[bennynihon]

Originally Posted by StainlessS

Look into what JM said, but this might be the immediate problem you had

    Global ydiff = ydiff + grainStrY # need to use Global when assigning to a global,

you'de also have to divide by the number of sampled frames.

I have ydiff defined as a global in my main script (outside of this function) to initialize it. That's also where I divide it by the total number of frames.

EDIT:
Think you may need to use the After_Frame=true arg to FrameEvaluate()
ie

  Y ? FrameEvaluate("global grainStrY = LumaDifference(GE_nr)" ,after_frame=true   ) : nop

I'll have to try this out. Thanks.