how to make movining object completly white for tracking

i have made real time motion detection code its working ok but when every thing is still i get image like this:

why isn't the back ground completly black?

when motion detected it like this:

which is fine

for tracking i do i need the moving object to be completely white??

here's my code:

plz help my project is getting delayed :(

[Image Analyst]

The bottom image does show as completely white where there is a difference, like you want, so what's the problem?