Background Subtraction

Hi

I am doing Background subtraction with Altera DE2 and Terasic D5M camera.

i want to use Frame difference algorithm but i have problem.

please help me if you know how to do that.

the formula is

| I_frame – I_background| > Th

then update background is

B(i + 1)= α * F(i)+ (1 - α) * B(i)

but it's not work for me