Special FX, Scatter

One effect that can look particularly effective if used correctly is scatter, which basically sets the colour of the current pixel to that of another pixel close by, and vice versa, effectively swapping the two pixels. The effect is called scatter because it gives a slightly "disintegrated" look to a picture as pixels are moved around - one pixel could potentially get swapped all the way to the other side of the screen, although it isn't likely!

The scatter algorithm works like this:

1. Go through each pixel

2. Get a random X value between some negative number and some positive number

3. Get a random Y value between some negative number and some positive number

4. If the values are over the width/height of the image or under 0, continue the loop with the next pixel

5. Otherwise, get the colour from the current pixel

6. Get the colour from the nearby pixel

7. Swap the two.

In PHP it is barely more difficult:

function scatter(&$image) {
    $imagex = imagesx($image);
    $imagey = imagesy($image);

    for ($x = 0; $x < $imagex; ++$x) {
        for ($y = 0; $y < $imagey; ++$y) {
            $distx = rand(-4, 4);
            $disty = rand(-4, 4);

            if ($x + $distx >= $imagex) continue;
            if ($x + $distx < 0) continue;
            if ($y + $disty >= $imagey) continue;
            if ($y + $disty < 0) continue;

            $oldcol = imagecolorat($image, $x, $y);
            $newcol = imagecolorat($image, $x + $distx, $y + $disty);
            imagesetpixel($image, $x, $y, $newcol);
            imagesetpixel($image, $x + $distx, $y + $disty, $oldcol);
        }
    }
}

To make your pixels scatter further, change the two values inside the rand() call.

Copyright ©2015 Paul Hudson. Follow me: @twostraws.