mental ray Architectural and Design Visualization Shader Library

62 3 Sun and Sky3.2 UnitsThe sun and sky work in true photometric units, but the output can be converted to somethingelse with the rgb unit conversion parameter.If it is set to 1 1 1, both the values returned by the mental ray shader API functionsmi sample light (for the sunlight) and mi compute avg radiance (for the skylight), when sentthrough the mi luminance function, can be considered as photometric illuminance values inlux.Since the intensity of the sun outside the atmosphere is calibrated as a 5900 degrees Kelvinblackbody radiator providing an illuminance of 127500 lux, this is very bright when seencompared to a more “classical” rendering where light intensities generally range from 0 to 1.The rgb unit conversion parameter is applied as a multiplier and could be set to avalue below 1.0 (e.g. 0.001 0.001 0.001) to convert the raw lux value to something more“manageable”.For convenience, the special rgb unit conversion value of 0 0 0 is internally set so that80000 lux (approximately the amount of light on a sunny day) equals the classical light levelof 1.0.An interesting alternative is to set rgb unit conversion to 0.318 0.318 0.318. Then thefinal rendered pixels in the image (when rendered with mia material or shaders following thatshading convention and when sent through the mi luminance function) are true photometricluminance values in candela per square meter 1 .These true luminance values then fit perfectly as input to the photographic tone mapperdescribed on page 87, with its cm2 factor set at 1.0.1 The value 0.318 (1/pi) originates from the illuminance/luminance ratio of a theoretically perfectLambertian reflector.