It's a rendering option that yields extremely realistic shading effects and is a lot less time consuming than radiosity.
With simple ray tracing ray's are cast from the camera according to the image size and sampling of the image, as it strikes an object it calculates the color according to the light in the scene.
With FG, first rays are cast from the light. Assuming we zero in on one ray that strikes an object from the light, when it collides with an object, a series of rays are diverted at random angles to calculate the light energy from the surrounding objects, this information is stored into what is called the photon map. The photon map is then consulted to add the effect of the bounced light in the ray-tracing process. This basically turns every object into a light source. Therefore each object in a scene influences the color of it's surroundings like in real life.
Here's a simple tutorial made with XSI 1.5.
Get a sphere and a cube, preferably poly but nurbs works just as well.
position and scale the cube so that it looks like a table for receiving shadows.
Remove the default light in the scene.
Create a new point light and adjust the color settings to 0. In Final Gathering it makes no difference where the light is positioned basically we only need it to fire off the photons necessary for the effect.
Duplicate the cube and translate it up. This object will act like the light source. Replace the default material with a constant material set to 1. All models in a scene with FG react like light sources, but since this one is really bright, it will be the light source.
Give the sphere a lambert shader make the sphere red 1,0,0 for diffuse and ambient. Do the same for the cube but this time with yellow 1,1,0 for diffuse and ambient. In this case we are giving a bright ambient to exagerate the effect. But it is not necessary with your regular scenes.
Draw a render region with the q-shortcut, right click on the render region's yellow crop line and select properties. Under the Photon tab, enable final gathering and pump up the Accuracy to 200. In XSI 2.0 there is an automatic compute button which gives great results with FG. The anti-aliasing was also increased.
You could see the red color from the sphere bleeding onto the cube's yellow to give orange hues. You can even add textures on the object used as a light source and view the effects.
You also have control on how bright an object will appear by modifying the material shader>indirect illumination parameter.
Final Gathering settings.
According to Mental Images, "Final Gathering, and other global illumination techniques, are at the edge of current technology, and subject to much research at mental images".
Pre-2.0:Though there's no official word on how to set up the FG settings, a general rule is that the Maximum Radius should be about 5%-10% of the scene's size (in SI units). The Minimum Radius should be about 10% of the Maximum Radius' value. On a particular small scene with just a chair I have 500 for accuracy, 0.5 for min and 5.0 set for max. For better results increase accuracy.
Mental Images completely rewrote the Final Gathering algorithms for
mental ray v3.0 included with XSI 2.0. Not only do the images render significantly faster, but always look much smoother with equal or lesser settings. There
is also an automatic compute feature that automatically sets the min/max radius
according to the scene.
Tutorial made by:
Manny Papamanos