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If irradiance mapping comes top of the list as the most widely used of V-Ray's GI engines in the primary bounce slot, it is probably safe to say that Light cache can make the same claim when it comes to being used as a secondary bounce engine. Now, just so you are certain where the Light cache controls can be found, let's go and open up our Render Settings window. Let's go into the Indirect Illumination tab, and with Light cache set as our Secondary bounce engine, let's go and open up the Light cache rollout, and you can see, we have all of these options, all of these controls that we can work with to create our GI solution from a secondary bounce engine perspective.
Now light cache, like irradiance mapping, is another view-dependant GI technique. This, of course, means that the same limitations apply; the solution is calculated only from the rendering camera's point of view, and so, if we move the camera, we will create holes in the solution that can only be filled by recalculating the light cache. If, again, we need animation in our scene, if we are working with animated cameras, or indeed objects, then there are a couple of options we can use. Again, we have this Use camera path option for cameras that are moving around the scene.
And again, if you come down to the Mode settings, and use the dropdown, you can see we have this Fly-through option. Again, this can help with any animation that is taking place in the scene. To calculate the lighting found in the environment, V-Ray's Light cache engine will trace large amounts of rays from the camera's point of view, and send them out into the scene. These rays are responsible for creating your light cache samples, the number of which will be controlled by your Light cache subdivs primitive. With every hit, or intersection of a ray and a surface, a sample is created.
Unlike the initial rays used in irradiance mapping, light cache rays will bounce, or reflect a number of times, even if they are set in the primary bounce engine slot. Illumination information from each of the subsequent bounces along the ray's path gets recorded, and then stored in a 3D point cloud structure, just, in fact, as a irradiance mapping, and photo maps do. If a bouncing ray hits an already created sample, then any further tracing of that particular ray gets canceled, and the information from the existing sample will be used instead.
This, as you can imagine, really speeds up the process hugely, as no unnecessary tracing of rays takes place in the system. This in turn allows light cache to calculate a lot of light bounces very quickly indeed, making it, of course, an excellent choice for lighting difficult spaces, such as interiors, which are typically much harder than exteriors to light with a global illumination solution. One interesting aspect of light cache is that there is no difference between a light cache computed as either a primary or a secondary bounce engine.
If we've saved a Light cache file to disk when it was computed with Light cache set as the primary bounce engine, we can safely load and use it with Light cache set as a secondary bounce engine. The lighting solution will always be identical. Of course, light cache does have some restrictions, or limitations. Like photo maps, light cache is not adaptive. The illumination is computed at a fixed resolution, which will be determined or set by the user prior to calculating the lighting solution.
Light cache also doesn't work very well with bump maps, so we may need to take special note of how any bump maps we have in our scenes are reacting to the lighting solution we're creating. In fact, light cache is not as good as any of the other GI engines for picking out any kind of fine details that may exist in our scene. So with a basic understanding, now, of how the Light cache system is working, let's again move on to trying it out in our test scene, and see how we can create a GI solution using this particular engine.
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