Example of color to grayscale conversion with ImageIO in Scala.

makeGray.scala

import java.io.File
import javax.imageio.ImageIO
import java.awt.Color

object makegray extends App {

   def pixels2gray(red: Int, green: Int, blue: Int): Int = (red + green + blue) / 3
  
   def makeGray(img: java.awt.image.BufferedImage): java.awt.image.BufferedImage = {
             val w = img.getWidth
             val h = img.getHeight
             for { w1 <- (0 until w).toVector
                   h1 <- (0 until h).toVector
                 } yield {
                     val col = img.getRGB(w1, h1)
      			         val red =  (col & 0xff0000) / 65536
      			         val green = (col & 0xff00) / 256
      			         val blue = (col & 0xff)
      			         val graycol = pixels2gray(red, green, blue)
                     img.setRGB(w1, h1, new Color(graycol, graycol, graycol).getRGB)  
                 }
             img
           }

  val myimg = ImageIO.read(new File("data/images/train/20.jpg")) 
  val myimgGray = makeGray(myimg)
  ImageIO.write(myimgGray, "jpg", new File("20gray.jpg"))
  
}