In a 3D scene, filled with layers in the Z axis, I build a "city". Far away in the background, I have some smoke coming out of a cheminee. This was made using lights as emitters with particular.
I am having rendering issues.
If the particular layer is at the lowest level in the timeline, something wired is happening and all the 3D layers in the forground are loosing their "hierachical attribut" and are now overlapping each others.
If I put the particular layer at the top, the smoke is overlapping all the 3D layers and doesn't have any depth of field.
The "obscure layer" can't help since I have several layers in the forground.
And for some reasons I can't want precompose anything
Pre-compose. In all honesty, there is no way around it. What you are doing is not Particular's fault, it's a limitation in AE itself - any 2D layer will "break" the rendering order of 3D layers. Put your city in a pre-comp and use it as a matte for the smoke.