There is a lot of naming convention confusion when it comes to global illumination. I’ll try to give you a short rundown of what the different things mean:
First off, there’s global illumination. This is a collective name for techniques for the interaction of diffuse surfaces. Basically, light bouncing around in the scene globally. Global illumination isn’t limited to any specific technique.
One of the many methods to achieve global illumination is photon mapping. Photon mapping is a fairly simple technique, which is why it’s fairly popular. It is a fairly fast technique, but it is nearly impossible to use it to achieve high quality global illumination with it. Getting a low-quality photon map will take seconds. Getting a medium-quality photon map will take minutes. Getting a high-quality photon map will take days. With photon mapping, the GI will essentially be made up of tens of thousands of highly inaccurate “spots” of light. By averaging these spots over a small area, you achieve noiseless results. The downside to this is that you lose details, and accuracy is still not very good.
Photon mapping goes like this:
1: Photon map is calculated in a pre-pass. The photon map is smoothed out, as well.
2: Rendering starts, and rays are fired from the camera into the scene.
3: When a ray hits a surface in the scene, the spot will be shaded by regular lights, as usual, and the data from the photon map.
The most primitive method of doing global illumination is the brute force method, sometimes called monte carlo or unbiased. This means that for every single pixel in the image, you fire a whole damned lot of rays randomly (or semi-randomly) into the scene. This is HIGHLY accurate, but slow as molasses. This is the method used primarily by renderers like Maxwell and FPrime. However, most renderers can use this method, though it’s generally not the default method. It goes like this:
1: Rays are fired from the camera into the scene. (Notice how it doesn’t need a pre-pass!)
2: A ray hits something in the scene.
3: From the spot in step 2, LOTS of rays are fired into the scene.
4: For every ray in step 3 that hits something, additional rays are fired for every light in the scene.
This means that rendering even a simple image needs a LOT of rays. A scene that renders in a couple of minutes without global illumination, and ten minutes with photon mapping, would take a few hours with brute force global illumination.
Because brute force rendering is so slow, a different technique was created: irradiance caching. Irradiance caching works by taking very accurate global illumination samples, but not at every pixel. On a flat surface, the global illumination is unlikely to change very rapidly, so it is unnecessary to calculate it at every pixel. With irradiance caching, you calculate it here and there, and then interpolate the results. The disadvantages of this technique is that it requires a pre-pass, just like photon mapping, and that it can lead to splotches and detail loss. With brute force global illumination, all you ever get is noise. Noise usually looks better than splotches.
This is where final gathering comes in. Because both brute force and irradiance caching GI can be fairly slow methods, and photon mapping has horribly poor quality, someone came up with a rather excellent idea:
Final gathering! It means that you first calculate a photon map, with all the splotches and detail loss and so on. Then you use a slower but more accurate method to calculate another bounce of global illumination. Essentially, it goes like this:
1: Photon map is calculated, as in the above example.
2: Rendering starts, and rays are fired from the camera.
3: A ray hits something in the scene, and regular shadow rays and such are fired, as usual. The photon map is ignored completely, because of the low quality.
4: Rays are fired into the scene from the spot in step 3. When these rays hit surfaces, the photon map is taken into account, but no shadow rays will be traced (the shadows are already calculated in the photon map).
The photon map is used to achieve a basic ambiance in the scene, and then you use the final gathering to smooth it out and bring back lost details. This is slower than just photon mapping, but MUCH higher quality. Also, since you do not have to fire shadow rays from the final gather rays, you save a LOT of time. Photon mapping + final gathering is a very popular technique. Final gathering works with both brute force rendering and irradiance caching.
Unfortunately, this is where the confusion starts. When mental ray users talk about final gathering, they’re actually talking about irradiance caching! In many renderers, irradiance caching is called final gathering, regardless of whether you actually use it for final gathering or not.
Okay, this didn’t turn out very short at all. I’m sure I’ve made a bunch of mistakes, as well. Feel free to correct me, if you find something inaccurate.
And as Vlado says, you’ll be wanting the “irradiance map”. Or possibly just the brute force method, if you want a slower but even better-looking render.