Spatial Domain :))

Spatial domain (Image Enhancement)

Definition

-          Techniques are based on direct manipulation of pixels in an image

-          “normal” image space

-          Changes in pixel positions correspond to changes in the scene

-          Distances in / correspond to real distances

-          Directly process the input image pixel array

-          An image processing operation transform the gray value of the pixels

-          In order to perform the transformation, the image must undergo 3 process

i.                     Point processing  - Gray values change without any knowledge of its surrounding

ii.                   Neighbourhood processing – Gray values change depends on the gray value in a small neighbourhood of pixels around the given pixel.

iii.                  Transform – Gray values are represented in a different domain but equivalent form; Fourier, wavelet.

Point processing

·         Neighbourhood =  1*1 pixel

·          g depends on only the value of f at (x,y)

·         T = gray level (or intensity or mapping) transformation function 

s = T(r) 

where

r = gray level of f(x,y)  

s =gray level of g(x,y)

                Arithmetic Operation

·         Act by applying a simple arithmetic functions

                  s = T(r)

to each gray level in the image

·          T is a function that maps r to s.

·         Additions, subtraction, scaling (multiplication & division), complement

Image Subtraction

g(x,y) = f(x,y) - h(x,y)

•            Is obtained by computing the difference between all pairs of corresponding pixels
•            Usefulness: enhancement of differences between images

                Image Negatives

The appearance of photographic negatives

•           It enhances white or gray detail on dark regions especially when black areas are dominant in size.

Identity function

•           what “goes in” , “comes out” the same

                              

Log transformation

•            bring up the details that are not visible due to large dynamic range of values 

 

Inverse Log Transformation

•          the opposite of Log transformation
•       used to expand the higher value pixels in an image while compressing darker-level values.

Power-Law Transformation

Gamma Correction

•           make linear input appear linear on displays
•           method: calibration pattern + interactive adjustment

-         

•      effect gamma on consumer photos