Define the basic GI method for your scene (see below for a description of each method):
•IR (Irradiance Cache) (Still Image): Default method. Typically, you can stick to using IR for still images.
•QMC
•IR/QMC (Still Image)
The advantages of this method are moderate render times combined with a very homogeneous and even dispersion of light.
The Irradiance Cache (IR) method attempts to determine which areas of a scene require more samples and which areas require fewer. The scene setup is analyzed before rendering is begun. During this “prepass”, the scene composition is scanned several times and test calculations are made to determine the optimal number of samples for each region of the image. This allows the number of overall calculations to be optimized and the render times for GI to be reduced, in part dramatically, as compared with the QMC method.
•Adjusting the Irradiance Cache calculation: Generally speaking, the predefined values in these additional settings don’t not have to be modified.
•Disadvantages of using Irradiance Cache: Finer details pertaining to light and shadow might be omitted, due to the limited number of shading points. This is where the QMC method has an edge. However, the IR method offers a Details Enhancement option that combines QMC and IR methods in order to compensate for this disadvantage.
See Details Enhancement.
The QMC method of Global Illumination calculation works under a so-called “brute force” principle. This means that, for each (!) object pixel in the image, a definable number of rays (“Sample Count”) can be “beamed” in a hemispherical pattern into the scene. This is not an adaptive process and the render time will definitely not be reduced.
This bears the advantage that it offers the most precise render results. Small details in shadows and shading can be rendered out that would otherwise not be noticeable if the IR method were used.
The abbreviation QMC stands for Quasi Monte Carlo. This sample mode randomly disperses calculation points throughout the scene. No preferential treatment will be given to, for example, luminous or highly detailed objects. In theory, this sample method offers the most exact results because every point in the image is calculated using the same effort. One disadvantage of using this method is that it increases render times dramatically, which rises linearly as the image resolution is increased. Furthermore, QMC renderings tend to produce unwanted noise, which can only be remedied using very high sample rates. On the other hand this sampling method produces a very realistic dispersion of light.
The QMC sampling method supports all Global Illumination properties, i.e. the application of specific Diffuse Depth, as well as the illumination of the scene using both light sources and materials.
Disadvantages of using QMC:
The render times are maxed out (much longer than with Irradiance Cache) and, since brightness and color values based on a finite and randomly distributed number of rays (“Sample Count”) are ascertained for each pixel, the images will have a slight graininess to them that can only be compensated for by increasing the sample count, which increases render time accordingly. Using Area Lights/GI Portals (indoor scenes, primarily illuminated by light falling through windows) can noticeably improve render quality (which also saves render time).