Reverse an image (stored in 2D array) vertically

Question

I'm dealing with an image that is stored in a 2D array, but the image is upside down. What I'm trying to do is flip it vertically. I need to swap the first row and the last row, the second row and the second to last row, and so on. Here is what I tried:

    public void mirror() {
    for (int i = 0; i < height; i++) {
        int[] firstRow = imageData[i];
        int[] secondRow = imageData[height - 1 - i];
        int[] tempRow = firstRow;
        firstRow = secondRow;
        secondRow = tempRow;
        imageData[i] = secondRow;
        imageData[height - 1 - i] = firstRow;

This runs without any errors, but the picture doesn't change at all.

The height variable is the height of imageData[][] which is the 2D array where the picture is stored.

Answer 1

You're swapping the rows twice .

for (int i = 0; i < height; i++)

Once i reaches values greater than height / 2 , you start to access rows that have already been swapped, and swap them back to their original positions. To prevent this, the loop should stop in the middle, ie when i reaches height / 2 .

You also have a slight logic error in your algorithm; you already assigned the value of secondRow to firstRow and vice versa, so those respective values are what should be the elements of imageData[i] and imageData[height - 1 - i] .

imageData[i] = firstRow;
imageData[height - 1 - i] = secondRow;

Answer 2

Your "swap" is a no-op. Ignoring secondRow for a minute:

int[] firstRow = imageData[i];
int[] tempRow = firstRow;
secondRow = tempRow; // == firstRow
imageData[i] = secondRow; // == tempRow == firstRow !

and on top of that you're (not) doing the swaps twice over.

Instead, try this:

for (int i = 0; i < height / 2; ++i) {
    int[] tempRow = imageData[i];
    imageData[i] = imageData[height - i - 1];
    imageData[height - i - 1] = tempRow;
}