unit vi dspa

8/10/2019 Unit Vi Dspa

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Unit VI

Image Formation and Display

By

Ankita Tidake


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Contents

Digital Image Structure

Camera and Eye

Television Video Signals Other Image Acquisition and Display

Brightness and Contrast Adjustments

Grayscale Transform Wraping


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Digital Image Structure

Pixel: A picture Element

A pixel is a single number between 0-255.

Grayscale: to view a visual image, the value ofeach pixel is converted into a grayscale

0 is black and 255 is white

Intermediate values are shades of gray.

Spatial Domain : Images have theirinformation encoded in the spatial domain,the image equivalent of the time domain.


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How are images represented in a

computer?


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Color Images


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A Simple model of image formation

The scene is illuminated by a single source.

The scene reflects radiation towards the camera.

The camera senses it via chemicals on film.


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Pinhole camera

This is the simplest device to form an image of a 3D scene

on a 2D surface.

Straight rays of light pass through a pinhole and form an

inverted image of the object on the image plane.

fXx

Z

fYy

Z


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Camera optics

In practice, the aperture must be larger to admit more light.

Lenses are placed to in the aperture to focus the bundle of rays

from each scene point onto the corresponding point in the

image plane


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Sample spacing : The value of each pixel in the digitalimage represents a small region in the continuousimage being digitized. For example, imagine that theVenus probe takes samples every 10 meters along the

planet's surface as it orbits overhead. This defines asquare sample spacing

Sampling grid: each pixel representing a 10 meter by10 meter area.

Sampling aperture: The region of the continuousimage that contributes to the pixel value is called thesampling aperture.


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Color Images


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A Color Image

A color image: Red, Blue, Green primary colors

256 levels for each primary colors

Each pixel color stored in three bytes (24 bits) of memory

16.7 million different possible colors.

Two types of color images: Binary Images

Use only single bit to represent each pixel

every pixel in a binary image must be one of two colors, usually blackor white.

Indexed Color Images

the data for each pixel consists of a palette index indicating which ofthe colors in the palette applies to that pixel

dithering must be applied to represent the missing colors and thisdegrades the image.


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CAMERA AND EYES


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Human Eye

A human eyeball is like a simple camera!

Sclera: outer walls, hard, like a light-tight box.

Cornea and crystalline lens (eyelens): the two

lens system.

Retina:at the back of eyeball, like the film.

Iris: like diaphragms or stop in a camera.

Pupil: camera aperture.

Eyelid: lens cover.


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Sclera (The white/non-transparent tissue surrounding the cornea)


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Aqueous humor and Vitreous humor

The Aqueous Humoris the clear liquid between the

cornea and the lens. It has the benefit of being fairly

homogenous and, as a result, the optical properties

are easily measured. (Le Grand, 1967) The space that it inhabits is called the anterior

chamber.

The Vitreous Humoris the clear liquid between the

lens and the retina.

The space that it fills is called the vitreous body.


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Functions?

Provides nourishment to the eyelens and cornea.

Cannot use the blood vessels:

Will block the light. Easy for surgical transplant.

Hold the shape of the eyeball.


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Focusing

The cornea and eyelens form a compound lens

system, producing a realinvertedimage on the

retina.

From air to cornea(n=1.376): large bending, the mainfocusing.

From cornea to eyelens(n=1.406), less focusing power.

(Eyelens can develop white cloudiness when getting old:

Cataracts.)

The eye has a limited depth of field. We cannot see

things close and far at the same time.


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Accommodation

The eye focusing is not done by change the distance

between the lens and retina. Rather, it is done bychanging the focal length of the eyelens! Ciliary

muscles help to change the shape of the lens:accommodation.

Muscles relax, long focal length, see objects far way;

Muscles tense, short focal length see objects close.

Normal eyes can see 25cm to infinity, however, if thecornea bulges too much or too little. The accommodation

does not help. (myopia or hyperopia)


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The Iris

When it is full open, it is about f/2 and f/3.

This happens at low light level.

When the iris has a small opening, it can cut

down the light intensity by a factor of 20.

However, the main function of stopping down

the iris is to increase the depth of field.


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Retina Structure

Light sensitive layeris made of photo-receptors: rods (120 millions) and cones (7

millions) which absorb the light. Plexiform Layer: nerve cells that process the

signals generated by rods and cones andrelay them to the optical nerve.

Choroid:carries mayor blood vessels tonourish the retina and absorb the light sothat it will not be reflected back (dark pupil!)


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Rods and Cones

Covers an area of 5 cm2. A baseball a mile

away gives an image covering one cone.

Cones: for more precise vision, need stronglight. help to see colors. Mostly distributed

in the center of the retina (fovea).

Rods: for peripheral and night vision.Sensitive to light. Mostly distributed away

from fovea.


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Sensitivity

Cones: slow, fine grain, like color film.

Need high level of light (photopic condition, day)

High density, high resolution.

Rods: fast, coarse grain, black & white film Low level of light (scotopic condition, at night)

No color is obvious.

Adaptation: Changing of retina sensitivity.


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Singal Processing

Trace the signal through the retina:

The retina is a seven-layered structureinvolved in signal transduction.

