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PxrEnvDayLight

Noon

Dusk

Dusk under haze

An environment light source that implements a simple analytical physical model for terrestial daylight under clear skies. The sky model is based on A Practical Analytic Model for Daylight by A.J. Preetham, Peter Shirley, and Brian Smits. The solar disk model is adapted from H.C. Hottel's A Simple Model for Estimating the Transmittance of Direct Solar Radiation Through Clear Atmospheres, with parameters curve fitted for turbidity from physical data and a simple limb darkening model.

In typical use you will create an envsphere as the geometry and attach this as a light source shader to it. To make the sky directly visible to the camera, add the PxrLightEmission material as the Bxdf as well.

Parameters

Intensity / Exposure

Scales the contribution of this light. With an intensity of 1, the model computes physically plausible results in cd/m^2 units. This makes it about 100k too bright for typical RGB rendering values.

RIB: float intensity

Default: 1.0e-5

RenderMan for Maya implements this shader as rmanDayLight, with an Exposure parameter in lieu of Intensity. Exposure specifies the intensity of the environment light as a power of 2. Increasing the intensity by 1 will double the energy emitted by the light source. A value of 0 produces an intensity of 1, -1 produces .5.

Default: -16

Direction

The apparent direction towards the center of the sun. The zenith is at +Y (for noon light) and the horizon is in the XZ plane (for sunrise/set). Note that the Y component must non-negative.

RIB: vector direction

Default: 0.1 1.0 0.1

Haziness

The turbidity of the sky. The lower limit of the model is 1.7 for an exceptionally clear sky, and 10, for an inversion, is the upper limit.

RIB: float haziness

Default: 2.0

Sky Tint

Tweak the sky's contribution and color. The default, white, gives results based on measured physical values.

RIB: color skyTint

Default: 1.0 1.0 1.0

Sun Tint

Tweak the sun's contribution and color. The default, white, gives results based on measured physical values.

RIB: color sunTint

Default: 1.0 1.0 1.0

Sun Size

Scale the apparent size of the sun in the sky. Leave at 1 for a realistic sun size with an 0.55 degree angular diameter.

RIB: float sunSize

Default: 1.0

Presets

RenderMan for Maya includes useful presets for specific times of day.

images/lighting/dayLightPresetEarlyAM.png

The Magic Hour (Early Morning)

Late Morning	High Noon
Late Afternoon	Evening