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When it comes to producing high-quality global illumination renders, render artists love to have options available to them in terms of the types of algorithms used to compute bounce lighting in the scene. The choice of course means that we can pick a GI method that suits the needs of the current project or the current phase of the project that we are working on. For instance, in the early stages of a project, we may be asked to produce some proof-of-concept renders. These don't need to be particularly polished, but they do need to convey the mood and feel of lighting, and possibly materials, in the scene reasonably well, and of course they must be quick and easy to produce.
On the other hand, when it comes to final renders, quality, not time, is generally the more important consideration. For each of these situations, many render engines require that we have a fairly extensive understanding of the GI algorithms at work and the myriad of settings controlling them, so that we can push our render settings either towards speed or towards high quality. Well, Twilight relieves us of the pain of such tweaking, while still offering plenty of choice regarding the GI algorithms used to compute our scene lighting and materials.
Simply put, we have three basic choices that can actually split into six separate options. These are accessed by coming into Twilight's Render dialog. Inside of the Render tab, we have our Preset options. We make our choice of GI engine or GI algorithm when we choose the Twilight Render preset that we're going to work with. The first of the GI options available that we will focus on is the combination of photon mapping and final gather, or irradiance mapping, to give it its official title.
Twilight uses this combination in all of the first seven of the easy presets. These are biased rendering options, meaning that the algorithms use some nonphysical cheats to get good GI in the scene as quickly as possible. These very different global illumination technologies are combined because when used separately, you never really get a complete GI solution from them, as each system has its own limitations. Photons, for instance, are really good at bouncing light around.
That makes them very useful on interiors, and they're pretty fast to calculate. The problem is that getting a smooth photon solution is not an easy thing to do. The number of photons required would usually be exceptionally high, and as these are stored in a 3D data map, we can eat up our RAM pretty quickly indeed. Irradiance mapping, on the other hand, is very good at creating a smooth or evenly spread GI solution, especially so on an interior scene. But it doesn't bounce light around anywhere near as well as photons do.
So whilst these two technologies do have their limitations when working on their own, they complement each other very nicely. They work exceptionally well and pretty speedily together. In fact, let's just demonstrate how speedily by coming to our Easy > 02.Low Preset and then taking a test render. As you can see, in no time at all--just 13 seconds--we get a very useable global illumination solution. Of course we don't mean useable in the sense of final render quality, but in the context of a quick test render, we can easily evaluate the direct lighting, the global illumination, and the materials that are set up in the scene, although of course we only have our default gray material applied at this moment in time.
Of course we don't want to give you the impression that Photon Mapping and Final Gather are for low-quality renders only. Certainly, as we start to work with the Easy > 04 and upwards preset, we can get some very high-quality renders out of this combination. In fact, if we just jump into Adobe Photoshop, we'll show you a render of our scene in its current state, taken using the Easy > 07 Preset. As you can see from our render, Photon Mapping and Final Gather are both perfectly capable of producing a very nice, very clean global illumination solution for us.
Any bits of noise, any blotchiness that is still present in the system would very easily be hidden once we apply materials to our scene. The brilliant thing about this particular render, if we just come up and examine its title, is that it only took one minute to complete, which is very, very fast. Now, of course things will slow down considerably once we add complex materials into the scene, but still, you can see that Photon Mapping and Final Gather are capable of producing a high-quality GI solution in very respectable render times.
Let's jump back into SketchUp and have a look at the next GI option available to us. This is Path Tracing. Well, actually, we get Path Tracing in two forms. To work with progressive Path Tracing, we will choose the easy 08 Preset. But if we just scroll down in our list to the Tech options and into this Path Tracing set, you can see we have a number of fixed Path Tracing options available to us also. The term Progressive, as used in the easy 08 Preset, refers to the fact that once we set a render going using this particular option, it will progressively continue to refine the rendered solution until we tell it to stop.
The Path Tracing part of the description, if we were to just simplify things down quite a bit, refers to the fact that light paths, or rays, are continuously being traced throughout the environment using this rendering option. So long as a ray has enough energy and continues to encounter surfaces that will bounce it--be they Diffuse or Specular-- the ray will just keep on being calculated. This is a physically accurate approach to image calculation, because no shortcuts are taken in the algorithms. Path tracing is an unbiased rendering solution.
Once again, if we jump into Photoshop, we have a render prepared using this particular preset. So let's just swap our image over to that particular render. You can see an example of progressive path tracing. This particular render was sampled over 15 passes. Now again, if you take a look at the title of the image, you can see that this took just five minutes to complete, which for a progressive unbiased rendering option, is pretty good. Of course, what we have here is not a final rendered solution; we have quite a bit of noise in our render that would need quite a bit more time to clean up.
But again, we can make a very good evaluation of the scene--its lighting, its light bounce, its materials--based on what we have. And if we just compare this to our Photon Map and Final Gather render, you can see that the light bounce has a much more natural feel to it. The easy 08 Preset that was used to render this particular image uses what I will refer to as standard path tracing. This means that rays are shot from the rendering camera's point of view and traced through our 3D environment. But if we come back into SketchUp and again, accessing our presets, if we just come down to the Tech option, you can see that we have, in our Progressive set, the Bidirectional Path Tracing option available to us also.
This is pretty much the same as path tracing in every respect, except that the rays are traced not just from the camera's point of view; they are also traced from light sources found in the scene, hence the Bidirectional label given to this particular algorithm. This too is a progressive unbiased rendering method. The final distinct option we have for light calculation in Twilight is the Metropolis Light Transport engine, or MLT for short. This is used in the easy 09 Preset.
It is basically a variation of progressive path tracing that uses some specialized algorithm changes that allow it to make path modifications during its calculation. It can even create new optional paths in the scene if they're required. Generally speaking, it is regarded, on paper, as the fastest of the unbiased rendering algorithms available, although mileage definitely does vary between implementations. Again, if we jump into Photoshop, we can show you a render taken using the easy 09 Preset.
Now, as you can see, it is considerably cleaner than the progressive path tracing version. There is a lot less noise present in the image. As with the progressive path tracing version, this particular render was given 15 passes in which to complete its rendering calculations. Besides the difference in quality, you will notice--if again we look at the titles--that there is a considerable difference in the render times: 16 minutes for the Metropolis Light Transport version, whereas our Progressive Path Tracing option took just five minutes. So, quite a jump in render times, but of course as we've seen, quite a jump in render quality also.
As with Progressive Path Tracing, Metropolis Light Transport shoots its rays from the rendering camera and then traces them throughout the scene. We can, if we want add, bidirectionality into the mix. We would do that by choosing the easy 10 Preset. Again, here we would find that our rays are calculated both from the rendering camera's point of view and from any light sources in the scene. So if we are a render artist who likes to have choice but perhaps doesn't have the time, or maybe even the desire, to become an expert parameter tweaker, well, Twilight has a GI option for every occasion.
Familiarity with the presets available of course will go a long way towards making the choice of which one to use in any given rendering scenario that much easier.
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