Image Thresholding#

Goal#

  • In this tutorial, you will learn Simple thresholding, Adaptive thresholding, Otsu’s thresholding etc.

  • You will learn these functions : cv.threshold, cv.adaptiveThreshold etc.

Simple Thresholding#

Here, the matter is straight forward. If pixel value is greater than a threshold value, it is assigned one value (may be white), else it is assigned another value (may be black).

We use the function: cv.threshold (src, dst, thresh, maxval, type)

Parameters

src

input array.

dst

output array of the same size and type and the same number of channels as src.

thresh

threshold value.

maxval

maximum value to use with the cv.THRESH_BINARY and cv.THRESH_BINARY_INV thresholding types.

type

thresholding type(see cv.ThresholdTypes).

thresholding type - OpenCV provides different styles of thresholding and it is decided by the fourth parameter of the function. Different types are:

Note

Input image should be single channel only in case of cv.THRESH_OTSU or cv.THRESH_TRIANGLE flags

Try it#

Adaptive Thresholding#

In the previous section, we used a global value as threshold value. But it may not be good in all the conditions where image has different lighting conditions in different areas. In that case, we go for adaptive thresholding. In this, the algorithm calculate the threshold for a small regions of the image. So we get different thresholds for different regions of the same image and it gives us better results for images with varying illumination.

We use the function: cv.adaptiveThreshold (src, dst, maxValue, adaptiveMethod, thresholdType, blockSize, C)

Parameters

src

source 8-bit single-channel image.

dst

destination image of the same size and the same type as src.

maxValue

non-zero value assigned to the pixels for which the condition is satisfied

adaptiveMethod

adaptive thresholding algorithm to use.

thresholdType

thresholding type that must be either cv.THRESH_BINARY or cv.THRESH_BINARY_INV.

blockSize

size of a pixel neighborhood that is used to calculate a threshold value for the pixel: 3, 5, 7, and so on.

C

constant subtracted from the mean or weighted mean (see the details below). Normally, it is positive but may be zero or negative as well.

adaptiveMethod - It decides how thresholding value is calculated:

Try it#