User Manual - twentynine
8.2
Image Processing Algorithms
8.2.1
Luminance Look-Up Table (LUT)
In image processing a Look-Up Table or LUT is used to transform input pixel values into desired output pixel
values by using mapping operations.
Essentially a luminance look-up table is a list with 2
n
entries, where n is the bit depth of the input image.
Each entry in the table has an index and represents an output luminance value. The indices are unique and
numbered continuously from 0 to 2
n
−
1
. Figure 88 shows an example of an 8 bit look-up table.
Lookup table
index
value
0
0
1
20
2
28
3
33
…
…
253
254
254
254
255
255
3
output
input
Figure 88: 8 bit look-up table
The index represents the luminance of a pixel in the image, which is exchanged by the value:
•
When an input pixel value has a value of 1, this value will be used as the index in the table. The
corresponding 8 bits output pixel value at index 1 will be 20.
•
When an input pixel value has a value of 3, this value will be used as the index in the table. The
corresponding 8 bits output pixel value at index 3 will be 33.
•
When an input pixel value has a value of 253, this value will be used as the index in the table. The
corresponding 8 bits output pixel value at index 253 will be 254.
Look-up tables are especially suited for point operations in image processing where output pixel values
depend only on the corresponding input pixel values. In the following a couple of application examples
using look-up tables are shown.
The first example shows a look-up table where each output pixel value is mapped to its exactly corresponding
input pixel value. The look-up table is a linear function (f(x) = x) and its graph is a 45
◦
straight line, shown in
Figure 89. Because of the one-to-one value mapping the output image is identical to the input image.
113
SMARTEK Vision | User Manual - twentynine | Doc. v1.0.2