It is the easiest method to set. Few parameters determine the quality of the Golbal Illumination. Instead high rendering times are to be expected.
Advantages
- It preserves all the details, such as small shadows of indirect light or complicated geometries. Thanks to the QMC very detailed and accurate images can be obtained.
- It is possible to solve flickering problems using the Direct computation (in other words the presence of “vibrating” pixels which can ruin the playback of the video)
- It is uses little RAM
- When using Motion blur, indirect light is correctly computed.
Disadvantages
- It is a very slow method, most of all for complex scenes, such as indoor ones.
- The real problem for this method is the presence noise, which involves the whole render, most of all in the shadowy areas. The only way to set it up is by increasing the values in the settings, which in turn increases computation times.
Irradiance Map (IM)
It is a method based on the Irradiance Caching. In this case the GI is computed in strategic points of the scene only. Usually these are in areas requiring more attention, like mesh areas. For example between very near polygons or very detailed geometrical shapes. It is up to the user to decide how many points must to be considered, where tp locate them, and how their interpolation has to be computed. The IM requires many more setup parameters to be adjusted compared to the QMC, and their correct knowledge, practice and experience can lead to high-quality results in a short time too.
Adcantages
- It is very quick to compute compared to the QMC GI, especially on scenes containing models with even surfaces.
- Noise is practically absent. This method’s main problem is the appearance of “stains” in certain areas, due to low settings.
- The IM can be saved in an opposite cache and reused when necessary, for instance in Fly-through animations. In this case it is to be remarked that the position of every object must be kept the same, for the color and lights intensity as well. The only objects which can be moved are, obviously, cameras. This idea will be explained up a head.
- One can use the IM for accelerating the computation of the VRayShadows produced by VRay lights. Usually VRayShadows by default use direct method of shadow computation and not approximated ones, for a better quality. This computation can become very long when many VRayLights are present. In this case the IM can be useful, enacbling one to speed up shadow computation.
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Disadvantages
- some details, such as small shadows generated by indirect light may not be rendered correctly, due to the interpolation of the GI
- during animations, when using low setting values, some flickering effects on the animation can occur
- it requires much more memory than the QMC GI
- indirect illumination is not correctly computes through objects to where Motion blur is applied. But often this defect is not evident enough to ruin the overall animation quality.
Photon map (PM)
This method computes the GI by emitting, from light sources, rays which then proceed to bounce within the scene. It is very useful for interior and/or semi-interior scenes, with many lights or little opening. Usually the PM does not produce stasfying results if used on its own. Instead, it is very useful if employed together with the QMC GI or the IM. In fact, by combining the speed and the original approximation of the PM with the previous methods, one is able to obtain excellent quality images in relatively short times.
Advantages
- the PM produces a very quick computation of the GI even if approximate and not so detailed.
- The PM can be saved and reused for speeding up the rendering of new views of the same scene and for Fly-through animations.
- The PM does not depend on the view, and, once computed, it is always available, whichever the zone of the scene to be rendered.
Disadvantages
- It is not worth using the PM as engine for primary bounces because of its lack of precision, most of all for very detailed scenes.
- It requires a lot of memory
- The PM needs real light sources for producing the GI. Self-lighting objects and environmental light (skylight) are not able to produce photons, therefore neither the GI.
Light Cache (LC)
Light Cache is a method for the approximate computation of the GI. It is basically very similar to Photon Map, but has managed to keep only its merits, solving the faults. The LC is created by emitting the rays from the observer’s frame towards the scene. The LC is an universal method for GI computation: it can be used both for indoor and outdoor scenes, and both for primary and secondary bounces, together with the IM and the QMC GI.
Advantages
- LC is very simple to set. Only rays deriving from the camera are used, contrarily to what happens in the Photon Map, wwhich must compute each single light source as well. In fact with the PM, it is necessary to set the number of photons for each source. This operation is pointless in the LC.
- LC works with any onject able o emit light, including self-lighting objects, non-photometric lights and skylight.
- LC produces correct result along corners in a room and near little objects. Instead, PM is not extremely efficient in corners, creating particularly dark areas.
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