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Display of Images

Display of Images. Prof. K. J. Hintz Department of Electrical and Computer Engineering George Mason University. Digital Image Displays. Psychophysiological Optical Impedance Matching Human visual system characteristics and capabilities Display device capabilities Dynamic Print.

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Display of Images

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  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

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