Tomas Slancik

raincole
Set POP Drag's air resistance without wind seems to do nothing

try higher values maybe? It defilinely should apply drag to Vellum Fluid particles

on Vellum Solver SOP you can also use Forces/Built In Wind Drag, which is linked to internal POP Wind, but regardless ow what POP Wind or POP Drag you use you should see the effect

if you render pure emissive volume it will have no Alpha, which is correct and it will also not block any other light

the comp should be simple, just Add over the plate, not sure how it's called in AE, in Nuke is called Plus
Assuming your soft assumes the image is already premultiplied (which it should) you can also use Over, since if Alpha is 0 then over is the same as Plus: A + B(1-alpha) = A + B(1-0) = A + B

raincole
What confuses me is that I can middle click the parameter to show its value even it uses stamps(), just like other expressions. But if stamps() is evaluated during cook, does it mean the value we see by middle clicking it is not necessarily meaningful?

Correct, the value you see is the 3rd argument of stamp*() function, which defines a default value shown when evaluated out of context

Doesn't have to be meaningless, it's certainly a placeholder