Shadows are time consuming to render, so it's a good idea to keep rendering in mind as you set up the shadows in your scene. In general, try to use the least memory-hungry type of shadow that you can to get the quality that you need.
For more information about shadows, see Shadows [Direct Illumination].
Generally, objects close to the camera need more detailed shadows than objects that are further away. Lighting and/or rendering near and far objects separately can often speed up rendering:
Selective Lights: Use selective lights to create shadows for groups of objects.
For example, you could use a selective area light to generate soft shadows for close objects, and use a selective point light in the same location to generate simpler shadows for distant objects.
Separate Passes: Use separate render passes for different types of shadows.
For example, you could create one pass with raytraced shadows for close objects and another pass with shadow-mapped shadows for distant objects.
Area lights create soft shadows with both an umbra (the full shadow where an object blocks all rays from the light) and a penumbra (the partial shadow where an object blocks some of the rays).
Use area lights in situations where the object receiving a shadow is far away from to the object casting the shadow. The area lights create better shadows than shadow maps, but will add to the rendering time.
For more information about area lights, see Creating Soft Shadows with Area Lights [Direct Illumination].
Shadow mapping, also known as "depth-mapped shadows," creates shadows more quickly, but less precisely than those created with raytracing. For more information about lights, see Creating Shadow-Mapped Shadows [Direct Illumination].
Use shadow maps in situations where the object receiving a shadow is close to the object casting the shadow.
If there is no transparency or colored shadow in the scene, use shadow maps instead of raytraced shadows.
Do not set the shadow map's Resolution value higher than the current screen resolution. It uses up memory and slows down rendering unnecessarily.
Shadow maps should not be used together with transparency maps. The resulting shadow will not be accurate.
To render a fast preview of a scene, the scanline renderer can use shadow maps to produce fast approximate renderings of your scene including shadows, without activating raytracing.
