import javax.imageio.ImageIO
import java.awt.Image
import java.awt.image.BufferedImage

//def image = ImageIO.read(new File('I:/test.jpg'))
def image = ImageIO.read(new File('i:/test6.png'))
def (x, y) = [image.getWidth(), image.getHeight()]
BufferedImage result = new BufferedImage(x, y, BufferedImage.TYPE_INT_RGB)
result.createGraphics().drawImage(image, 0, 0, null)

def createPermutation(x, y) {
    def RNG = new Random([x, y].hashCode())
    def permutation = [*0..<x*y]
    permutation.size().times() {
        int randomIdx = RNG.nextInt(x*y)
        int tmp = permutation[it]
        permutation[it] = permutation[randomIdx]
        permutation[randomIdx] = tmp
    }
    return permutation
}

boolean direct = false
def perm = createPermutation(x, y)

(0..<x).each { x0 ->
    (0..<y).each { y0 ->
        int destIdx = perm[x0*y + y0]
        (x1, y1) = [destIdx/y, destIdx%y] as int[]
        if (direct) result.setRGB(x0, y0, image.getRGB(x1, y1))
        else result.setRGB(x1, y1, image.getRGB(x0, y0))
    }
}

ImageIO.write(result, "png", new File('i:/test7.png'));