I think focusing on proposals is a the key aspect that people pay attention to. I think if you have good foot prints like you do, the other stuff can fill itself in.
So this is an interesting question. The building shell rule is great if you are ok with a random texture as opposed to the one that the model might have came with. I have used the building shell rule to make zoning breakdowns of the building, but also to have higher resolution textures than some planimetric models come with because they look better at street level. I will say that once you apply a rule like building shell to a 3D models the number of options you have increase.
Generally there are 3 options that I know for most applying rules to most building/land use models:
1. Lot rules that make a building from a simple property lot, includes setbacks.
2. Footprint rules make buildings from the building outline and extrude it up, texture it, and manipulate it for certain purposes.
3. Shell Rules that essentially allow you change how the imported building looks like by splitting it into floors, texturing it, and even color coding it by land use function.
What is interesting about this hierarchy of rules, is that the closer you get to a shell rule, the less flexibility the rule has about changing the geometry of the original initial shape. This is largely because the ability to both orient yourself spatially in CGA (the programming language for CityEngine) and make decisions about what information you can get from the shape becomes increasingly complex with more edge cases. So a lot rule has a lot of flexibility to fill in setbacks, orient to the street, and do a lot of interesting things from a basic starting geometry. Foot print rules have a lot of flexibility vertically, but because there can be so many different edges on the geometry it can sometimes be a pain to reliably orient the foot print to other assets (it is possible though, there are many geometry functions that can help). And finally shell rules rarely if ever change the original geometry of the building because a planimetric building is already pretty close to reality.
So to answer the question, what you are doing with CityEngine and which model choices are better depends on what you are trying to do. Maybe you want the flexibility of lots to make custom proposals, or you have work flow that achieves a similar goal with footprints (see Bergmann Associates for examples in Rochester NY). And on the flip side maybe you want the realism that comes with planimetric models (and have money to buy them), and you just want something that you can put into a City Model. It depends on the end goal. 😃
David Wasserman, AICP
