Its a siple aritmetic operator... You can find it in most image applications such as photoshop. Just put the images on top odf eachorther and then set teh other ayer to ddifference. And viola you se the difference.
But because ps normalizes the dta to a 1 0 range you cant se if thre subtracted layer is kless that theother so what you want to do is layer another layer added on top wich is the diff the other way areound. Now the end result might be wery subtle so you might want to boost the image witha alevels layer.
I use this a lot to analyze differences in images, when i tweak the render globals for optimum perforance.
