Im trying to render a city scene animation with a road, buildings and 10 cars. The camera never moves. Would I just render the buildings, vegetation and sky once and then the road, cars and car shadows on its own layer? And whats the best way to capture the shadows on moving objects such as the cars? I read about a VrayProperties node and adding multiple objects into one VrayProperties node but is that done in the outliner or elsewhere or do i need to create a vray property node for every single object? It wasnt clear. Or would a materialwrapper be better?
To save time I would render out your still camera shot first. Then just render the area that has the animation and comp them together, use a mask in After Effects or Davinci with a soft edge to blend.
sphere fade would work for this, if you don’t want to mess with shadow passes and that stuff. just link a sphere fade to each car, adjust it so that you see as much of the ground as needed to get the shadowing, and place that render over your still frame background with no cars.
one down side to doing it this way, if for example you later change the texture on the road in your BG pass, you now need to render out the cars again with the sphere fade since the ground will be partially in those renders too. Using shadow passes, you could change the road texture and not need to re-render anything else unless the road is significantly different and the difference is visible in the reflections of the cars.
Sphere fade is an easy way however there is a limitation with that workflow. If you need to color correct the city+roads then you would have to also somehow color correct the roads which are revealed by sphere fade for the car layer, may not be ideal.
We deal with such shots all the time, here is our workflow:
- add whole city + roads everything to vray obj properties set - call it city or scene object properties.
- add your moving cars to a different vray obj properties set - call it car object properties.
For city scene layer just layer override all cars hidden (like a group visibility override). However if the cars are being reflected in the buildings you may need to render the whole city animation (that depends).
For car layer, set the city obj properties to: matte surface check, alpha contribution -1. This will ensure that any car that went behind a building will be occluded by that building.
For shadow layer (from cars to road) duplicate the car layer and check in city object properties affect alpha and shadows on. In this instance the city will act as a shadow catcher from cars, anything that may catch a shadow. I would also in the city properties disable casts shadow, so that other objects inside the city set it self would not cast shadows onto itself and result in doubling up of the shadows.
So in this case you will end up with 1 frame render of the city. Animation render of the cars (which reflect the city and are occluded by it), and a shadow layer.
Yeah I also forgot to mention that for the shadow layer you need to set the car obj properties to primary visibility off (so only shadow gets output)