The concept is "shrinking arrows", as Computica describes above.
But as usual, vertex animations (including for quivers) is a mess because each animation frame must share not only the number of vertexes, but also their internal ordering, and this is what makes it difficult to get it right.
In other words, something that often goes wrong in the above method is that each frame will have different internal orderings when you export it, due to the internal working of the modelling software you use (which will assume that the ordering is irrelevant).
A way to get around this problem is to design your vertex animation entirely in the external application (using shrunk arrows trick) , then export it as MD3 and import it. Unfortunately, many 3D modelling software are not designed to produce vertex animations.
A alternative way is to use the special "quiver mode" in OpenBRF. I added that, a long time ago, in order to to help for assembling vertex animations for quivers. It has been a long time since I last touched it but it should still work. Let's see...
1. First, activate it, under [Settings] => [On assemble vertex animation] => [Quiver mode]
2. Now produce and import the quiver Obj (or whatever) with all arrows in it, as a "static mesh".
3. Then, remove (not shrink) one arrow (using wings 3D or whatever you use), save it again (as Obj or whatever), and import it in OpenBRF as a Frame.
INFO: what is happening here is that whatever is present in the 1st frame but is absent in another frame will be shrunk by OpenBRF. But for the rest, every frame will be an exact copy of the 1st, meaning that the vertex order will be correct by construction. Basically you are using the new Obj-s only to tell OpenBRF which part of the 1st frame must be shrunk in order to produce the other frames.
4. Repeat 3 until empty quiver.
5. Final touches: now you need to make sure the frame ordering and timings of your new quiver is ok. Edit timings manually, following what Computica says above (or copy-paste all timings from an existing quiver model, with [edit]->[paste timings]).
To quickly make sure that frame order is the one you want, you can [Right-click] on the quiver, [Seaparte all frames], reorder all frames at will (with alt-up and alt-down), then select them all, right click again, "assemble in a vertex animation".
Hope this helps!
