The General Parameters rollout for both photometric and standard lights lets you turn shadow-casting on or off for the light, and choose which type of shadow the light uses.
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Create panel > 
(Lights) > Create a light. > General Parameters rollout > Shadows group > Shadow type
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Select a light >
Modify panel > General Parameters rollout > Shadows group > Shadow type
Shadow Types and the Active Renderer
The renderer you use also affects your choice of a shadow type. The scanline renderer does not generate mental ray Shadow Map shadows, while the mental ray renderer does not support Advanced Ray-Traced or Area shadows (but it can generate area shadows: See the "Notes," below).
The Quicksilver hardware renderer always casts shadow-mapped shadows. These shadows are adaptive, and adjust to the scale of the scene: They never show the jagged edges that can appear on scanline-rendered shadow-mapped shadows if you choose an inappropriate shadow map size. The Quicksilver renderer supports most of the shadow controls associated with light objects. The exception is that it does not support patterned shadows (Shadow Parameters 
Map). Area lights can cast soft-edged shadows.
The iray renderer always casts ray-traced shadows. It ignores the shadow controls for light objects: With the iray renderer, all geometry is shadow-casting and all shadows are physically accurate.
| Shadow type | Scanline renderer | mental ray renderer |
|---|---|---|
| Advanced Ray-Traced | Yes | No |
| mental ray Shadow Map | No | Yes |
| Area | Yes | No |
| Shadow Map | Yes | Yes |
| Ray-Traced | Yes | Yes |
The following table describes the advantages and disadvantages of each shadow type:
| Shadow Type | Advantages | Disadvantages |
|---|---|---|
| Advanced Ray-Traced | Supports transparency and opacity mapping. Uses less RAM than standard ray-traced shadows. Recommended for complex scenes with many lights or faces. |
Slower than shadow maps. Does not support soft shadows. Processes at every frame. |
| Area Shadows | Supports transparency and opacity mapping. Uses very little RAM. Recommended for complex scenes with many lights or faces. Supports different formats for area shadows. |
Slower than shadow maps. Processes at every frame. |
| mental ray Shadow Maps | Can be quicker than ray-traced shadows with the mental ray renderer. | Not as accurate as ray-traced shadows. |
| Ray-traced Shadows | Supports transparency and opacity mapping. Processes only once if there are no animated objects. |
Can be slower than shadow maps. Does not support soft shadows. |
| Shadow Maps | Produces soft shadows. Processes only once if there are no animated objects. Fastest shadow type. |
Uses a lot of RAM. Does not support objects with transparency or opacity maps. |
Notes:
- When the scanline renderer encounters a light set to “mental ray Shadow Map” shadows, the scanline renderer does not generate shadows for that light.
- When the mental ray renderer encounters a light set to Advanced Ray-Traced or Area shadows, it generates ray-traced shadows instead. (And it displays a warning to this effect.)
- While the mental ray renderer does not use the Area shadow type, it can generate area shadows: use a photometric light set to a light shape other than Point. See Shape/Area Shadows Rollout.
- When you use photometric lights with shadow maps, a hemispherical shadow map is created for the entire light sphere. To capture sufficient detail with complex scenes, the map resolution must be very large. For best results with photometric lights, use ray-traced shadows instead of shadow maps.
- Results from light tracing, including shadow casting, are based on 3ds Max geometry. A staircase effect can appear in the penumbra area of shadows, even when the transitions in the shadow-casting surface appear to be gradual. To avoid this effect, you can increase the density of the shadow-casting mesh.