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#### Gamma, Hurter and Driffield (H&D), and Sigmoid functions

This section describes the non-linear LUT functions that you can apply to images. They include:

Gamma function

The Gamma function is affected by the parameter called the Gamma coefficient. By specifying the Gamma coefficient, you can modify the display values, until the optimal image presentation is obtained.

To apply the Gamma function to images, see Steps for applying a non-linear LUT function.

Hurter and Driffield (H&D) function

The H&D curve by Hurter and Driffield shows the relationship between the light/X-Ray exposure and optical density.

In Disc, the H&D curve is implemented by a modified sigmoid function, to represent the relationship between optical density and display values. This modified curve is called the H&D function.

H&D function is affected by two parameters:

| • | The Toe coefficient shifts the function left and right, enabling you to view different tissue types.

| • | The Shoulder coefficient changes the steepness (also called slope) of the function, enabling you to modify the image contrast.

By specifying the Toe and Shoulder coefficients, you can modify display values for different procedure types or body regions, until the optimal image presentation is obtained.

To apply the H&D function to images, see Steps for applying a non-linear LUT function.

Sigmoid function

The Sigmoid function is affected by its curvature. By modifying the curvature, you can modify the display values for different procedure types or body regions, until the optimal image presentation is obtained.

To apply the Sigmoid function to images, see Steps for applying a non-linear LUT function.

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