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DIGITAL IMAGE PROCESSING

INTRODUCTION
Pictures are the most common and convenient means of conveying or transmitting information.
A picture is worth a thousand words. Pictures concisely convey information about positions, sizes and inter-relationships between objects. They portray spatial information that we can recognize as objects.
Pictures concisely convey information about positions, sizes and inter-relationships between objects. They portray spatial information that we can recognize as objects.
Human beings are good at deriving information from such images, because of our innate visual and mental abilities. About 75% of the information received by human is in pictorial form.

DIGITAL IMAGE
A digital image is typically composed of picture elements (pixels) located at the intersection of each row i and column j in each K bands of imagery.
Each pixel is associated a number known as Digital Number (DN) or Brightness Value (BV), that depicts the average radiance of a relatively small area within a scene (Fig. 1)
A smaller number indicates low average radiance from the area and the high number is an indicator of high radiant properties of the area .

COLOR COMPOSITES
While displaying the different bands of a multispectral data set, images obtained in different bands are displayed in image planes (other than their own) the color composite is regarded as False Color Composite (FCC).
A color infrared composite Ëœstandard false color compositeâ„¢ is displayed by placing the infrared, red, green in the red, green and blue frame buffer memory (Fig. 2).

IMAGE RECTIFICATION
Geometric distortions manifest themselves as errors in the position of a pixel relative to other pixels in the scene and with respect to their absolute position within some defined map projection.
If left uncorrected, these geometric distortions render any data extracted from the image useless

REASONS OF DISTORTIONS
For instance distortions occur due to changes in platform attitude (roll, pitch and yaw), altitude, earth rotation, earth curvature, panoramic distortion and detector delay.
Rectification is a process of geometrically correcting an image so that it can be represented on a planar surface (Fig. 3).

IMAGE ENHANCEMENT TECHNIQUES
Image enhancement techniques improve the quality of an image as perceived by a human
Spatial Filtering Technique
Contrast Stretch

Contrast
Contrast generally refers to the difference in luminance or grey level values in an image and is an important characteristic. It can be defined as the ratio of the maximum intensity to the minimum intensity over an image.

Contrast Enhancement
Contrast enhancement techniques expand the range of brightness values in an image so that the image can be efficiently displayed in a manner desired by the analyst

Linear Contrast Stretch
The grey values in the original image and the modified image follow a linear relation in this algorithm.
. A density number in the low range of the original histogram is assigned to extremely black and a value at the high end is assigned to extremely white.

SPATIAL FILTERING
Low-Frequency Filtering in the Spatial Domain
Image enhancements that de-emphasize or block the high spatial frequency detail are low-frequency or low-pass filters.
The simple smoothing operation will, however, blur the image, especially at the edges of objects.

High-Frequency Filtering in the Spatial Domain
High-pass filtering is applied to imagery to remove the slowly varying components and enhance the high-frequency local variations
Thus, the high-frequency filtered image will have a relatively narrow intensity histogram


CONCLUSIONS
So, with the above said stages and techniques, digital image can be made noise free and it can be made available in any desired format. (X-rays, photo negatives, improved image, etc)