Image Processing Algorithms; Luminance Look-Up Table (Lut) - Smartek UCC User Manual

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
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
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
Figure 89. Because of the one-to-one value mapping the output image is identical to the input image.
n
entries, where n is the bit depth of the input image.
n−1
. Figure 88 shows an example of an 8 bit look-up table.
Lookup table
index
0
1
2
3
input
3
...
253
254
255
Figure 88: 8 bit look-up table
113
value
0
20
28
output
33
...
254
254
255
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