Physical Sky


This shader implements a variation of the Hosek-Wilkie sky radiance model, including the direct solar radiance function. You can plug it into the environment, or to the color input of a Skydome light (which benefits from efficient importance sampling, sending more rays to bright areas around the sun) or add it directly as an environment shader. Currently, this shader is invisible to GI diffuse and specular rays. To use it as a light source you must attach it to a skydome_light with sufficient resolution to capture the small solar disk.

This is an experimental shader. Although it has been tested extensively, please be warned before using it in production.

Due to the inherent nature of the Physical Sky shader, there will be a horizon line, and depending on the camera position, the ground plane below the horizon will always appear black.


Physical sky attributes

A video tutorial that shows how to use the Physical Sky can be viewed here.

The physical sky uses a polar coordinate system. Elevation has an angle between 0 degrees to 180 degrees (sunrise to sunset). The sun is at the zenith at 90 degrees. The azimuth has an angle between 0 degrees to 360 degrees.

It is not recommended that you use the Physical Sky with the Ai Sky shader. You should connect the Ai Physical Sky to either the Ai SkyDome light or directly to the Environment.

 

Turbidity

Turbidity determines the overall aerosol content (dust, moisture, ice, fog) of the air. It is used to easily define sky appearance and affects the color of the sun and sky.

Turbidity values range from 1 to 10:

2 - Yields a very clear, Arctic-like sky.
3 - A clear sky in a temperate climate.
6 - A sky on a warm, moist day.
10 - A slightly hazy day.

Ground Albedo

The amount of light reflected from the planet surface back into the atmosphere. This is an RGB value between 0 and 1, where 0 0 0 would correspond to a black ground, and 1 1 1 would be a white ground. Note, that this is a subtle effect (noticeable in the top corners of the images below).

Elevation

The angle between the sun and the observable horizon. The range is between 0 and 180 (90 to 180 is a mirror reflection of 0 to 90).

Left - Sun Elevation: 0 to 48 degrees (azimuth 0 to 360 degrees). Right - Sun Elevation: 0 to 20 degrees.

Azimuth

The angle of the sun around the horizon. Measured from the north, increasing towards the east (0 to 360 degrees).

Intensity

Intensity is a scalar multiplier for the sky radiance. This value is similar to the Sky Tint. However, the Sky Tint uses RGB values whereas Intensity uses scalar values (easier to adjust).

Sky Tint

Allows you to set a color that 'tints' the sky color. The sky tint should be used for subtle coloration. For example making the sky a slightly deeper blue color.

Sun Tint

Allows you to set a color that 'tints' the color of the sun. This an RGB multiplier that should be used for subtle coloration. Any non-subtle coloration will give physically inaccurate results and will produce a strange inconsistency between the sun and the sky colors (unless both tints are exactly the same).

Sun Size

Sets the size of the visible sun disk. It is possible to change the size of the sun for 'artistic' reasons. However, 0.51 is the solid angle (degrees) of the sun, as seen from the earth. Increasing this value increases the area size of the sun and will, therefore, create softer shadows.

Enable Sun

The toggle switch that enables/disables the visibility of the sun.

Orientation XYZ

In certain cases, it may be useful to change the orientation of the Physical Sky. For example, your scene up direction is mapped to +Z instead of +Y.

These controls can be found in the 'Extra Attributes' of the Physical Sky shader.

Y-Up axis (default values)

Z-Up axis

Fireflies

Fireflies can appear when rendering scenes with glass surfaces and the Physical Sky shader. They are caused by the bright sun disc from the Physical Sky that is connected to the Background. A work around is to connect a different Physical Sky to the Background that has Enable Sun disabled.

Background is deprecated and will be removed in a future release. You should instead connect the Physical Sky to the Skydome light.

 

 

Further examples

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