Searched a bit for a function. Im looking for some kind of preroll function that starts before a loop. for example:

|| Preroll (when clip starts) 3 sec ||: :||

so for example if the whole clip is 10 sec long, it starts with a 3 seconds long preroll and then a continuously loop for 7 seconds.

is this possible? any hint of where to find more info or anyone who can give me some guidance for this?

|| Preroll (when clip starts) 3 sec ||:  :||

so for example if the whole clip is 10 sec long, it starts with a 3 seconds long preroll and then a continuously loop for 7 seconds. 

possible-with-preroll-and-postroll-in-loops

This basic patch will do it; the only problem i foresee is if you want to envelope your looping section: this will silence your preroll.

How can I implement it to use it with video and rslider? can groove~ myFile also work with a videofile?

No, [groove~] is used to access an audio file buffer; I suggest you look under Help/Jitter Tutorials

possible with preroll and postroll in loops?