mental ray Architectural and Design Visualization Shader Library

3.8 Sky- and Environment Portals 71declare shader "mia_ciesky" (boolean "on"default on,color "rgb_unit_conversion" default 0.0001 0.0001 0.0001,# Luminance parametersboolean "auto_luminance"default on,scalar "zenith_luminance" default 0.0,scalar "diffuse_horizontal_illuminance" default 0.0,# CIE specific parameters. 0 = clear sky, 1 = overcastscalar "overcast" default 0.0,# Sun directionvector "sun_direction",light "sun",)# Y vectorboolean "y_is_up",version 1apply environment, textureend declareThe shader shares many parameters with the mia physicalsky and they work the same, thisincludes parameters on, rgb unit conversion, sun direction and y is up. See page 66 fordetails.The following parameters are unique to the mia ciesky shader:• auto luminance. When on, the luminance of the sky can either be defined by settingthe zenith luminance parameter (a candela per square meter measurement) or bysetting a known diffuse horizontal illuminance value (in lux) - not both.• When overcast is zero, the CIE Clear Sky model is used. When overcast is 1.0, the CIEOvercast model is used. In-between values is a linear interpolation of the two models.3.8 Sky- and Environment Portals3.8.1 The ProblemA classic problem in computer graphics is lighting a scene solely through indirect light, likefrom a sky, or other “environment” light from an acquired HDRI or similar.This is accomplished in mental ray using Final Gathering (henceforth abbreviated as FG),and is done by tracing a large number of “FG rays” to see which hit the environment (or other