OpenCV GrayScale Conversion Tutorial


In order to convert an colour image to GrayScale image using OpenCV, first thing you need to do is to read image into BufferedImage and convert it into Mat Object. Its syntax is given below:

File input = new File("digital_image_processing.jpg");
BufferedImage image =;
//convert Buffered Image to Mat.

Then you can transform an image from RGB to Gray scale format by using method cvtColor in the Imgproc class. Its syntax is given below:

Imgproc.cvtColor(source mat, destination mat1, Imgproc.COLOR_RGB2GRAY);

The method cvtColor takes three parameters which are the source image matrix , the destination image matrix and the colour conversion type.

Apart from the cvtColor method, there are other methods provide by the Imgproc class. They are listed below:

Sr.NoMethod & Description
1cvtColor(Mat src, Mat dst, int code, int dstCn)
Converts an image from one colour space to another.
2dilate(Mat src, Mat dst, Mat kernel)
Dilates an image by using a specific structuring element.
3equalizeHist(Mat src, Mat dst)
Equalizes the histogram of a grayscale image.
4filter2D(Mat src, Mat dst, int ddepth, Mat kernel, Point anchor, double delta)
Convolves an image with the kernel.
5GaussianBlur(Mat src, Mat dst, Size ksize, double sigmaX)
Blurs an image using a Gaussian filter.
6integral(Mat src, Mat sum)
Calculates the integral of an image


The following example demonstrates the use of Imgproc class to convert an image to GrayScale.

import java.awt.image.BufferedImage;
import java.awt.image.DataBufferByte;
import javax.imageio.ImageIO;
import org.opencv.core.Core;
import org.opencv.core.CvType;
import org.opencv.core.Mat;
import org.opencv.imgproc.Imgproc;
public class Main {
   public static void main( String[] args )
   try {
      System.loadLibrary( Core.NATIVE_LIBRARY_NAME );
      File input = new File("digital_image_processing.jpg");
      BufferedImage image =;	

      byte[] data = ((DataBufferByte) image.getRaster().
      Mat mat = new Mat(image.getHeight(), image.getWidth(), CvType.CV_8UC3);
      mat.put(0, 0, data);

      Mat mat1 = new Mat(image.getHeight(),image.getWidth(),CvType.CV_8UC1);
      Imgproc.cvtColor(mat, mat1, Imgproc.COLOR_RGB2GRAY);

      byte[] data1 = new byte[mat1.rows()*mat1.cols()*(int)(mat1.elemSize())];
      mat1.get(0, 0, data1);
      BufferedImage image1=new BufferedImage(mat1.cols(),mat1.rows()

      File ouptut = new File("grayscale.jpg");
      ImageIO.write(image1, "jpg", ouptut);
      } catch (Exception e) {
         System.out.println("Error: " + e.getMessage());


When you run the above example, it would convert an image name digital_image_processing.jpg to its equivalent GrayScale image and write it on hard disk with name grayscale.jpg.

Original Image

OpenCV GrayScale Conversion Tutorials

GrayScale Image

OpenCV GrayScale Conversion Tutorials