averageluma vs. Ydifference

[martina]

Hi,
can anyone explain me the difference between averageluma and Ydifference... I mean, Ydifferencefromprevious gives me the luma difference between the current and the previous frame, right?
How come, if I compute the difference between the averageluma of the current frame and the averagelume of the previous frame, I hav a different result than Ydifferencefromprevious?

Suppose I'm at the frame i.

abs(averageluma(i-1)-averageluma(i))<>YDifferencefromprevious

Why do they differ?

thanks
martina

[IanB]

Two sets of numbers 1 2 3
3 2 1For both the average is 2
The Absolute Difference is 2 0 2 so the average of the absolute difference is 4/3