Threshold & Histogram Section

Threshold

Sets the brightness boundaries between which pixels are considered.

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Reset

Clears the upper and lower thresholds. No pixels are considered.

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Undo

Undoes the last change made to the threshold values.

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Redo

Restores the last undone change to the threshold values.

Color Model

Only visible if the image is a color image, see Color Model.

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RGB

In RGB mode you can define the threshold values for the red, green and blue color channels.

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HLS

In HLS mode you can define the threshold values for hue, saturation and lightness.

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Low

Sets the lower threshold. Only pixel values above this value are considered. The range of possible values depends on the bit depth of the image.

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High

Sets the upper threshold. Only pixel values below this value are considered. The range of possible values depends on the bit depth of the image.

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Invert

Only pixels outside the threshold boundaries are considered, i.e. those pixels below the lower threshold and above the higher threshold.

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Full Range

Sets the lower threshold to 0 and the upper threshold to the highest value (depending on bit depth). The entire range of pixel values is considered.

Histogram

In the histogram you can change the lower and upper threshold value for the activated value. Drag the lower or upper adjustment handle or shift the entire highlighted area between the lower and upper threshold value.

Click

Click in the image on the regions that you want to define as objects. The threshold values are adapted according to the pixel intensities at the clicked position in the image.

Automatic

The threshold values are determined automatically from the histogram. During setup only the part of the image displayed in the viewport is taken for the calculation of the threshold. After the automatic calculation of the threshold values you can further modify the threshold values found interactively by selecting Click for threshold value definition.

Pick Behavior

Only visible, if you have selected Click.

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+

Enables you to expand the currently segmented regions by the gray values/colors of the objects subsequently clicked on.

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-

Enables you to reduce the currently segmented regions by the gray values/colors of the objects subsequently clicked on.

Tolerance

Only visible, if you have selected Click.
Specifies how many additional pixel values are included in the selection based on their brightness. A higher value means that more pixel values similar to the selected one are included. A lower value means that only the exact pixel value selected is included.

Neighborhood

Only visible, if you have selected Click.
Specifies how many additional pixel values are included in the selection based on their physical proximity to the selected pixel. A higher value means that more pixels surrounding the selected pixel are included. The threshold boundaries are adapted so that all the pixel values of these neighboring pixels are included. A lower value means that the boundaries are adapted based on only the pixels directly next to the selected pixel.

Method

Only visible, if you have selected Automatic.
Selects the algorithm that is used to automatically detect the threshold boundaries. The most suitable algorithm depends on your precise requirements. The value also depends on the bit depth of the image. The following methods are available:

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Otsu

The pixel values below the threshold are designated as background and those above the threshold as foreground. It iterates through all possible threshold values and calculates the variance of the pixel intensities of the background and foreground pixels for each value. The threshold is set at the value that minimizes both variances. This method is particularly suited to light objects on a dark background.

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Maximum Peak

Separates background and foreground pixels at the maximum value of the histogram.

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Iso-Data

The pixel values below the threshold are designated as background and those above the threshold as foreground. An initial threshold value is chosen, and the mean pixel intensity of the foreground and background pixels is calculated. These two mean values are averaged and the result serves as the input threshold for the next calculation. The process is repeated until the threshold value no longer changes.

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Triangle Threshold

The algorithm constructs a line between the peak of the highest frequency pixel intensity and the lowest pixel intensity. The distance between the line and the histogram is computed for all values along the line. The pixel intensity where the line is longest is used as the threshold. This method is particularly suited when the foreground pixels only have a weak peak in the histogram.

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Three Sigma Threshold

Calculates the pixel value that occurs most frequently. The standard deviation of the values in the peak is calculated. The threshold is set to the pixel intensity that is the sum of the average peak value and three times the standard deviation.