This is a custom action for creating reverse reverb/delay swells, on the grid, from a source sound in your song. A common use is on vocals. For example in the Travis Scott song at 00:12 -
https://youtu.be/e14IHe035mc?t=10
I've seen some other actions for this, but they didn't work as I wanted. I had a DOP preset set up for this in Cubase, and couldn't figure out an equivalent in Reaper, but as usual, once I put some time in I realised I could actually do it better in Reaper. I have a few of these set up now for different reverb and delay variations.
All you need to do is cut the snippet you want to effect, then run the custom action, it extends the item out to the next 2 measures, mutes everything except the part you originally selected, reverses that first part then glues the whole thing together so you have the reversed part followed by empty silent space, it then adds the FX chain to the item (reverb/delay, whatever you've set it to) and reverses it again. This double reversing means that the direction of that original snippet you started with is preserved, so if you're using a word from a vocal for instance, then that word will be preserved and not reversed in the final reverb swell, but you still get the reversing effect of the reverb/delay swelling up.
It also does it all on the grid so that timing is preserved and it's easy to line up the effected swelling sound up with the original sound if it's connecting into it, which is helpful for me.
It renders the item into a new take so you have the non-reversed affected item on the previous take. I like that, but if you don't then you can add in another 'glue' action at the end.
Before it will work you'll need to save an FX chain with your preferred reverb/delay plugin (for my main reverse reverb chain I use a combination of valhalla plate around 90% wet, and shimmer around 30% wet, for example). Assign that FX chain to an FX chain slot in the resources window, then reference that same slot in the custom action below -
Custom: Reverse Reverb
Item navigation: Move cursor to end of items
SWS/BR: Save edit cursor position, slot 01
Item navigation: Move cursor to start of items
Move edit cursor forward one measure (no seek)
Move edit cursor forward one measure (no seek)
Xenakios/SWS: Trim/untrim item right edge to edit cursor
SWS/BR: Restore edit cursor position, slot 01
Item: Split items at edit cursor (select right)
Item properties: Mute
SWS: Select previous item, keeping current selection (across tracks)
Script: Item - Glue items (preserve stuff).lua
SWS/S&M: Resources - Paste FX chain to selected items, slot 3 (assign your slot here)
Script: Item - Glue items (preserve stuff).lua
Item: Reverse items to new take
Hopfully someone finds this helpful. I'll certainly be using it a lot, so I thought I'd share it.