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Characteristics and LUT

uEye Camera Manual Version 3.80

Characteristics and LUT

When perceiving or imaging a scene, the form of the imaging characteristic is crucial for displaying the differences in brightness. With image processing (e.g. applications such as edge detection and character recognition), linear characteristics are generally required. The human eye, on the other hand, perceives differences in brightness based on a logarithmic characteristic, which often approximates a gamma characteristic in practice. All three forms will be shown in the following.

Linear characteristic

If a system (e.g. a camera with a conventional CCD sensor) yields double the output value for double the brightness, the system features a linear characteristic:

Figure 44: Imaging with linear characteristic

Figure 44: Imaging with linear characteristic

Gamma characteristic

Gamma characteristics (or gamma curves) are named after the Greek formula symbol γ. Gamma curves are power functions of the form

and are often used in photography or image display on computer screens. A gamma value of 1 generates a linear characteristic again. A curve with the value γ = 2.2 used for computer screens is shown in the figure below.

Figure 45: Imaging with gamma characteristic

Figure 45: Imaging with gamma characteristic

Such a gamma characteristic brightens dark areas of an image, which corresponds more to the perception of the human eye. In light areas of an image, the differences in brightness are condensed for this.

Bild_LUT_Lin

Bild_LUT_Gamma

Figure 48: Image with linear characteristic (left) and gamma characteristic (right)

Logarithmische characteristic

The effect of the logarithmic characteristic is even stronger. Here, the characteristic follows the function

The following diagram illustrates how very large jumps in brightness in light areas of a scene only cause small changes in image brightness. This explains why image sensors with a logarithmic characteristic, in particular, are ideal for imaging scenes with very high dynamic range.

Figure 46: Imaging with logarithmic characteristic

Figure 46: Imaging with logarithmic characteristic

Look Up Table (LUT)

With Look Up Tables (LUT) it is easy to apply characteristic curves to digital images. A LUT is a table which assigns an output value to every possible input value. The figure below shows a LUT which would binarize an image: For an 8 bit image, for example, this LUT would replace all pixels with gray values 0...127 with value 0 and all pixels with gray values 128...255 with value 255.

Using LUTs has the advantage that calculations can be done very fast. Typical applications of LUTs are enhancing image contrast, or gamma characteristics. GigE uEye cameras can perform LUT operations on the image already in the camera (see Pixel Preprocessing in GigE uEye Cameras).

Figure 47: LUT characteristic for binarizing an image

Figure 47: LUT characteristic for binarizing an image


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