machinevisiontoolbox.Image.threshold

Image.threshold(t=None, opt='binary')

Image threshold

Parameters:

  • t (scalar, str) – threshold value

  • option (str, optional) – threshold option, defaults to ‘binary’

Returns:

thresholded image

Return type:

Image

Apply a threshold t to the image. Various thresholding options are supported:

Option

Function

'binary'

\(Y_{u,v} = \left\{ \begin{array}{l} m \mbox{, if } X_{u,v} > t \\ 0 \mbox{, otherwise} \end{array} \right.\)

'binary_inv'

\(Y_{u,v} = \left\{ \begin{array}{l} 0 \mbox{, if } X_{u,v} > t \\ m \mbox{, otherwise} \end{array} \right.\)

'truncate'

\(Y_{u,v} = \left\{ \begin{array}{l} t \mbox{, if } X_{u,v} > t \\ X_{u,v} \mbox{, otherwise} \end{array} \right.\)

'tozero'

\(Y_{u,v} = \left\{ \begin{array}{l} X_{u,v} \mbox{, if } X_{u,v} > t \\ 0 \mbox{, otherwise} \end{array} \right.\)

'tozero_inv'

\(Y_{u,v} = \left\{ \begin{array}{l} 0 \mbox{, if } X_{u,v} > t \\ X_{u,v} \mbox{, otherwise} \end{array} \right.\)

where \(m\) is the maximum value of the image datatype.

If threshold t is a string then the threshold is determined automatically:

threshold

algorithm

'otsu'

Otsu’s method finds the threshold that minimizes the within-class variance. This technique is effective for a bimodal greyscale histogram.

The triangle method constructs a line between the histogram peak and the farthest end of the histogram. The threshold is the point of maximum distance between the line and the histogram. This technique is effective when the object pixels produce a weak peak in the histogram.

'triangle'

Example:

>>> from machinevisiontoolbox import Image
>>> img = Image([[1, 2, 3], [4, 5, 6], [7, 8, 9]])
>>> img.threshold(5).image
array([[  0,   0,   0],
       [  0,   0, 255],
       [255, 255, 255]], dtype=uint8)
Note:

  • The threshold is applied to all color planes

  • If threshold is ‘otsu’ or ‘triangle’ the image must be greyscale, and the computed threshold is also returned.

References:

  • A Threshold Selection Method from Gray-Level Histograms, N. Otsu. IEEE Trans. Systems, Man and Cybernetics Vol SMC-9(1), Jan 1979, pp 62-66.

  • Automatic measurement of sister chromatid exchange frequency” Zack (Zack GW, Rogers WE, Latt SA (1977), J. Histochem. Cytochem. 25 (7): 741–53.

  • Robotics, Vision & Control for Python, Section 12.1.1, P. Corke, Springer 2023.

Seealso:

threshold_interactive threshold_adaptive_ otsu opencv.threshold