CS9261 – Lecturer series 1

Image Fundamentals

Prepared by Mohanalin

For FX

Fundamentals

These slides show

Importance of images

Image acquisition systemsBasic concepts of digital images Evolution of image processing

Overview of Image processing

Picture of earth’s moon taken by space probe in 1964. Picture made with a television camera (vidicon), transmitted to the earth by analog modulation, and digitized on the ground.

Images….What are they? Why it is important?

Medical Images

Satellite Images

Radar Image

Ancient times: Documentation and manual drawing

Photography: Documentation eg. Astronomy, particle physics

Imagine counting the stars manually

Imagine counting the hydrogen particles in a gas chamber

Then came the PCs which revolutionized the field

So how do images form?

Evolution of Images

Fundamentals Let’s look at eye

Fundamentals

We are mostly interested in the retina:

• consists of cones and rods• Cones• color receptors• About 7 million, primarily in the retina’s

central portion • for image details

• Rods• Sensitive to illumination, not involved in

color vision• About 130 million, all over the retina• General, overall view

Fundamentals

Distribution of cones and rods:

Fundamentals

The human eye is sensible to electromagnetic waves in the ‘visible spectrum’ :

Fundamentals

The human eye is sensible to electromagnetic waves in the ‘visible

spectrum’ , which is around a wavelength of

0.000001 m = 0.001 mm

Fundamentals

The human eye

• Is able to perceive electromagnetic waves in a certain spectrum

• Is able to distinguish between wavelengths in this spectrum (colors)

• Has a higher density of receptors in the center

• Maps our 3D reality to a 2 dimensional image !

Fundamentals

…or more precise:

maps our continous (?) reality to a (spatially) DISCRETE 2D image

Fundamentals

Some topics we have to deal with:

• Sharpness• Brightness

• Processing of perceived visual information

Fundamentals

Sharpness

The eye is able to deal with sharpness in different distances

Fundamentals

Brightness

The eye is able to adapt to different ranges of brightness

Fundamentals

Processing of perceived information: optical illusions

Fundamentals

optical illusions:

Digital Image Processing does NOT (primarily) deal with cognitive

aspects of the perceived image !

Image Acquisition

There are two parts to the image formation process:

The geometry of image formation, which determines where in the image plane the projection of a point in the scene will be located.

The physics of light, which determines the brightness of a point in the image plane as a function of illumination and surface properties.

General purpose Image acquisition and processing system consists of 4 essential components:

An image acquisition system…e.g ccd camera, video recorder etc…

A frame grabber – to convert the electrical signal to digital Image

A PC to store

IP s/w to process

Fundamentals

Acquisition

Fundamentals

Acquisition

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.

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.

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

Fundamentals

Typical sensor for images:

CCD (Charge Couple Devices) CMOS (complement metal oxide semiconductor)

• Use in digital cameras• Typical resolution 1024 x 768

(webcam)

Fundamentals

CCD

Fundamentals

CCD

Fundamentals

CCD: 3.2 million pixels !

CCD (Charged-Coupled Device) cameras

Tiny solid state cells convert light energy into electrical charge.

The image plane acts as a digital memory that can be read row by row by a computer.

Frame grabber

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

The frame grabber digitizes the signal and stores it in its memory (frame buffer).

Image digitization

• Sampling means measuring the value of an image at a finite number of points.

• Quantization is the representation of the measured value at the sampled point by an integer.

Image digitization (cont’d)

Image quantization(example)

• 256 gray levels (8bits/pixel) 32 gray levels (5 bits/pixel) 16 gray levels (4 bits/pixel)

• 8 gray levels (3 bits/pixel) 4 gray levels (2 bits/pixel) 2 gray levels (1 bit/pixel)

Image sampling (example)

original image sampled by a factor of 2

sampled by a factor of 4 sampled by a factor of 8

Digital image

• An image is represented by a rectangular array of integers.

• An integer represents the brightness or darkness of the image at that point.

• N: # of rows, M: # of columns, Q: # of gray levels– N = , M = , Q = (q is the # of bits/pixel)– Storage requirements: NxMxQ (e.g., N=M=1024, q=8, 1MB)

(0,0) (0,1) ... (0, 1)

(1,0) (1,1) ... (1, 1)

... ... ... ...

( 1,0) ( 1,1) ... ( 1, 1)

f f f M

f f f M

f N f N f N M

2 n 2m 2 q

Image file formats

• Many image formats adhere to the simple model shown below (line by line, no breaks between lines).

• The header contains at least the width and height of the image.

• Most headers begin with a signature or “magic number” - a short sequence of bytes for identifying the file format.

Common image file formats

• GIF (Graphic Interchange Format) -

• PNG (Portable Network Graphics)

• JPEG (Joint Photographic Experts Group)

• TIFF (Tagged Image File Format)

• PGM (Portable Gray Map)

• FITS (Flexible Image Transport System)

Comparison of image formats

PGM format

• A popular format for grayscale images (8 bits/pixel)

• Closely-related formats are:

– PBM (Portable Bitmap), for binary images (1 bit/pixel)

– PPM (Portable Pixelmap), for color images (24 bits/pixel)

» ASCII or binary (raw) storage

ASCII vs Raw format

• ASCII format has the following advantages:– Pixel values can be examined or modified very easily using a standard text editor.– Files in raw format cannot be modified in this way since they contain many unprintable

characters.

• Raw format has the following advantages:– It is much more compact compared to the ASCII format.– Pixel values are coded using only a single character !

Fundamentals

What is an image ?

Fundamentals

A 2D grayvalue - image is a 2D -> 1D function,

v = f(x,y)

Fundamentals

As we have a function, we can apply operators to this function, e.g.

H(f(x,y)) = f(x,y) / 2

Operator Image (= function !)

Fundamentals

H(f(x,y)) = f(x,y) / 2

6 8 2 0

12 200 20 10

3 4 1 0

6 100 10 5

Fundamentals

Remember: the value of the cells is the illumination (or brightness)

6 8 2 0

12 200 20 10

3 4 1 0

6 100 10 5

Fundamentals

As we have a function, we can apply operators to this function…

…but why should we ?

some motivation for (digital) image processing

Over view of IP

Finding Depth in images Classification of objects Motion estimation Extraction of features Restoration Exploring dynamic process Finding shapes Visual Inspection and so onnnnnnnn

Fundamentals

• Image Enhancement

Fundamentals

• Image Analysis / Recognition

Fundamentals

Digital vs. Analogue Images

Analogue:Function v = f(x,y): v,x,y are REAL

Digital:Function v = f(x,y): v,x,y are INTEGER