How to get pixels (RGB values) of an image using Java OpenCV library?

A digital image is stored as a 2D array of pixels and a pixel is the smallest element of a digital image.

Each pixel contains the values of alpha, red, green, blue values and the value of each color lies between 0 to 255 which consumes 8 bits (2^8).

The ARGB values are stored in 4 bytes of memory in the same order (right to left) with blue value at 0-7 bits, Green value at 8-15 bits, Red value at 16-23 bits and, alpha at 24-31 bits.

Retrieving the pixel contents (ARGB values) of an image −

To get the pixel values from an image −

• Loop through each pixel in the image. i.e. run nested loops traversing the height and width of the image.

• Get the pixel value at every point using the getRGB() method.

• Instantiate the Color object by passing the pixel value as a parameter.

• Get the Red, Green, Blue values using the getRed(), getGreen() and getBlue() methods respectively.

Example

Following java, example reads the contents of every pixel of an image and writes the RGB values to a file −

import java.io.File;
import java.io.FileWriter;
import java.awt.Color;
import java.awt.image.BufferedImage;
import javax.imageio.ImageIO;
public class GetPixels {
   public static void main(String args[])throws Exception {
      FileWriter writer = new FileWriter("D:\Images\pixel_values.txt");
      //Reading the image
      File file= new File("D:\Images\cat.jpg");
      BufferedImage img = ImageIO.read(file);
      for (int y = 0; y < img.getHeight(); y++) {
         for (int x = 0; x < img.getWidth(); x++) {
            //Retrieving contents of a pixel
            int pixel = img.getRGB(x,y);
            //Creating a Color object from pixel value
            Color color = new Color(pixel, true);
            //Retrieving the R G B values
            int red = color.getRed();
            int green = color.getGreen();
            int blue = color.getBlue();
            writer.append(red+":");
            writer.append(green+":");
            writer.append(blue+"");
            writer.append("\n");
            writer.flush();
         }
      }
      writer.close();
      System.out.println("RGB values at each pixel are stored in the specified file");
   }
}

Output

RGB values at each pixel are stored in the specified file

You can also retrieve the RGB values from a pixel using the shift operators −

To do so −

• Right, shift to the beginning position of each color i.e. 24 for alpha 16 for red, etc.

• The shift right operation may impact the values of other channels, to avoid this, you need to perform bitwise and operation with 0Xff. This masks the variable leaving the last 8 bits and ignoring all the rest of the bits.

int p = img.getRGB(x, y);
//Getting the A R G B values from the pixel value
int a = (p>>24)&0xff;
int r = (p>>16)&0xff;
int g = (p>>8)&0xff;
int b = p&0xff;