colored dots closer together than the eye can resolve appear to be single color • Dithering

22. Additive Color • Mechanism used on TV, color monitors • Red, Green, and Blue • All colors added together produce white

23. Subtractive Color • Control What Color Is Reflected • White • all colors reflected • Red • only red is reflected, all other colors absorbed (subtracted) • Black • no colors are reflected

24. Printing Color, CYMK • Color printing uses 4 pigments • Cyan (blue-green) • Yellow • Magenta (purple-red) • blacK • Fixed dot size • Lighter shades are produced by leaving more dots white (unprinted)

25. Dithering • To Produce Colors Beyond Simple Mix of Primary Colors (8 Colors), Specific Combinations of Yellow, Cyan, and Magenta Dots Are Used • Dots Are Not Placed in Same Spot, but Close Together (Dithering) • Eye Blends Discrete, Separately Colored Dots Into Single Color

26. Half-tone for Grey

27. Color Dithering

28. Paper Characteristics • Uncoated Paper • Rough surface, scatters light • Blurs image • Inks “bleed” into paper • Coated Paper • Smoother surface reflects light more precisely • Coating prevents color bleed • Thickness Measured by Weight in Pounds

29. Laser Printer • Laser Beam Forms Bit Pattern on Rotating Drum by Causing Charge to Leak off Where Illuminated • Exact Position on Drum Is Set by Spinning Mirror Which Redirects Beam Along Horizontal Line. • As Exposed Portions of Drum Rotate Further, They Contact Black Powder (Toner) • Electrically Charged Toner Particles Are Attracted to Dots on Drum Written by Laser

30. Laser Printer • Drum contacts paper and electrical charge on paper transfers toner to paper • Drum passes “corona” wire to returns entire surface of drum to negative electrical charge • Paper passes to fuser • Fuser uses heat and pressure to melt and press wax within toner into paper

31. Laser Printer

32. Color Laser • Single-pass, Multi-issue • Solid-ink • Dye-sublimation • Multi-pass • Thermal-wax • Heat transfers wax to wide ribbon then to paper • 4-pass color laser • 4 printer engines, each applies own toner to cartridge

33. Color Laser Printing • Laser Beam Writes Pattern on Electrically Charged Drum • CYM or K Is Picked up From Cartridge on Each Pass and Deposited on Transfer Belt • When All 4 Toners Have Been Placed on Transfer Belt, It Is Passed Over Paper • Fuser Melts/presses Toner Into Paper

34. Tandem Color Laser • Alternative to CYMK Cartridge • Use Separate Lasers and Drums for Each Color Which Allows All Colors to Be Applied to Secondary Belt at Once

35. Color Ink-Jet • Inexpensive • Slow • Simultaneous Issue of 4 Colors • Colors can run if paper gets wet

36. Color Ink-Jet Printers • Print head, including Ink filled cartridge moves horizontally across paper surface • Each of 4 cartridges (CYMK) has 50 ink-filled firing chambers

37. Color Ink-Jet Printers • Ink Flows Into Firing Chamber • Quickly Heated Ink Vaporizes Creating Bubble • Vapor Bubble Pushes Ink Out Nozzle • Droplet Is Propelled Toward Paper by Bursting Bubble • Removal of Heat Collapses Bubble and Pulls More Ink Into Firing Chamber

38. Color Ink-Jet Printers

39. Solid-Ink Color Printers • Example: Tektronix Phaser 350 • Ink Solid at Room Temperature Distributed in 4, Wax-like Uniquely Shaped Blocks • Printer Melts Ink Into Reservoirs in Print Head • Base of Nozzle Contains Piezoelectric Membrane • To Fill Nozzle, Piezoelectric Membrane Wall Extends Out, Forcing Ink in • Amount of Ink Controlled by Amount of Flexure

40. Solid-Ink Color Printers