Radiosity Tag

The radiosity tag extends the camera object with the possibility to render your scene with indirect illumination (radiosity). Cheetah3D also offers the faster ambient occlusion algorithm for fast IBL (Image Based Lighting) renderings in combination with the HDRI tag.

The classical raytracing algorithm just calculates the direct illumination which comes from a light source. But in reality a big contribution of our environment lighting comes from light scattered by other objects. For example the sun light coming through a window just illuminates a small area of the floor/wall. The rest of the room and the ceiling will be illuminated indirectly.

Rendering a scene with full indirect illumination is now as easy as adding a radiosity tag to your camera object. You don't have to change anything else. You'll probably want to fine tune the properties of the radiosity tag because these properties are highly dependent on the scene geometry and the light setup.

Attention: You can only add a radiosity tag to a camera object.

Properties

  • Type: There are two different types for calculating indirect diffuse illumination.
    • Ambient occlusion: This isn't actually a way to calculate indirect diffuse illumination. It just fakes it's behavior. It doesn't calculate how much light is scattered from other objects to the current pixel, it just calculates what percentage of the pixel is occluded by the scene. The main advantage of AO is that it is much faster than the exact solution. It is also very well suited for creating fast and nice looking outdoor HDRI renderings. See the images in the Samples property section.
    • Radiosity: If you chose Radiosity, Cheetah3D calculates the indirect diffuse illumination with a Quasi Monte Carlo Path tracing algorithm. This algorithm also offers effects which can be seen in the real world like color bleeding (see the ceiling of the box scenes below).
  • Intensity: Defines the intensity of the indirect illumination. The actual indirect illumination will be multiplied by this factor. It can therefore be used to fine adjust the intensity of the indirect illumination.
  • Samples: The number of samples which will be used to evaluate the indirect illumination. If you use too few samples the image will become quite cloudy and patchy. See the left images below for a demonstration. How many samples are actually needed depends strongly on the scene and lighting complexity. For simple scenes, 200 samples can be more than enough while for complex scenes up to 10000 samples could be necessary.
samples=25
samples=400
  • Diffuse scatterings: Defines how many indirect diffuse scatterings will be considered. More scattering orders will create more physically correct images but the rendering time will also increase dramatically. There are also more samples necessary if you use higher scattering orders because the noise increases.
    This parameter doesn't have any effect if you use ambient occlusion.
Without radiosity
diffuse scatterings=1
diffuse scatterings=2
diffuse scatterings=3
  • Specular scatterings: If you set specular scatterings to zero only the diffuse lightning components of the surrounding environment will be gathered. That is ok if there are no reflective or transparent objects in the scene. But if you render a scene with some reflecting or transparent objects like the metal ring below you lose some serious amount of lightning. To also gather this light increase the specular scatterings value. For reflective objects 1 is enough. For transparent objects values >1 might be necessary. But be careful. The specular scatterings have a serious impact on the rendering time.
    This parameter doesn't have any effect if you use ambient occlusion.
    specular scatterings=0
    specular scatterings=1
  • error: Defines how exact the indirect illumination will be interpolated. The lower this number the more exact the final image will become. If you set this number lower than 0.05 the caching algorithm will be disabled, which will cause a dramatic increase in rendering time. It is therefore not recommended to set this value lower than 0.05. There are only some exceptions where disabling the cache is really desired.
  • rmin: The minimum distance between caching samples.
  • rmax: The maximum distance between caching samples.

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