Sky Light

To lighten outdoor scenes you often just want to illuminate the scene by the sun and the sky. You might now think what is the difference between the sun and the sky. Doesn't all the light come from the sun? And yes, you are right. But in the context of the sky light object the sun light is considered as two parts.

  1. The light coming straight from the sun without scattering. This part of the light is approximated by a distant light.
  2. The sky light is light which has been scattered by the atmosphere. It can be considered as a huge hemispherical area light.

When illuminating a scene with a sky light the light source will always use it's first sample for the direct sun light. All other samples will be used to approximate the sky illumination.

To render a scene illuminated by a skylight you can either use a plain sky light object and set it's samples property to maybe 32. This will create a nice smooth rendering.

But especially if you want to render a scene with radiosity you can also use the image based lightning techniques to gather the illumination of the sky. Therefore set the shadow samples to 1 to get just the direct sun illumination. Afterwards turn on the geometry property. If you now render the scene with the radiosity tag. The sky illumination will be gathered by the radiosity algorithm. This is generally considerably faster than using just the plain sky light object.

Scene just illuminated by the direct sun light
shadow samples = 1
render time = 2 sec.
Scene illuminated by sun+sky
shadow samples = 32
render time = 42 sec.
Scene rendered with ambient occlusion
shadow samples = 1
render time = 5 sec.

As you can see on the images above. The third method offers the best quality in the shortest time.

Attention: Using more than one sample and the radiosity tag will cause that the sky illumination will be gathered twice which makes the scene much brighter than wanted.

Properties

  • intensity: The intensity of the sky light. The intensity normally doesn't have to be changed.
  • turbidity: Sets the turbidity of the atmosphere. This value should be between 2-10.
  • radius: This property is only needed when rendering scenes with fog. It determines the "distance to the sky".
  • geometry: Turn this property on if you want to use the skylight as a background.
  • latitude: Since the position of the sun depends on your position on the earth you have to tell Cheetah3D where the scene should be located on the earth. Sets the latitudal position of the scene [-180°, 180°]
  • longitude: Sets the longitudal position of the scene [-180°, 180°]
  • date: Time and date. This determines the current position of the sun at the sky. The date is set in the following format "hh:mm MM/YY"
  • shadow type: Use this property to choose the shadow calculation type. See the description of the various modes in the Light object.
  • shadow color: The color of the shadow.
  • shadow samples: Number of samples at which the sky light will be evaluated. The more samples you use the smoother the shadows will become. The rendering time will increase substantially with more samples. For a pure skylight 16-32 light samples should be fine.

© 2001-2010 Martin Wengenmayer. All rights reserved.