display of images prof. k. j. hintz department of electrical and computer engineering george mason...
TRANSCRIPT
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