Digital Image Processing

Lecture 2(Image processing fundamentals)

Bu-Ali Sina UniversityComputer Engineering Dep.

Fall 2009

Outline

Human Visual SystemThe best vision model we have!Knowledge of how images form in the eye canhelp us with processing digital imagesWe will take just a whirlwind tour of the humanvisual system

Structure Of The Human EyeThe lens focuses light from objects onto theretinaThe retina is covered withlight receptors calledcones (6-7 million) androds (75-150 million)Cones are concentratedaround the fovea and arevery sensitive to colourRods are more spread outand are sensitive to low levels of illumination

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Blind-Spot ExperimentDraw an image similar to that below on a pieceof paper (the dot and cross are about 6 inchesapart)

Close your right eye and focus on the crosswith your left eyeHold the image about 20 inches away fromyour face and move it slowly towards youThe dot should disappear!

Image Formation In The EyeMuscles within the eye can be used to changethe shape of the lens allowing us focus onobjects that are near or far awayAn image is focused onto the retina causingrods and cones to become excited whichultimately send signals to the brain

Brightness Adaptation & Discrimination

The human visual system can perceiveapproximately 1010 different light intensitylevelsHowever, at any one time we can onlydiscriminate between a much smaller number –brightness adaptationSimilarly, the perceived intensity of a region isrelated to the light intensities of the regionssurrounding it

Brightness Adaptation & Discrimination(cont>)

Brightness Adaptation & Discrimination(cont>)

An example of simultaneous contrast

Brightness Adaptation & Discrimination(cont>)

For more great illusion examples take a look at: http://web.mit.edu/persci/gaz/

Light And The Electromagnetic Spectrum

Light is just a particular part of theelectromagnetic spectrum that can be sensedby the human eyeThe electromagnetic spectrum is split upaccording to the wavelengths of different formsof energy

Reflected LightThe colours that we perceive are determinedby the nature of the light reflected from anobjectFor example, if whitelight is shone onto agreen object mostwavelengths areabsorbed, while greenlight is reflected fromthe object

White Light

ColoursAbsorbed

Green Light

Sampling, Quantization And Resolution

In the following slides we will consider what isinvolved in capturing a digital image of a real-world scene

– Image sensing and representation– Sampling and quantization– Resolution

Sampling

Sampling (aliasing)

Sampling

Image Sampling And Quantisation

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Digital Image Representation

· A typical image formation system consists of an “illumination”source, and a sensor.· Energy from the illumination source is either reflected orabsorbed by the object or scene, which is then detected by thesensor.· Depending on the type of radiation used, a photo-converter(e.g., a phosphor screen) is typically used to convert the energyinto visible light.· Sensors that provide digital image as output, the incomingenergy is transformed into a voltage waveform by a sensormaterial that is responsive to the particular energy radiation.· The voltage waveform is then digitized to obtain a discreteoutput.

Image Sensing and Acquisition

Image Sensing and Acquisition

Image Sensing and Acquisition

Image Sensing and Acquisition

Image Sensing and Acquisition

– I = f(x,y)– I: intensity (or color)– (x,y): Position or Coordination– When (x,y) and I are finite and discrete

quantities -→ digital image– pixels, picture elements, image elements

Digital Image, Mathematical Definition

Digital Image Representation

Image Sensing and Acquisition

Effect of spatial resolution

Effect of spatial resolution

Effect of spatial resolution

Effect of graylevel quantization

Effect of spatial resolution

Effect of graylevel quantization

Some Basic Relationships Between Pixels,eighbors of a Pixel

Some Basic Relationships Between PixelsConnectivity

(a) 4-adjacency. Two pixels p and q with values from V are 4-adjacent if q isin the set �4(p).

(b) 8-adjacency. Two pixels p and q with values from V are 8-adjacent if q isin the set �8(p).

Some Basic Relationships Between PixelsDistance

Chapter 2: Digital Image Fundamentals

Rotation of Pixels

SummaryWe have looked at:

– Human visual system– Light and the electromagnetic spectrum– Image representation– Image sensing and acquisition– Sampling, quantisation and resolution

Next time we start to look at techniques forimage enhancement