Fog shader - scratchable surfaces

For a long time I've wanted to make a interactive scratchable surface and it seems that I'm almost there.  The nice people of the Alexandra Institute posted an article on a technique to create steam/fog which you can scratch away.  Based on this article I started coding in openFrameworks and together with that post I created a "scratchable" surface app.  The app. shows an image and slowly fades away using a gradient image. This gradient is used by the shader as a "value" map.  The values are used to check how fast the fog should grow on a certain x/y coordinate.