Because of the way that final gathering calculates illumination, it allows you to light a scene without actually using any active lights. Instead, you can use bright, constant-shaded objects to "bounce" light into the scene (much like the way that photographers use reflectors). To create more complex lighting, you can color or texture these reflector objects. This technique is often used with a bright-blue "reflector" to simulate indirect illumination from the sky.
Alternatively, you can use environment maps to surround the scene with a texture that final gathering uses to calculate illumination. This technique can create extremely realistic lighting, especially if the environment map uses a High Dynamic Range (HDR) image as its source. For more information about using HDR images with environment maps, see Using Environment Maps to Light a Scene [Image-Based Lighting].
In either case, only a single light with little or no color or intensity is needed to enable the calculation of secondary (reflection, refraction, and transparency) rays.
Typically, setting up final-gathering-only lighting involves the following steps:
Bring down the intensity and/or color of the lights in your scene to 0 (or close to it). This allows you to see the final gathering effect by itself as you set it up.
For best results, bring the scene ambience down to 0 (or close to it) as well. For more information about setting lights and scene ambience, see the Ambient Lighting Property Editor.
Set up your final gathering "reflectors."
These are the objects that light your scene instead of lights. You can position small reflector objects as you would position lights or use larger reflective objects like hemispheres or large grids.
To avoid noise and other artifacts, especially while rendering sequences, don't add too much color to the material of the objects you want to use as lights and increase their size instead.
You may try to use several different colored cards (3D objects with a bright constant material) for light sources instead of just one. This encourages the light bouncing effect and gives more complexity to the scene.
Apply an environment map to the render pass that you are using for final gathering, as described in Using Environment Map Shaders [Texturing].
For the most realistic results, use the Environment shader with an HDR image, as described in Using Environment Maps to Light a Scene [Image-Based Lighting].
Enable final gathering and adjust the settings in the Render Manager (or render region options), as described in the Final Gathering Workflow Overview. This allows you to control the quality of the final gathering effect.
If necessary, you can add small amounts of direct lighting and ambience to enhance raytraced effects like shadows and reflection. This works best if you position your lights in the same place as your reflector objects.
