I'm looking to use PI to create explosions emitting from within a 3d object such as a building or similar.
Having the explosion on the upper layer just hides the object, having it on the lower layer hides the explosion and using blockers would mean applying them to hundreds of pieces of shrapnel across hundreds of frames, whilst moving them on each frame to keep them in place.
Itried adding the background image on 2 layers and putting the explosion in the middle one and changing the transparency of the upper layer. That created double the shrapnel and just looked a mess.
I would render out the 3D objects in two separate passes as PNGs or TGAs with alpha channels. One with just the building, a second sequence with just the debris, and then place them on two layers. Then drop the explosion between them.
I did this but it did look like a 2d explosion sandwiched in the middle.
However, I then applied a further explosion on the top layer and used the visibility option to create a fake transparency.
I think it looks preety good for my first effort.....
I used Ulead Cool3d prodution studio for the 3d work and originally for the fire and smoke.