Image Processing Reference
In-Depth Information
4
Point Processing
Sometimes when people make a movie they lower the overall intensity in order to
create a special atmosphere. Some overdo this and the result is that the viewer cannot
see anything except darkness. What do you do? You pick up your remote and adjust
the level of the light by pushing the brightness button. When doing so you actually
perform a special type of image processing known as point processing .
Say we have an input image f(x,y) and wish to manipulate it resulting in a dif-
ferent image, denoted the output image g(x,y) . In the case of changing the bright-
ness in a movie, the input image will be the one stored on the DVD you are watching
and the output image will be the one actually shown on the TV screen. Point pro-
cessing is now defined as an operation which calculates the new value of a pixel in
g(x,y) based on the value of the pixel in the same position in f(x,y) and some
operation. That is, the values of a pixel's neighbors in f(x,y) have no effect what-
soever, hence the name point processing. In the forthcoming chapters the neighbor
pixels will play an important role. The principle of point processing is illustrated in
Fig. 4.1 . In this chapter some of the most fundamental point processing operations
are described.
4.1
Gray-Level Mapping
When manipulating the brightness by your remote you actually change the value of
b in the following equation:
g(x,y)
=
f(x,y)
+
b
(4.1)
Every time you push the '
' brightness button the value of b is increased and vice
versa. The result of increasing b is that a higher and higher value is added to each
pixel in the input image and hence it becomes brighter. If b> 0 the image becomes
brighter and if b< 0 the image becomes darker. The effect of changing the bright-
ness is illustrated in Fig. 4.2 .
+

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