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

Digital images are stored as 2D arrays of pixels, which are the smallest elements of digital images.

Each pixel contains alpha, red, green, and blue values. Each color value ranges from 0 to 255 and occupies 8 bits (2^8).

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

Retrieve the pixel content (ARGB value) of the image -

To get the pixel value from the image -

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

• Use the getRGB() method to get the pixel value of each point.

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

• Use the getRed(), getGreen(), and getBlue() methods to get the red, green, and blue values.

Example

The following is an example from Java that reads the contents of each 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 use the shift operator to retrieve RGB values ​​from pixels

To do this,

• shift right, moving the starting position of each color to the corresponding position, such as 24 for alpha, 16 for red, etc.

• The right shift operation may affect the values ​​of other channels. To avoid this, you need to perform a bitwise AND operation with 0Xff. This masks the variable, keeping only the last 8 bits, and ignoring all the rest.

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;

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