transparency when using multiple passes?

this is probably a stupid question, but i could not find sth useful when searching: i use multiple passes to simulate lighting - the 1st pass renders depth only, then i enable additive blending and render one pass for each light. works fine, but how can i simulate transparency now (for billboards etc)?

there was a thread here already, (or it was gdalgos list). For bigger objects it was suggested to treat as opaque do the additive lighting in another rendertarget, then blend the image back on the framebuffer with transparency.

(or it was gdalgos list)
what’s that?


I looked up the thread there its in the archive:

it started at the July and goes for a couple days.…0607&viewday=15