View source for Terragen 4 Global Illumination

=== Prepass ===

'''GI cache detail:''' The GI cache records the lighting at many points across the image. These are the dots you see in the pre pass; each dot corresponds to one sample. Each sample stores light from many directions. GI cache detail affects the spacing of the samples in image space during the pre pass. Then, in the final pass the lighting at any part of the scene is a weighted average of nearby samples, or in other words a filtered interpolation. The lower the detail, the coarser the approximation. If the detail is too low, this usually shows itself as a blurriness or lack of clarity in the GI. Low GI cache detail can also cause some parts of the scene to be completely missed in the pre pass, and this could result in no GI (black) in those areas. This is more likely to be a problem with vegetation or objects that are small and sparse in the image. (This can happen even at high GI cache detail too, but is less likely.) GI cache detail affects the size of the GI cache in memory (and on disk if saving to a GI cache file) and it affects the amount of time taken to render the pre pass. It can also affect render time for the final pass, but much less so than "GI sample quality".

'''GI sample quality:''' At each sample point in the GI cache, many rays are cast outwards in a sphere to capture the lighting at that point. GI sample quality affects how many rays are cast outwards from each sample point. The actual number of rays is much greater than the setting (usually at least 6x), but in general a higher setting for GI sample quality results in more rays. If there's a lot of contrast with respect to the direction the GI is received from, higher sample quality may be needed to avoid what might be described as "blotchiness" or sometimes completely missing important sources of light. The default value of 2 may be enough for some landscape renders, but for high quality final renders you may wish to increase this and for architectural renders we recommend at least 6. GI sample quality also affects how much data is stored at each sample. Therefore both GI cache detail and GI sample quality affect the size of the GI in memory and the amount of time taken to render the pre pass. GI sample quality also affects render time for the final pass much more than GI cache detail does, as that primarily affects the pre pass time.

'''GI blur radius:''' Each GI sample point affects a spherical region of space in the final image, and where multiple sample spheres overlap they are averaged together. The radius of these spheres depends on various things that are decided by the renderer, but you can control a minimum radius that is enforced to ensure that enough samples overlap on every part of the image. GI blur radius affects this minimum radius in image space. Although it is in image space it is ''not'' measure in pixels. It is '''relative to the spacing of the GI prepass samples''' that you see in the prepass, but not including the extra samples created by "Supersample prepass". The default value is 8. Lower values may give the impression of greater detail in GI but can lead to increased blotchiness in images and flickering in animations. Increased GI blur radius can lead to smoother GI but apparent detail will likely be reduced. We believe that 8 is a good compromise for most renders.

''In Terragen 3 & 4'': Because the radius is relative to the spacing of the prepass samples, if you increase "GI cache detail" then the blur area in ''image space'' will be reduced. It works this way so that "GI cache detail" changes overall detail without affecting the quality of the interpolation.

''In Terragen 2'': Because the radius is relative to the spacing of the prepass samples, if you increase "Detail" (Render settings) or "GI relative detail" then the blur area in ''image space'' will be reduced. It works this way so that "Detail" and "GI relative detail" change overall detail without affecting the quality of the interpolation.

'''Supersample prepass:''' Supersample prepass takes more closely-packed samples in the prepass so that it is less likely to miss small or narrow objects that don't have any other objects nearby. If you can see that the prepass is missing objects and there are no other objects nearby (e.g. blades of grass in the foreground, or narrow tree trunks), you should probably enable this. Missed objects in the prepass can result in the shadows (areas where GI is important) being too dark. However, supersample prepass reduces the quality of each sample, which can reduce the overall quality of the GI, so it is turned off by default.