Histogram Thresholding

Histogram thresholding is used by the Object Extraction, Background and Image Factory (segmentation) tools to select a range of pixel values that should match to what you want to select in the image.

Thresholding is a technique for converting a grayscale or color image to a binary image based upon two threshold values. The histogram presents the frequency of intensity values of a channel in an image (see Image Types). The threshold values (min and max) can be applied by moving the arrows above the histogram or by using an automatic threshold method.

Note: The histogram represents the intensity values of the visible part of the image only. After moving the image, use the Refresh button to update the histogram.

Note: The Image Factory tool offers some image preprocessing functions (e.g. to reduce noise).

Image: Select one image in the list.

Channel: Select the RGB or IHS channels for a color image (see Image Types)

Inverse: Inverse the selection values.

- +: Shrink or stretch the histogram vertically.

Auto: In the combo box, select an automatic histogram threshold method. Then click on the Auto button to find the appropriate max threshold value (min is always equal to 0).

Iterative bisection finds the midpoint in an iterative fashion.
Max variance maximizes the ratio of the between-class variance to the within-class variance.
Max entropy maximizes the entropy.
Min error minimizes the probability of error.
Min fuzziness minimizes the fuzziness.

Refresh: Refresh the histogram according to the visible part of the image.

Selection Color: Change the selection color on the image and between the arrows of the histogram.

Color Picker

This tool offers an alternative method to select a range in the histogram by picking colors in the image. The selection results of the min and max values of the pixels, which are displayed in the dialog at the left of the picker button .

: Click on this button, then move the cross cursor on the image and finally click on the desired area to make a new selection.

: Undo the selection (only the last selection can be restored).

: Clear the current selection.

Selection: When Add is selected, the new selection does not replace the previous.

Size: Set the size of the picker.

Tolerance: Increase the min and max picked values.

Note: Adjusting the slider after the selection has no effect.