Point Processing - Introduction to Video and Image Processing - page 43

Fig. 4.1 The principle of point processing. A pixel in the input image is processed and the result
is stored at the same position in the output image

Fig. 4.2 If b in Eq. 4.1 is zero, the resulting image will be equal to the input image. If b is a
negative number, then the resulting image will have decreased brightness, and if b is a positive
number the resulting image will have increased brightness

An often more convenient way of expressing the brightness operation is by the
use of a graph, see Fig. 4.3 . The graph shows how a pixel value in the input im-
age (horizontal axis) maps to a pixel value in the output image (vertical axis). Such
a graph is denoted gray-level mapping . In the first graph, the mapping does ab-
solutely nothing, i.e., g( 142 , 42 ) f( 142 , 42 ) . In the next graph all pixel values

are increased ( b> 0), hence the image becomes brighter. This results in two things:
i) no pixel will be completely dark in the output and ii) some pixels will have a value
above 255 in the output image. The latter is no good due to the upper limit of an
8-bit image and therefore all pixels above 255 are set equal to 255 as illustrated by
the horizontal part of the graph. When b< 0 some pixels will have negative values
and are therefore set equal to zero in the output as seen in the last graph.
Just like changing the brightness on your TV, you can also change the contrast.
The contrast of an image is a matter of how different the gray-level values are. If we
look at two pixels next to each other with values 112 and 114, then the human eye
has difficulties distinguishing them and we will say there is a low contrast. On the
other hand if the pixels are 112 and 212, respectively, then we can easily distinguish

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