Threshold operation

double threshold(InputArray src, OutputArray dst, double thresh, double maxval, int type)

Applies a fixed-level threshold to each array element

Parameters:

• src – input array (single-channel, 8-bit or 32-bit floating point).
• dst – output array of the same size and type as src.
• thresh – threshold value.
• maxval – maximum value to use with the THRESH_BINARY and THRESH_BINARY_INV thresholding types.
• type – thresholding type

• THRESH_BINARY

  

• THRESH_BINARY_INV

  

• THRESH_TRUNC

  

• THRESH_TOZERO

  

• THRESH_TOZERO_INV

  

Find an example in OpenCV documentaion.

Steps:

1. Load an image
2. Create a window to display results
3. Create Trackbar to choose type of Threshold
4. Call the function "Threshold_Demo" to perform threshold operation.

Functions:

threshold, createTrackbar, imread, cvtColor, namedWindow, waitKey.

Example:

------------

#include "opencv2/imgproc/imgproc.hpp"
#include "opencv2/highgui/highgui.hpp"
#include <stdlib.h>
#include <stdio.h>

using namespace cv;

int threshold_value = 0;
int threshold_type = 3;;
int const max_value = 255;
int const max_type = 4;
int const max_BINARY_value = 255;

Mat src, src_gray, dst;
char* window_name = "Threshold Demo";

char* trackbar_type = "Type: \n 0: Binary \n 1: Binary Inverted \n 2: Truncate \n 3: To Zero \n 4: To Zero Inverted";
char* trackbar_value = "Value";

void Threshold_Demo( int, void* );

int main( int argc, char** argv )
{
  /// Load an image
  src = imread( "shape.jpg", 1 );

  /// Convert the image to Gray
  cvtColor( src, src_gray, CV_RGB2GRAY );

  /// Create a window to display results
  namedWindow( window_name, CV_WINDOW_AUTOSIZE );

  /// Create Trackbar to choose type of Threshold
  createTrackbar( trackbar_type,
                  window_name, &threshold_type,
                  max_type, Threshold_Demo );

  createTrackbar( trackbar_value,
                  window_name, &threshold_value,
                  max_value, Threshold_Demo );

  /// Call the function to initialize
  Threshold_Demo( 0, 0 );

  /// Wait until user finishes program
  while(true)
  {
    int c;
    c = waitKey( 20 );
    if( (char)c == 27 )
      { break; }
   }
}


void Threshold_Demo( int, void* )
{
  /* 0: Binary
     1: Binary Inverted
     2: Threshold Truncated
     3: Threshold to Zero
     4: Threshold to Zero Inverted
   */

  threshold( src_gray, dst, threshold_value, max_BINARY_value,threshold_type );

  imshow( window_name, dst );
} 

------------

Result:

Sources:
http://docs.opencv.org/doc/tutorials/imgproc/threshold/threshold.html