DIGITAL IMAGE FUNDAS.ppt

Digital Image Processing
DIGITAL IMAGE FUNDAMENTALS
Prepared by
M. HARI KRISHNA

References
1. Rafael C. Gonzalez and Richard E. Woods, “Digital Image
Processing”, Pearson Education, Inc., Second Edition, 2004.
2. Anil K. Jain, “Fundamentals of Digital Image Processing”,
Prentice Hall of India, 2002.
3. William K. Pratt, “Digital Image Processing”, 2nd Edition,
Wiley & Sons, Inc., 1991.

• What is an Image?
Picture, photograph
Visual data
Usually two or three dimensional
What is a digital image?
An image which is “discretized,”, i.e., defined on a
discrete grid
Two-dimensional collection of light values (or gray
values)

Basic Terminology
What is a digital image?
• A representation of the original image by a discrete set of data
points
• Each of these data points is called a picture element, commonly
referred to as a “pixel”

Digital Image Generation
Generating a digital image
• Place a regular 2D grid over an image and read the colour value
• at the intersections of the grid => set of data points for that image

What is digital image processing?
Digital image processing is the study of representation
and manipulation of pictorial information by a
computer.
Improve pictorial information for better clarity
(human interpretation)
Automatic machine processing of scene data
(interpretation by a machine/non-human, storage,
transmission)
What is image interpetation?
Assign meaning to an ensemble of recognized
objects

Digital Image
Digital image = a multidimensional
array of numbers (such as intensity image)
or vectors (such as color image)
Each component in the image
called pixel associates with
the pixel value (a single number in
the case of intensity images or a
vector in the case of color images).
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Important steps in a typical image processing system
Step 1: Image acquisition
Step 2: Discretization / digitalization; Quantization ; Compression
Step 3: Image enhancement and restoration
Step 4: Image segmentation
Capturing visual data by an image sensor
Convert data into discrete form; compress for efficient
storage/transmission
Improving image quality (low contrast, blur noise)
Partition image into objects or constituent parts

ELEMENTS OF VISUAL PERCEPTION
Human Visual Perception

The Human Eye
• Diameter: 20 mm
• 3 membranes enclose the eye
– Cornea & sclera
– Choroid
– Retina

The Choroid
• The choroid contains blood vessels for eye nutrition
and is heavily pigmented to reduce extraneous light
entrance and backscatter.
• It is divided into the ciliary body and the iris
diaphragm, which controls the amount of light that
enters the pupil (2 mm ~ 8 mm).

The Lens
• The lens is made up of fibrous cells and is
suspended by fibers that attach it to the ciliary
body.
• It is slightly yellow and absorbs approx. 8% of
the visible light spectrum.
• The lens contains 60-70% water, 6% of fat.

The Retina
• The retina lines the entire posterior portion.
• Discrete light receptors are distributed over
the surface of the retina:
– cones (6-7 million per eye) and
– rods (75-150 million per eye)

Cones
• Cones are located in the fovea and are sensitive to
color.
• Each one is connected to its own nerve end.
• Cone vision is called photopic (or bright-light vision
or day vision).
• There are three types of cones: Red, Green, and Blue

Rods
 Rods are giving a general, overall picture of the field
of view
 They see only brightness (not color) and are not
involved in color vision.
• Several rods are connected to a single nerve and are
sensitive to low levels of illumination (scotopic or
dim-light vision or night vision).

MESOPIC VISION
• Intermediate region of illumination – between
dim and bright light
• Both rods and cones are active

Receptor Distribution
• The distribution of receptors is radially
symmetric about the fovea.
• Cones are most dense in the center of the
fovea while rods increase in density from the
center out to approximately 20% off axis and
then decrease.