Light enters from the GCL side first, and mustpenetrate all cell types before reaching the rodsand cones.

The outer segments of the rods and cones

transduce the light and send the signal throughthe cell bodies of the ONL and out to their axons.


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In the OPL photoreceptor axons contact the

dendrites of bipolar cells and horizontal

cells. Horizontal cells are interneuronswhich aid in signal processing

The bipolar cells in the INL process input

from photoreceptors and horizontal cells,and transmit the signal to their axons.


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In the IPL, bipolar axons contact ganglioncell dendrites and amacrine cells, another

class of interneurons. The ganglion cells of the GCL send their

axons through the OFL to the optic disk tomake up the optic nerve. They travel all the

way to the lateral geniculate nucleus.


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Fovea

The foveadefines the center of the retina, and

is the region of highest visual acuity. The fovea

is directed towards whatever object you wish

to study most closely - this sentence, at themoment. In the fovea there are almost

exclusively cones, and they are at their highest

density.


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Processing Time

Latency:it takes a bit time for the cells in

retina to respond to a flash of light.

Persistence of response: the response does

not stop at the instant the flash stops.

1/25 second at low intensity, 1/50 second at high

intensity.

The persistence allows as to see moving thingsclearly.


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Cameras

Charge Coupled Devices (CCDs)

CCDs work by converting light into a pattern of

electronic charge in a silicon chip.

This pattern of charge is converted into a videowaveform, digitized and stored as an image file on

a computer.


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CCD Analogy

A n number of buckets (Pixels) are distributed across a field (FocalPlane of a telescope) in a square array.

The buckets are placed on top of a series of parallel conveyor beltsand collect rain fall (Photons)across the field.

The conveyor belts are initially stationary, while the rain slowly fillsthen buckets (During the course of the exposure).

Once the rain stops (The camera shutter closes) the conveyorbelts start turning and transfer the buckets of rain , one by one , toa measuring cylinder (Electronic Amplifier) at the corner of thefield (at the corner of the CCD)


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RAIN (PHOTONS)

BUCKETS (PIXELS)

VERTICAL

CONVEYOR

BELTS

(CCD COLUMNS)

HORIZONTAL

CONVEYOR BELT

(SERIAL REGISTER)

MEASURING

CYLINDER

(OUTPUT

AMPLIFIER)

CCD Analogy


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Exposure finished, buckets now contain samples of rain.


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Conveyor belt starts turning and transfers buckets. Rain collected on the vertical conveyoris tipped into buckets on the horizontal conveyor.


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Vertical conveyor stops. Horizontal conveyor starts up and tips each bucket in turn intothe measuring cylinder .


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`

After each bucket has been measured, the measuring cylinderis emptied , ready for the next bucket load.


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A new set of empty buckets is set up on the horizontal conveyor and the processis repeated.


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Charge Collection in a CCD.


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pix

el

boundary

Charge packetp-type silicon

n-type silicon

SiO2 Insulating layer

Electrode Structure

pi

xel

bo

undary

inc

oming

photons

Photons entering the CCD create electron-hole pairs. The electrons are then attracted towards

the most positive potential in the device where they create charge packets. Each packet

corresponds to one pixel


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PotentialEnergy

Conventional Clocking 1

Surface electrodesCharge packet (photo-electrons)

P-type siliconN-type silicon

Insulating layer

Charge packets occupy potential minimums


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PotentialEnergy



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PotentialEnergy



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PotentialEnergy



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PotentialEnergy



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PotentialEnergy



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PotentialEnergy



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PotentialEnergy



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PotentialEnergy


Charge Transfer in a CCD 1.


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In the following few slides, the implementation of the conveyor belts as actual electronic

structures is explained.

The charge is moved along these conveyor belts by modulating the voltages on the electrodespositioned on the surface of the CCD. In the following illustrations, electrodes colour coded red

are held at a positive potential, those coloured black are held at a negative potential.

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Charge packet from subsequent pixel enters

from left as first pixel exits to the right.



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d


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Camera and Eye

You can watch the Eyes vs Camera animation

by following the link

https://www.youtube.com/watch?v=CL61MbyYsd4


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TELEVISION VIDEO SIGNALS


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The horizontal and vertical synchronization (sync)pulses mixed with the actual picture informationis called composite video.

Frame: Each second of standard video contains30 complete images called Frames.

Frame contains 525 lines:

480-486 lines contains video information

39-45 lines are reserved To keep television circuits synchronized with the video

signal.

F bb


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Frame grabber

Usually, a CCD camera plugs into a computer board (framegrabber).

The frame grabber digitizes the signal and stores it in its

memory (frame buffer).


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Image digitization

Samplingmeans measuring the value of an image at a finite numberof points.

Quantizationis the representation of the measured value at thesampled point by an integer.

O h I A i i i d Di l


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Other Image Acquisition and Display

Images can be acquired :

An entire frame at a time

By line scanning e.g. fax m/c, airport x-ray baggage

scanner Point by point e.g. microwave image of Venus

Printed images are divided into two categories:

Grayscale Halftone


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a. Brightness too High b. Brightness too low

c: Contrast too high d: Contrast too low

G S l T f ti


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Gray Scale Transformation

a. Original IR Image b. With gray-scale transform

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