Okay, a lookup table is where you get one value, and find out the next value based on that value.

var new_value = lookup_table[old_value];

Give sphere three or four arrays (lookup tables) and it can perform this operation on an image:

pixel.red   = red_lookup[pixel.red];
pixel.green = red_lookup[pixel.green];
pixel.blue  = red_lookup[pixel.blue];
pixel.alpha = red_lookup[pixel.alpha];

EvaluateSystemScript("time.js");

function Clamp(value, min, max)
{
  return Math.max(min, Math.min(value, max));
}

/**
 * This lookup does nothing, null, nada.
 * It's useful for color channels that you don't want to change.
 */
function NullLookup()
{
  var null_lookup = new Array(256);
  for (var i = 0; i < 256; i++) {
    null_lookup[i] = i;
  }
  return null_lookup;
}

/**
 * This lookup adds 'value' to each color value
 * it clamps the color within the range: 0 <-> 255
 */
function BrightenLookup(value)
{
  var brighten_lookup = new Array(256);
  for (var i = 0; i < 256; i++) {
    brighten_lookup[i] = Clamp(i + value, 0, 255);
  }
  return brighten_lookup;
}

function ApplyLookupTest()
{
  var original_surface = GrabSurface(0, 0, GetScreenWidth(), GetScreenHeight());

  var surfare = original_surface.clone();
  surfare.applyLookup(0, 0, surfare.width, surfare.height, BrightenLookup(5), BrightenLookup(5), BrightenLookup(5), NullLookup());
  surfare.blit(0,0);
  FlipScreen();
  Delay(1000);
}


Thus, the screen just got slightly brighter for a second.

Now since the only line in the code for ApplyLookupTest that changes is:

  surfare.applyLookup(0, 0, surfare.width, surfare.height, BrightenLookup(5), BrightenLookup(5), BrightenLookup(5), NullLookup());


That's the only line I'll mention from now on.

/**
 * Invert lookup, i.e. new_color = 255 - old_color;
 */
function ReverseLookup() {
  var reverse_lookup = new Array(256);
  for (var i = 0; i < 256; i++) {
    reverse_lookup[i] = 255 - i;
  }
  return reverse_lookup;
}

  surfare.applyLookup(0, 0, surfare.width, surfare.height, ReverseLookup(), ReverseLookup(), ReverseLookup(), NullLookup());


Will invert the red, green and blue channels.
The resulting image will look very weird.
This is called Filter->Negative Image->RGB in the editor.