It's been a rather headachey day so far. A little bit haphazard probably the result of not ever working with a scene this complex before and in the past and some last minute troubles. I've always rendered my scenes in one go, this one requires that I do multiple renders, some now, some next week, and this has been the cause of anxiety. However, I'd rather sort and come across these issues out rather than when it comes to render. So apologies for clogging up inboxes with my woes!
Thankfully I think all my issues are now resolved, and I can continue with populating the rest of the scene! The trees still need to be rendered on a seperate pass, however when doing this directly the trees were not casting shadows and it looked a tad strange. Thankfully I've overcome this by turning off the primary visibility of the trees but still retaining the shadow, like so.
Basically there are going to be 4 passes. One for the backdrop, one for the main scene with all the shadows etc and one for the trees, birds and crowd animation. Also, if there's time I'd like to run an ambient occlusion pass as well.