How to understand detrend_df_f function, please help me

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I want to ask a basic question

how to understand the detrend_df_f function i see the original article(elife) and is difficult for me to understand the function. in my opinion the dfof can be simply understand by the F(t)-F(0)/F(0)，and i want to know the F(0) in this detrend_df_f function mean. is that the dynamic range of baseline for the specific F(t)? or static or markered? i mean how to define the F(0), is the first 500 frame value or the 500 frame around the F(t)? Thanks a lot!