Track mattes, front side, back side rendering, lots of tricks. To give you a recipe it would help to see a screenshot of your comp.
As to your particle question: Trapcode Particular (a third-party plugin available from Red Giant) is a very powerful particle plugin for After Effects and it allows you to use a depth pass (which you can create in C4D) to determine the visibility of particles around actual 3d objects. It can also use layers in AE's fake 3d space as obscuration layers for the particles.
You could fake such interactivity with 3d models in Particle World by using the depth pass with some color correction applied as a luma matte. However, I don't know a good way to fake interactivity with layers in AE's fake 3d space.
As to your first question, you could fake interactivity with AE's layers and your 3d model by using the depth pass from C4D as a luma matte for your AE layers. If you have a lot of layers, it might make sense to precomp the layers and use a copy of the camera in the precomp.
There may be some other suggestions from other folks, but it looks to me like you may need to rethink your workflow. For example, what kind of "control" do you need over your elements in AE. Are you talking about timing of the animation or are you just needing to control the contents of the layer? The reason I'm asking is that you could use planes in C4D and render out object buffers for the planes.
For example, where I work we do a pretty complex slideshow every month or so. The camera flies around a 3d world with ropes, barbed wire, mud, etc. and looks at various picture frames on the ground. The picture frames are precomps in AE with the slides and I've generated an object buffer for the planes in C4D. I used the external compositing tag in C4D to generate the position of those planes for use in AE. In addition to the object buffer, I also rendered a depth pass, a reflection pass, and a shadow pass. When I composite in After Effects, I use Particular to generate a cloudy/smokey atmosphere and use the depth pass to integrate it with the 3d scene, I use the object buffer as a luma matte for the AE precomps so that the ropes, barbed wire, etc. go over the top of them and behind them as appropriate, and I use the reflection and shadow passes so that the picture frames seem to be a part of the scene. It works beautifully and I can change the content of the picture frames on the ground in AE all I want.
Thanks Rick. I was worried that the answer was going to be a lot of fakery. I also need to convert it to stereoscopic 3D which makes it harder to pull off the tricks. I wished it was as simple as objects appearing in the 3D space.
Hi Szalam, thanks for the help! I am definitely going to look into getting Trapcode Particular asap.
I haven't yet worked with a depth pass but it sounds very interesting. It may just solve some of the challenges I have.
As mentioned to Rick, the plan is to make the finished article stereoscopic 3D, which does make all the tricks for faking and pre-comping of layers a lot harder to do. Similar to your job this is required for an animated slideshow and as you say I may need to rethink my workflow to achieve my goal. I had assumed the AE 3D space was no different from the C4D one so thought it would easily intergrate - it's a shame that it isn't a bit more intuitive between the two programs.
Thanks again for the answer, Rob
Graphic Rob wrote:
I had assumed the AE 3D space was no different from the C4D one so thought it would easily intergrate - it's a shame that it isn't a bit more intuitive between the two programs.
Well, it's way better than trying to use another 3d app with After Effects! Considering that CC is the first time that C4D was included with After Effects, one can only hope that the integration will become better over time.
Graphic Rob wrote:
Similar to your job this is required for an animated slideshow and as you say I may need to rethink my workflow to achieve my goal.
I guessed right! I would suggest using planes in C4D and using the external compositing tag on them (and set it to create solids in AE) and the compositing tag (to create an object buffer for them). That will make integration with After Effects much easier.