IMAGE ENHANCEMENT

Er. Prem Ranjan M.Tech.(SWE)

Roll No.- 01/SWE/2015

OverviewIntroductionImage enhancement

Contrast enhancement Linear contrast stretching Non Linear contrast stretching

Image EnhancementDefinition: It is the process of improving the

visual quality of the image by manipulating the gray levels of the pixels for a specific application.

Processing of image to bring out specific features of an image.

Highlight certain characteristics of image.

Contrast Enhancement Contrast is the difference in colour that makes an object (or its

representation in an image or display) distinguishable.

Contrast enhancement or stretching : Stretching the gray levels in an image to increase the contrast between the features.

Used to increase the dynamic range of gray levels in the image

Example:Number of gray levels possible in an 8-bit system = 256 (0 to 255).The image pixel values are stretched to use the entire gray level range i.e. from

0 to 255.

Contrast Enhancement

1. Linear Contrast Enhancement

2. Non-Linear Contrast Enhancement

Linear Contrast Stretching …

Min-max stretching:-

Corresponds to the maximum number of display

levels

0-255 in an 8-bit system0-127 in a 7-bit system

in

The basic purpose of contrast enhancement is to expand the original brightness values to make use of the full range of the radiometric scale of the sensor.

Landsat TM Band-5 image and Histogram

Displayed in an 8-bit systemImage is vagueDN values range from 60-1580-59 and 159-255 are not utilized

DN values are stretched to 0-255Contrast is improvedLight tones appear lighterDark tones appear darker

Linearly stretched Landsat TM Band-5 image and Histogram

Linear Contrast Stretching…

Non-linear Contrast StretchingDN values are not stretched linearly to

uniformly occupy the entire display range.Histogram-equalized stretch

Piece-wise linear stretch

Logarithmic, Power law or Gaussian stretch

Histogram-Equalized Stretch Image Histogram a graphical representation of the brightness value that

comprise an image. The brightness value are displayed along x-axis and frequency of occurrence of each these values on y-axis of the graph.

DN values are enhanced based on their frequency in the original image

DN values corresponding to the peaks of the histogram are assigned to a wider range

More display levels to the higher frequency region gives better information enhancement

Original histogram Adjusted histogramTransformation

Histogram-Equalized Stretch… DN values are enhanced based on their frequency in the original

image

Example: Enhancement in an 8-bit display system

k

j

jst N

nDN

0

255

DNst = Enhanced DN valuenj = Number of pixels having DN values in the jth range, in the input imagek = Maximum number of DN ranges in the input imageN = Total number of pixels in the input image

Histogram Equalization Stretch…

Landsat ETM+ band-5 image &

Histogram

Landsat ETM+ band-5 image

after histogram equalization stretch

& Histogram

Most of the image enhancement procedures are subjective, due to which a procedure well suited to one application or one user, may not be suitable to another application for another user.

“ One man’s enhancement is another man’s noise.”

Thank You!!!