DIGITAL IMAGE

Basic Image Concepts

An image is a spatial representation of an object

An image can be thought of as a function with resulting values of the light intensity at each point over a planar region

In computers, this function must be sampled at discrete intervals

Basic Image Concepts

An image can be represented as a matrix of pixels

Intensity at each pixel is represented by an integer (called a gray scale level) into which the color of the image is encoded

If there are just 2 intensity values, 0 and 1 represent black and white (1 bit)

If 8-bit integers, color levels range from 0 (black) to 255 (white)

Basic Image Concepts

Spatial Resolution - number of pixels in the matrix (pixels X pixels). Affects amount of detail that can be displayed

Image size - can be very large. E.g. 640X480 image with 8bits/pixel 307,200 8bit integers 2,457,600 bits

Basic Image Concepts

Image Format - a stored image is an array of values Each value represents data associated with

a pixel in the image For color, this value may be:

3 numbers - intensities of red, green and blue components of color at that pixel

3 numbers - indices to tables of RGB intensities 1 number - index to table of color triples 1 number - index to other color models, such as

CMY, YIQ, HSB, etc

COLOURS

Number of Colours 1 Bit per pixel = 2 Colours 4 Bit per pixel = 16 Colours 8 Bit per pixel = 256 Colours 16 Bit per pixel = 65536 Colours 24 Bit per pixel = 16.7 Million Colours

Colour Resolution: No. of pixels at any given time on the screen

Colour Monitor Video Modes

640 x 480 800 X 600 1024 x 768 1600 x 1200

Dot Pitch Actual difference between two pixels on

the monitor. Refresh Rate: The speed with which

every line is illuminated one after another and it is measured in Hz.

Color Fundamentals

Commonly used color models RGB = Red, Green, Blue -

additive primary colors (color monitors)

CMY = Cyan, Magenta, Yellow - subtractive primary colors plus black (color printers)

Color Fundamentals

Dithering Added to sample data for the purpose of

minimizing quantization error a photo may contain millions of colors but an 8 bit color palette contains only 256

colors so a mapping must be done at each pixel to

determine the “closest” available color in the palette

a mathematical process

Colour Properties

Hue distinguishes among colors such as red, green, and yellow.

Saturation refers to how far color is from a gray of equal intensity.

Lightness embodies the achromatic notion of perceived intensity of a reflecting object.

Brightness is used instead of lightness for a self-luminous object such as CRT.

Types of Graphics

Raster Graphics: Technique of dividing the entire image area into

pixels or small dots, and record the image data. High storage required Difficult to process

Vector Graphics Technique of storing the graphics using lines,

arcs, circles or basic shapes and sizes. Less Storage Easy to Modify Difficult to represent the actual picture

Image Editing

Resampling or Resizing Dithering Flipping Rotating Selection Masking Zooming Colour Correction Anti-aliasing

Images Filters

Blur Pinch Punch Sketch Edge