2D layers interfere with how 3D layers interact between/amongst themselves. 3D interaction encompasses shadow casting and layer encroachment, which defines how 3D layers are able to cut across each other.
If you have a 3D text layer casting shadows onto a 3D ‘floor’ layer and you then insert a 2D layer between the text and ‘floor’ layers (in the Timeline hierarchy), then the shadow will no longer be available.
Now that you have a better idea as to how 2D layers affect 3D interaction, it’s time to cover another important concept – layer hierarchy within the Timeline.
With 2D-only comps, the structure works from top-down. With 3D-only layers within a comp, the Timeline hierarchy is irrelevant as layers are rendered based on their distance from the Active Comp camera. This of course precludes Adjustment Lights from the scenario.
With a combination of 2D and 3D layers, things get more interesting. You should start analyzing the Timeline hierarchy from the bottom and work your way up.
2D layers on top of other 2D and 3D layers will always be rendered as an overlay, on top. As well, 2D layers below other 2D and 3D layers will be rendered below.
If you conduct simple tests, you should have a better grasp of the mechanics.
What becomes a lot trickier is when you have precomps with a combination of 2D and 3D layers. In such a situation, I can’t provide a solution that works best under all circumstances. It really depends on what the final movie should look like and what you have in the precomps and how they are stacked in the Timeline.
HTH
RoRK
Intensive AE & Mocha Training in Asia.