First Steps

What should you do if you want to light a scene using Global Illumination? Which workflow leads me quickly to the best results?

1. Activate Global Illumination in the Render Settings ("Effects … " button).
   
2. Set Preset to an appropriate setting. Any subsequent test renderings must be done at a reasonable rate of speed. Otherwise the test render phase could turn into an arduous process. Follow these steps to make sure this does not happen:
   
   
   • Render at lower resolutions (e.g., 320 x 240 is often sufficient to check the brightness of a scene or part of a scene) or:
     
   • Use the Interactive Render Region (IRR) at a lower resolution.
     
   • First, concentrate on the brightness, light dispersion and similar elements.
     Determine if you need additional light sources. The Gamma value (General tab) can be increased, if necessary (keep in mind also the Color Mapping parameter here and here).
     
   • Once you are satisfied with the lighting of your scene you can turn your attention to the GI quality. First, check to see if you can implement Portals or Polygon/Area Lights. This can improve the render quality and shorten the render time enormously.
     
   • Now begin increasing the GI quality by defining a higher quality Preset - or by adjusting individual settings.
     

Error Correction

GI shadows not rendered precisely enough?

For Portals and Polygon Lights activate the Force Per-Pixel option.

Spotty renderings?

In most cases, raising the Samples value (General tab) will help.

Rendering takes too long?

Are the Samples, Records Density or values too high? Is one or more of the following activated (these options improve image quality but slow rendering): Distance map for Polygon Lights/Portals activated? Refractive Caustics activated (Options tab)?

Have you activated Primary Method and Secondary Method Quasi-Monte Carlo (QMC) modes? 2 times QMC produces the best quality but is by far the slowest GI mode.

Are the values for Samples or Record Density too high? Is Force Per-Pixel enabled for polygon lights/portals? Check your Refractive Caustics settings.

Maybe you’ve enabled the saving and loading of overly large radiosity maps (Cache tab).

Unfortunately there are innumerable more reasons for slow rendering.