LUT Content Definition

This method defines the transfer function of a lookup table by means of four parameters: Contrast, Brightness, Visibility and Negative.

The Contrast and Brightness parameters provide controls similar to the brightness and contrast controls of a television monitor.

The Visibility parameter provides control to smoothly reshape the transfer function to cover the full input range.

The Negative parameter allows transforming an image into its negative image.

This method defines the transfer function of a lookup table by means of two parameters: Emphasis and Negative.

It allows transforming an image using a power-law expression also known as γ – Gamma – function.

The Negative parameter allows transforming an image into its negative image.

This method defines a double threshold transformation law by means of five parameters: SlicingLevel, SlicingBand, LightResponse, BandResponse, DarkResponse.

This method defines the transfer function of a lookup table in a tabular form.

The Brightness parameter exclusively applies to the Response Control lookup table definition method.

It implements a control similar to the brightness control of a television monitor.

Brightness	Note
-1.00	Min. Brightness value: darkest output. The whole input range data gets transformed into the full black. This rule applies for any chosen Contrast value.
0.00	Default Brightness value. The mid-level input level of 0.5 is transformed as the same output level of 0.5. This is true for any value of the other parameters. Any increase in the brightness towards +1.00 results into a lighter output. Any decrease of the brightness towards -1.00 results into a darker output.
+1.00	Max. Brightness value: lightest output. The whole input range data gets transformed into the full white. This rule applies for any chosen Contrast value.

Effect of Brightness when all other controls are set to their default value:
Contrast = 1.00; Visibility = 0.00; Negative = FALSE

The Contrast parameter exclusively applies to the Response Control lookup table definition method.

It implements a control similar to the contrast control of a television monitor.

The slope of the transformation law is the gain, which is non-linearly controlled from the Contrast parameter.

Mathematically, the relationship is:

Gain = 10^{2 x(Contrast-1)}

Contrast	Gain	Note
0.00	0.01	Min. Contrast value; smallest gain
1.00	1	Default Contrast value;unity gain
2.00	100	Max. Contrast value; largest gain

To achieve a required given gain, the contrast control should be set to:

Contrast = 1 + (log₁₀ Gain)/2

If the required gain is expressed in decibels (dB):

Contrast = 1 + Gain(dB)/40

Effect of Contrast when all other controls are set to their default value:
Brightness = 0.00; Visibility = 0.00; Negative = FALSE

The Visibility parameter exclusively applies to the Response Control lookup table definition method.

The operation of Contrast and Brightness parameters occasionally removes some part of the input dynamics. Very dark regions of the image can be transformed into full black, and become invisible. This holds true for very bright regions, clipping to full white.

The Visibility parameter has been created to smoothly reveal these hidden parts in the image.

Visibility	Gain	Note
0.00	0.01	Minimum and default Visibility value. This generates the piecewise linear transformation curves. Choosing values closer to +1 generates smoother curves.
1.00	100	Max. Visibility value.

Effect of Visibility for typical values of Contrast and Brightness parameters assuming that Negative = FALSE

The Negative parameter applies to both the Response Control and the Emphasis lookup table definition methods.

This control allows transforming an image into its negative image, where the lightest areas of the image appear darkest and the darkest areas appear lightest.

Negative	Note
FALSE	Default value.
TRUE	The transformation table is mirrored around a vertical axis in the graphs. This swaps the black and white values, and gives rise to a photographic negative effect.

Effect of Negative for typical values of other controls

The Emphasis parameter exclusively applies to the Emphasis lookup table definition method.

It allows transforming an image using a power-law expression:

Output = Input^γ

The γ – Gamma – exponent is mathematically linked to Emphasis by:

γ = 10^-Emphasis

Emphasis	Gamma	Note
1.00	0.1	Max. Emphasis value; smallest γ value
0.00	1	Default Emphasis value; linear law
-1.00	10	Min. Emphasis value; largest γ value

To achieve a required given γ, Emphasis should be set to:

Emphasis = - log₁₀γ

Emphasis effect for typical values of Emphasis and both values of Negative

SlicingLevel, SlicingBand, LightResponse, BandResponse and DarkResponse parameters exclusively apply to the Threshold lookup table definition method.

As shown on the next figure, the parameters set defines a double threshold transformation law.

Parameter	Minimum Value	Default Value	Maximum Value
Slicing Level	0.00	0.50	1.00
Slicing Band	0.00	0.50	1.00
LightResponse	0.00	0.75	1.00
BandResponse	0.00	0.50	1.00
DarkResponse	0.00	0.25	1.00

Double threshold transfer function

Note: SlicingLevel specifies the mean value of both thresholds in the input range.