The Threshold command performs a threshold operation on a grayscale image.

The img parameter is the input image. It must be of type GrayImage.

The optional thr parameter specifies the threshold. If not specified, a threshold is computed using Otsu's method.

The effect of the threshold operation can be specified by the method option, by specifying whether to threshold pixels above the threshold, below the threshold, or both.

The options high and low specify the new intensity values.

low = float

Specifies the value that all pixels that fall below the threshold are set to.  The default value is 0.0.

high = float

Specifies the value that all pixels that fall above the threshold are set to.  The default value is 1.0.

method = above, below, or both

Specifies how the threshold operation is performed. Specifying method = above causes all the pixels that fall above the threshold to be set to the value specified by the high option, and the pixels that fall below the threshold retain their value. Specifying method = below causes all the pixels that fall below the threshold to be set to the value specified by the low option, and the pixels that fall above the threshold retain their value. Specifying method = both, which is the default value, causes all the pixels that fall below the threshold to be set to the value specified by the low option, and all the pixels that fall above the threshold to be set to the value specified by the high option.

Otsu, N. "A threshold selection method from gray level histograms." IEEE Trans. Systems, Man and Cybernetics, Vol. 9. (1979).