Display of Images

Prof. K. J. HintzDepartment of Electrical and

Computer EngineeringGeorge Mason University

Digital Image Displays

Psychophysiological Optical Impedance Matching Human visual system characteristics

and capabilities Display device capabilities

Dynamic Print

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

Human Visual System

Intensity Resolution Approximately 40 grey levels (~ 5 bits)

Retinal Edge-enhancement Allows Detection of More LevelsMore Sensitive to High Spatial Frequencies Than LowMore Sensitive to Low Intensities Off-axis

Human Visual System

Acuity Minimum visible

1 arc-second Minimum separable

1 arc-minute

* Geldard, The Human Senses, Wiley, 1972

Human Visual System (Color)

Normal color range violet ( = 380 nm) to red ( = 760 nm)156 total discriminable hue steps1 to 6 nm shift required to discriminate, varies with Purkinje phenomenon Colors matched at high intensity do

not match at low intensity

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)

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)

Simultaneous Contrast

Center is same grey level

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

Display Calibration

Turn Contrast and Intensity to Lowest SettingIncrease Intensity to Just Below Observable RasterIncrease Contrast to See All Shades of Test PatternColor more complex Printed scale accompanies software along with

sequence of steps for color and intensity calibration

Gaussian Intensity Distribution About Mean Position

Letting R be the 1/2 Intensity Radius

Display Spot

222-2=),( R/)y+x(yxp

222

= =),( +- r-yx eeyxp

Static Display Criteriafor Flat Field

Desire to Have Uniform Grey Scale Produced by an Array of DotsBest Flat-field Occurs With Horizontal and Vertical Spacing Between 1.55 R and 1.65 R for Gaussian Intensity Distribution

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

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

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

Permanent Displays

Binary Printers Produce Grey Scale by Dithering or Half-toningCRT Cameras Photograph Phosphor Display

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

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

Printing Terminology

Resolution Measured in Dots Per Inch, dpi 75: minimal 300: LaserJet 1440: Professional print quality

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

Additive Color

Mechanism used on TV, color monitorsRed, Green, and BlueAll colors added together produce white

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

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)

Dithering

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

Half-tone for Grey

Color Dithering

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

Laser Printer

Laser Beam Forms Bit Pattern on Rotating Drum by Causing Charge to Leak off Where IlluminatedExact 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

Laser Printer

Drum contacts paper and electrical charge on paper transfers toner to paperDrum passes “corona” wire to returns entire surface of drum to negative electrical chargePaper passes to fuserFuser uses heat and pressure to melt and press wax within toner into paper

Laser Printer

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

Color Laser Printing

Laser Beam Writes Pattern on Electrically Charged DrumCYM or K Is Picked up From Cartridge on Each Pass and Deposited on Transfer BeltWhen All 4 Toners Have Been Placed on Transfer Belt, It Is Passed Over PaperFuser Melts/presses Toner Into Paper

Tandem Color Laser

Alternative to CYMK CartridgeUse Separate Lasers and Drums for Each Color Which Allows All Colors to Be Applied to Secondary Belt at Once

Color Ink-Jet

InexpensiveSlowSimultaneous Issue of 4 ColorsColors can run if paper gets wet

Color Ink-Jet Printers

Print head, including Ink filled cartridge moves horizontally across paper surfaceEach of 4 cartridges (CYMK) has 50 ink-filled firing chambers

Color Ink-Jet Printers

Ink Flows Into Firing ChamberQuickly Heated Ink Vaporizes Creating BubbleVapor Bubble Pushes Ink Out NozzleDroplet Is Propelled Toward Paper by Bursting BubbleRemoval of Heat Collapses Bubble and Pulls More Ink Into Firing Chamber

Color Ink-Jet Printers

Solid-Ink Color Printers

Example: Tektronix Phaser 350Ink Solid at Room Temperature Distributed in 4, Wax-like Uniquely Shaped BlocksPrinter Melts Ink Into Reservoirs in Print HeadBase of Nozzle Contains Piezoelectric MembraneTo Fill Nozzle, Piezoelectric Membrane Wall Extends Out, Forcing Ink inAmount of Ink Controlled by Amount of Flexure

Solid-Ink Color Printers