The Fovea
• The fovea is circular (1.5 mm in diameter) but can be
assumed to be a square sensor array (1.5 mm x 1.5
mm).
• The density of cones: 150,000 elements/mm2 ~
337,000 for the fovea.
• A CCD imaging chip of medium resolution needs 5
mm x 5 mm for this number of elements

BLIND SPOT
• Blind spot is the region of emergence of the
optic nerve from the eye
• Place on the retina where optic nerve
connects, and which consists of no light
sensors

Image Formation in the Eye
• The eye lens (if compared to an optical lens) is
flexible.
• It gets controlled by the fibers of the ciliary
body and to focus on distant objects it gets
flatter (and vice versa).

• Distance between the center of the lens and
the retina (focal length):
– varies from 17 mm to 14 mm (refractive power of
lens goes from minimum to maximum).
• Objects farther than 3 m use minimum
refractive lens powers (and vice versa).

• Example:
– Calculation of retinal image of an object
17
100
15 x

mm
x 55
.
2


• Perception takes place by the relative
excitation of light receptors.
• These receptors transform radiant energy into
electrical impulses that are ultimately
decoded by the brain.

LUMINANCE
OR
INTENSITY
• Emitting or reflecting light from the object
• The luminance of an object is independent of
the luminance of the surrounding objects

BRIGHTNESS
• Brightness is the perceived luminance
• Depends on the luminance of the surround
• Two objects with different surroundings could
have identical luminance but different
brightness

CONTRAST
• Contrast is the difference in visual properties that
makes an object (or its representation in an image)
distinguishable from other objects and the
background
• Contrast is determined by the difference in the color
and brightness of the light reflected or emitted by an
object and other objects within the same field of
view .

Brightness Adaptation of Human Eye (cont.)
Simultaneous contrast. All small squares have exactly the same intensity
but they appear progressively darker as background becomes lighter.

HUE
• The hue of a color refers to its “redness”, “greenness”
and so on.
• A hue refers to the gradation of color within the optical
spectrum, or visible spectrum, of light.
• "Hue" may also refer to a particular color within this
spectrum, as defined by its dominant wavelength,
• or the central tendency of its combined wavelengths. For
example, a light wave with a central tendency within
565-590 nm will be yellow.

SATURATION
• Saturation refers to the intensity of a specific hue.
• In art or when working with colors, saturation is the
amount of color a certain color has. For example,
black and white have no saturation and bright red
has 100% saturation

MACH BAND EFFECT
• The spatial interaction from an object and its
surroundings creates a phenomenon called
mach band effect.
• The visual system tends to undershoot or
overshoot around the boundary of regions of
different intensities

In area A, brightness perceived is darker while in area B is
brighter. This phenomenon is called Mach Band Effect.
Brightness Adaptation of Human Eye (cont.)
Position
Intensity
A
B

• Although the intensity of the stripes is
constant, we actually perceive a brightness
pattern that is strongly scalloped, especially
near the boundaries
• These scalloped bands are called mach bands

RGB Model
• An additive colour model, based on three components:
red(R), green (G) and Blue (B).
• 8 bits are usually used for each colour (=> 24 bits total)
• These can be stored (in memory) as 3 separate colour
planes ....
• ... or in an ‘interlaced form’, with each pixel represented
with 3 bytes (24bits) sequentially.

But!!!!!
However
Not all colours can be represented by the RGB system
It is probably good enough to fool the human vision system

The RGB Cube
B
R
G
Red
Green
Blue Cyan
Yellow
Magenta

The HSI Model (an alternative to RGB)
• Hue, H, specifies the dominant pure colour perceived
by the observer (e.g. red, yellow, blue)
[i.e. a representation of the frequency/wavelength]
• Saturation, S, specifies the degree to which a pure
colour has been diluted by white light to produce
observed colour.
• Intensity, I, is related to the perceived brightness of
the colour.
N.B. Decoupling (separating) intensity from colour is
very useful in image manipulation and analysis as
increased/decreased lighting in a scene (more or less)
only effect this parameter.

