# Light Attribute Define

## Defines
#define | XSIMR_LIGHTDATA_LABEL 2860411

## Detailed Description

In Softimage, you can set lights to affect different portions of the lighting calculation. Normally, a lighting model, such as Phong, computes a diffuse component and a specular component as part of the lighting calculation. However, users can specify that a light should not contribute to the diffuse component. All four combinations (diffuse light, specular light, both, neither) are acceptable.

To pass this information to the shaders, Softimage associates a piece of user data with the light. The shader can then extract this user data from the light, and perform the appropriate calculations.

To find the user data, the shader must ask for the head of the list of user data associated with the light. The shader then searches through this list for the user data with the magic number `XSIMR_LIGHTDATA_LABEL`. If it finds it, it extracts the settings using the xsimrLightData struct. If it does not find the data, it implies that both specular and diffuse computations should be performed.

See also:
  xsimrLightFlags, xsimrLightData

Since:
  7.0

## Define Documentation

#define XSIMR_LIGHTDATA_LABEL 2860411

The magic number for querying light userdata from a mental ray shader.