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11.4 Grey Level Transformation (LUT)
11.4 Grey Level Transformation (LUT)
Grey level transformation is remapping of the grey level values of an input image to new
values. The look-up table (LUT) is used to convert the greyscale value of each pixel in an image
into another grey value. It is typically used to implement a transfer curve for contrast
expansion. The camera performs a 12-to-8-bit mapping, so that 4096 input grey levels can be
mapped to 256 output grey levels. The use of the three available modes is explained in the next
sections. Two LUT and a Region-LUT feature are available in the camera (see Section 11.4.4).
The LUT is implemented as a 12-to-8 bit LUT to be compatible with other Pho-
tonfocus cameras. Bits 0 & 1 of the 12 bit LUT input data are set to random
values.
The output grey level resolution of the look-up table (independent of gain,
gamma or user-definded mode) is always 8 bit.
There are 2 predefined functions, which generate a look-up table and transfer it
to the camera. For other transfer functions the user can define his own LUT file.
Some commonly used transfer curves are shown in Fig. 11.2. Line a denotes a negative or
inverse transformation, line b enhances the image contrast between grey values x0 and x1.
Line c shows brightness thresholding and the result is an image with only black and white grey
levels. and line d applies a gamma correction (see also Section 11.4.2).
a
y = f ( x )
x
x
m a x
x
0
x
1
y
m a x
b
c
d
Figure 11.2: Commonly used LUT transfer curves
11.4.1 Gain
The ’Gain’ mode performs a digital, linear amplification with clamping (see Fig. 11.3). It is
configurable in the range from 1.0 to 4.0 (e.g. 1.234).
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