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As we've already noted, in the real world, light doesn't just bounce once. After being emitted from a source, it will, for all intensive purposes, just keep on bouncing. Not that we are saying it stays infinitely bright, mind you. After an unspecified number of bounces, or after a certain distance traveled, the level of light energy will become so low, that is really is no longer contributing illumination to the environment that can be perceived by the human eye. The energy is still active, of course; the falloff or decay rate of light is such that, mathematically, it can be said to actually never reach absolute zero.
Now, I'm sure you can imagine that performing such an infinitely reducing calculation, in a manner completely true to the laws of physics, tracing every potential light path and bounce, well, that would be extremely expensive; extremely slow in terms of computation required. Thankfully, V-Ray has been designed in such a way so as to give us a very high level of flexibility and control regarding the way we calculate our global illumination solutions. In part, this is why we see V-Ray dividing its indirect illumination calculations into two parts; primary, and secondary bounces.
If we just come into our Render Settings window, we want to come across to our Indirect Illumination tab, and if we come down, with the GI Systems enabled, you can see we have Primary, and Secondary Bounce options, and we get to choose the type of GI engine that we use in that particular slot. Now, the Primary bounce engine controls only the first bounce of light. This initial bounce occurs only where surfaces are visible to the rendering camera. That can be either directly in its field of view, as we see inside our viewport here, or it can be through the material properties of reflections and/or refractions.
Using Primary bounce only, we can get some basic bounce light, or GI in the scene, instantly, of course, giving us a more realistic lighting solution. At this point, we are not really allowing the light to continue to bounce, nor are we calculating even an initial light bounce for areas of the scene not visible to the rendering camera. So for instance, in our scene here, you can see we have a back area, a backroom as it were, that travels behind this piece of geometry; behind the wall. Now, this area would not be included in this initial light bounce calculation.
Naturally, this would leave us quite a long way short of producing an accurate, or photo real lighting solution. For that, we would need to copy the real world behavior of light, as we've outlined it, and enable our Secondary bounce engine. This engine controls, naturally enough, every GI ray not counted as a primary bounce. Calculating the secondary bounces, of course, becomes more costly in terms of render time, as these are the ones that will be performing continuous intersurface reflections, or bounces.
Naturally, V-Ray allows us a measure of control over how much bouncing we allow. Just bear in mind that the more accurately we mimic real world light behavior by allowing more bounces, the more accurate, and probably believable, our lighting solutions will become. So we're armed, now, with an understanding of at least the basic workings of GI. We also understand that V-Ray splits its GI lighting calculations into two parts; primary, and secondary bounces. Time, then, to move on to looking at V-Ray's GI engines themselves.
Our first stop will be irradiance mapping.
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