How to flip an image horizontally

Question

HiI was wondering how to flip and image horizontally, for a practce task I was given a code that reads an image, inverting it to an image indicating it's brightness from 0-5, I had to flip an image.

This is my code of my reading an image and drawing it

public int[][] readImage(String url) throws IOException
{
    // fetch the image
    BufferedImage img = ImageIO.read(new URL(url));

    // create the array to match the dimensions of the image
    int width = img.getWidth();
    int height = img.getHeight();
    int[][] imageArray = new int[width][height];

    // convert the pixels of the image into brightness values
    for (int x = 0; x < width; x++)
    {
        for (int y = 0; y < height; y++)
        {
            // get the pixel at (x,y) 

            int rgb = img.getRGB(x,y);
            Color c = new Color(rgb);
            int red = c.getRed();
            int green = c.getGreen();
            int blue = c.getBlue();

            // convert to greyscale
            float[] hsb = Color.RGBtoHSB(red, green, blue, null);                
            int brightness = (int)Math.round(hsb[2] * (PIXEL_CHARS.length - 1));

            imageArray[x][y] = brightness;
        }
    }
    return imageArray;
}

public void draw() throws IOException
{
    int[][] array = readImage("http://sfpl.org/images/graphics/chicklets/google-small.png");
    for(int i=0; i<array.length; i++)
    {
        for(int pic=0; pic<array[i].length; pic++)
        {
            if(array[pic][i] == 0)
            {
                System.out.print("X");
            }
            else if(array[pic][i] == 1)
            {
                System.out.print("8");
            }

            else if(array[pic][i] == 2)
            {
                System.out.print("0");
            }       

            else if(array[pic][i] == 3)
            {
                System.out.print(":");
            }

            else if(array[pic][i] == 4)
            {
                System.out.print(".");
            }

            else if (array[pic][i] == 5)
            {
                System.out.print(" ");
            }

            else 
            {
                System.out.print("error");
                break;
            }   

        }
        System.out.println();
    }
}    

and this is the code I tried to create to horizontally flip it,

void mirrorUpDown()
{
    int[][] array = readImage("http://sfpl.org/images/graphics/chicklets/google-small.png");
    int i = 0;

    for (int x = 0; x < array.length; x++)
    {
        for (int y = 0; y < array[i].length; y++)
        {{
                int temp = array[x][y]; 
                array[x][y]= array[-x][y]; 
                array[array[i].length-x][y]=temp;
            }
        }
    }

}    

I get an error

 unreported exception java.io.IException;
 must be caught or declared to be thrown

Answer 1

I'd actually do it by this way...

    BufferedImage flip(BufferedImage sprite){
        BufferedImage img = new BufferedImage(sprite.getWidth(),sprite.getHeight(),BufferedImage.TYPE_INT_ARGB);
        for(int xx = sprite.getWidth()-1;xx>0;xx--){
            for(int yy = 0;yy < sprite.getHeight();yy++){
                img.setRGB(sprite.getWidth()-xx, yy, sprite.getRGB(xx, yy));
            }
        }
    return img;
}

Just a loop whose x starts at the end of the first image and places its rgba value on the flipped position of the second image. Clean, easy code :)

Answer 2

Sorry about posting this here over a year later but it should aid someone at a stage

   try{
   java.awt.image.BufferedImage bi = javax.imageio.ImageIO.read(getClass().getResource("Your image bro.jpg")) ;
 int[] h = bi.getRGB(0, 0, bi.getWidth(), bi.getHeight(), null, 0, bi.getWidth());
 int [] h1 = new int[h.length];
 System.out.println(""+h.length);
  for(int j = 0;500>j;j++){
   for(int i = 500;i>0;i--){
         h1[j*500+(500-i)] = h[(j*500)+(i-1)];
   }
  }
 bi.setRGB(0, 0, bi.getWidth(), bi.getHeight(), h1, 0, bi.getWidth());
      }
      catch(Exception e){e.printStackTrace();}

Lets break the code down

java.awt.image.BufferedImage bi =javax.imageio.ImageIO.read(getClass().getResource("Your image bro.jpg"));

Tries to read the image and stores the read image into the BufferedImage variable bi

  int[] h = bi.getRGB(0, 0, bi.getWidth(), bi.getHeight(), null, 0, bi.getWidth());
  int [] h1 = new int[h.length];

instantiate two arrays, h is the original RGB Array and h1 will be the horizontally flipped RGB array.

for(int j = 0;500>j;j++){
 for(int i = 500;i>0;i--){
  h1[j*500+(500-i)] = h[(j*500)+(i-1)];
 }
}

Lets look at something in particular more closely

  h1[j*500+(500-i)] = h[(j*500)+(i-1)];

Images are scanned from position 0;0 to x.length;y.length but it is scanned in a coninual array. Thus we use a psuedo-array to manipulate the flipping of the image. j*500 references the Y values and (500-i) references the x values.

 bi.setRGB(0, 0, bi.getWidth(), bi.getHeight(), h1, 0, bi.getWidth());

Finally, the image gets stored back into the BufferedImage variable.

Note that the 500 constant is referencing your x resolution of the image. For example, 1920 x 1080 sized image uses a max value of 1920. The logic is yours to decide.

Answer 3

How is your image supposed to know it should get it's data from imageArray ?

instead, you should access the raster of your image and modify the data in it.

void flip(BufferedImage image) {
         WritableRaster raster = image.getRaster();
         int h = raster.getHeight();
         int w = raster.getWidth();
         int x0 = raster.getMinX();
         int y0 = raster.getMinY();
         for (int x = x0; x < x0 + w; x++){
             for (int y = y0; y < y0 + h / 2; y++){
                 int[] pix1 = new int[3];
                 pix1 = raster.getPixel(x, y, pix1);
                 int[] pix2 = new int[3];
                 pix2 = raster.getPixel(x, y0 + h - 1 - (y - y0), pix2);
                 raster.setPixel(x, y, pix2);
                 raster.setPixel(x, y0 + h - 1 - (y - y0), pix1);
             }
         }
         return;
    }

Answer 4

The function mirrorUpDown() , add a throws IOException there.

Also the function from which you are calling these methods, does that handle exception, does that code enclosed in a try catch block or the function is also set to throw IOException (one of either should be there)