Black
The HSI Cone
White
Intensity
Red
Cyan
Blue Magenta
Green Yellow
Hue
Saturation

A Simple Image Model
• Image: a 2-D light-intensity function f(x,y)
• The value of f at (x,y)  the intensity
(brightness) of the image at that point
• 0 < f(x,y) < 

• Nature of f(x,y):
– The amount of source light incident on the scene
being viewed
– The amount of light reflected by the objects in the
scene

• Illumination & Reflectance components:
– Illumination: i(x,y)
– Reflectance: r(x,y)
– f(x,y) = i(x,y)  r(x,y)
– 0 < i(x,y) < 
and 0 < r(x,y) < 1
(from total absorption to total reflectance)

• Sample values of r(x,y):
– 0.01: black velvet
– 0.93: snow
• Sample values of i(x,y):
– 9000 foot-candles: sunny day
– 1000 foot-candles: cloudy day
– 0.01 foot-candles: full moon

Create an image
To create a digital image, we need to convert the continuously sensed
data into digital form.
This involves:
Sampling:
Digitizing the coordinate values (resolution)
Depends on density of sensor in an array
Limited by optical resolution
Quantization (bits/pixel)
•Digitizing the amplitude values
Pixel: short for picture element

Sampling & Quantization
• The spatial and amplitude digitization of f(x,y)
is called:
– image sampling when it refers to spatial
coordinates (x,y) and
– gray-level quantization when it refers to the
amplitude.

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Digital Image Image Elements
(Pixels)

• The digitization process requires decisions
about:
– values for N,M (where N x M: the image array)
and
– the number of discrete gray levels allowed for
each pixel.

• Usually, in DIP these quantities are integer powers of
two:
N=2n M=2m and G=2k
number of gray levels
• Another assumption is that the discrete levels are
equally spaced between 0 and L-1 in the gray scale.

• If b is the number of bits required to store a
digitized image then:
– b = N x M x k (if M=N, then b=N2k)

• How many samples and gray levels are
required for a good approximation?
– Resolution (the degree of discernible detail) of an
image depends on sample number and gray level
number.
– i.e. the more these parameters are increased, the
closer the digitized array approximates the original
image.

• How many samples and gray levels are
required for a good approximation? (cont.)
– But: storage & processing requirements increase
rapidly as a function of N, M, and k

• Different versions (images) of the same object
can be generated through:
– Varying N, M numbers
– Varying k (number of bits)
– Varying both

• Conclusions:
– Quality of images increases as N & k increase
– Sometimes, for fixed N, the quality improved
by decreasing k (increased contrast)
– For images with large amounts of detail, few
gray levels are needed

Dithering
• Full-color photographs may contain an almost infinite range of color values
• Dithering is the attempt by a computer program to approximate a color
from a mixture of other colors when the required color is not available
• Dithering is the most common means of reducing the color range of
images down to the 256 (or fewer) colors seen in 8-bit GIF images

• Most images are dithered in a diffusion or randomized
pattern to diminish the harsh transition from one color to
another
• But dithering also reduces the overall sharpness of an
image, and it often introduces a noticeable grainy
pattern in the image
• This loss of image detail is especially apparent when full-
color photos are dithered down to the 216-color
browser-safe palette.

TWO DIMENSIONAL MATHEMATICAL
PRELIMINARIES

71
Review: Matrices and Vectors
Definitions (Con’t)
A column vector is an m × 1 matrix:
A row vector is a 1 × n matrix:
A column vector can be expressed as a row vector by using
the transpose:

72
Review: Matrices and Vectors
Some Basic Matrix Operations
• The sum of two matrices A and B (of equal dimension),
denoted A + B, is the matrix with elements aij + bij.
• The difference of two matrices, A B, has elements aij  bij.
• The product, AB, of m×n matrix A and p×q matrix B, is an
m×q matrix C whose (i,j)-th element is formed by
multiplying the entries across the ith row of A times the
entries down the jth column of B; that is,

BLOCK MATRIX
• Any matrix whose elements are matrices
themselves is called a block matrix.
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DIGITAL IMAGE FUNDAS.ppt

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