EC-433 Digital Image Processing

Lecture 2

Digital Image Fundamentals

Dr. Arslan Shaukat

1

Fundamental Steps in DIP

Image Acquisition

An image is captured by a sensor (such as a monochrome

or color TV camera) and digitized.

If the output of the camera or sensor is not already in

digital form, an analog-to-digital converter digitizes it.

Pre-processing such as scaling.

Image Enhancement

To bring out detail which is obscured, or simply to

highlight certain features of interest in an image.

Image Enhancement

Enhancing objects of interest

Image Restoration

Improving the appearance of an image

Tend to be based on mathematical or probabilistic models

of image degradation

Distorted Image Restored Image

Color Image Processing

Gaining in importance because of the significant increase

in the use of digital images over the Internet

Wavelets

Foundation for representing images in various degrees of

resolution.

Used in image data compression and pyramidal

representation (images are subdivided successively into

smaller regions)

Compression

Reducing the storage required to save an image or the

bandwidth required to transmit it.

Example is JPEG (Joint Photographic Experts Group)

image compression standard.

Morphological Processing

Tools for extracting image components that are useful in

the representation and description of shape.

Image Segmentation

Image processing system tries to separate objects from

the image background

Output of the segmentation stage is raw pixel data,

constituting either the boundary of a region or all the

points in the region itself.

Segmentation

Segmentation

http://people.csail.mit.edu/xgwang/SLDA.html

http://www.umiacs.umd.edu/~mishraka/downloads/iccv2009_activeSeg.pdf

Representation & Description

Representation make a decision whether the data

should be represented as a boundary or as a complete

region.

– Boundary representation focus on external shape

characteristics, such as corners and inflections.

– Region representation focus on internal properties, such as

texture or skeleton shape.

Description, also called feature selection, deals with

extracting attributes that result in some quantitative

information of interest or are basic for differentiating one

class of objects from another.

Representation & Description

Recognition

Recognition the process that assigns a label to an

object based on the information provided by its

descriptors.

Knowledge Base

A problem domain detailing regions of an image

where the information of interest is known to be located.

Help to limit the search

Not All the Processes are Required

Example: Postal Code Problem

Human Visual Perception

Developing a basic understanding of human visual

perception

Interest lies in the mechanics and parameters related to

how images are formed in the eye.

Human Eye

The eye's lens, iris & cornea form

the optical system to projects an

upside down image onto retina

The iris is a membrane in the eye,

responsible for controlling the

diameter and size of the pupil and

the amount of light reaching the

retina.

The pupil is an opening located in

the centre of the iris of the eye that

allows light to enter the retina

Retina contains the light sensors

Retina – The Sensor Chip

Cones -- Responsible for bright-light vision

(Photopic Vision)

• 6 to 7 million

• Resolve fine details

• High visual resolution

Rods -- Responsible for dim-light vision

(Scotopic Vision)

• 75 to 150 million

• Lesser details – overall picture

• Monochrome vision

Image Formation in the Eye

The distance between the lens and the retina is fixed.

Focal length needed to achieve proper focus is obtained

by varying the shape of the lens.

Brightness Discrimination

The ability of the eye to

discriminate between

changes in brightness is of

considerable interest.

I is a uniform illumination

on a flat area large enough to

occupy the entire field of

view.

ΔIc is the change in the

object brightness required to

just distinguish the object

from the background

Weber's ratio: ΔIc/I

If ΔIc/I is small

Good brightness discrimination

If ΔIc/I is large

Poor brightness discrimination

Weber Ratio

Brightness discrimination is poor (the Weber ratio is large) at low

levels of illumination, and it improves significantly (the Weber

ratio decreases) as background illumination increases.

At low levels of illumination vision is carried out by rods, whereas

at high levels (showing better discrimination) vision is the function

of cones.

Brightness Discrimination

Brightness is not a simple

function of intensity.

Visual system tends to

undershoot or overshoot

around the boundary of

regions of different

intensities.

The intensity of the stripes is

constant but we actually

perceive a different

brightness pattern.

Simultaneous Contrast

All the small squares have exactly the same intensity, but

they appear to the eye progressively darker as the

background becomes brighter.

Region’s perceived brightness does not depend simply on

its intensity.

Human Perception Phenomena

Imaging Source --- Illumination

– Sunlight, X-Rays

Reflectance

– Certain wavelengths absorbed; certain reflected

Imaging Surface

– Sensor or Eye

– The reflected or transmitted energy is focused onto a photo

converter (e.g., a phosphor screen), which converts the energy

into visible light

Components of an Image Formation

Image Sensing and Acquisition

Incoming energy is

transformed into a

voltage by the

combination of input

electrical power and

sensor material

The output voltage

waveform is the

response of the

sensor(s), and a digital

quantity is obtained from

each sensor by digitizing

its response.

Image Acquisition

Image Acquisition

Collect the incoming energy and focus it onto an image

plane.

If the illumination is light, the front end of the imaging

system is a lens, which projects the viewed scene onto the

lens focal plane.

The sensor array, which is coincident with the focal

plane, produces output voltage.

Digital and analog circuitry sweep these outputs and

convert them to an analog signal, which is then digitized

by another section of the imaging system.

The output is a digital image.