3 : Render out the scene, and you will have your scenery with shadows moving in a single layer for your final composition.
Characters, shadows and background in one layer
Background and shadows only layer
Save a different file for your characters, this time turning the scenery layers off. Render the character layers and you will have another layer for your final composition.
4 : Composite both layers in AfterEffects or any other similar software, capable of importing sequenced image files and reading the Alpha Channel of the Targa files.
If rendering the shadows and scenery together, is still taking too much time you can render your shadows in a separate layer. To render shadows separately follow the next steps.
Case 2: Shadows - only layer -
To render shadows only
1 : The object that is casting the shadows should have the primary visibility stat unchecked. Maybe also you could uncheck the visible in reflections and visible in refractions stats too.
2 : The object that receives the shadows needs to have the Receive shadows and Primary Visibility stats checked.
3 : Assign a Use Background material to the object that receives the shadows, this material allows you to disappear the object but still receive the shadows.
If you view the alpha channel of the object you will see the shadows.
Alpha channel of shadows layer
4 : Import into your composite project between the scenery layer and the characters layer.
When you want to hide something behind an object, for example, if your character is holding an object and you want to render both character and object separately, you should set 0 reflectivity, reflection limit, and shadow mask in the Use Background material attributes. This will create a hole in RGB and Alpha channels.
Experiment and find which way of rendering best suits your needs.
If your camera doesn’t move, you can always render just one frame of your scenery, a complete sequence of the moving shadows and a complete sequence of the moving characters.
About compositing :
Now that you have your final rendered layers it’s time to merge them together.
For this you will need any compositing capable software like Adobe After Effects or Adobe Premiere. This software can import image sequences and respect the alpha channels.
This step is really easy:
1 : Import your images as image sequence and premultiplied matted if asked
2 : Drag the sequences onto your time-line
3 : Place them in order
4 : Render again as a Targa sequence or movie file
Before rendering you will be able to correct the colour, brightness, contrast and other attributes of your layers, add effects or multiply a layer many times.
Combining your 3D and compositing softwares allows you to correct or create things that could lead to prolonged render times with your 3D software alone.
