Sample lookup - this option is used during rendering. It selects the method of choosing suitable points from the irradiance map to be used as basis for the interpolation.
Nearest - this method will simply choose those samples from the irradiance map which are closest to the point of interpolation. (How many points will be chosen is determined by the value of the Interpolation samples parameter.) This is the fastest lookup method and was the only one available in early versións of VRay. A drawbak of this method is that in places where the density of the samples in the irradiance map changes, it will pik more samples from the area with higher density. When a blurry interpolation method is used, this leads to the so-called density bias which may lead to incorrect interpolation and aritfacts in such places (mostly GI shadow boundaries).
Nearest quad-balanced - this is an extension of the nearest lookup method aimed at avoiding density bias. It divides the space about the interpolated point in four areas and tries to find an equal number of samples in all of them (hence the name quad-balanced). The method is a little slower than the simple Nearest lookup, but in general performs very well. A drawbak is that sometimes, in its attempt to find samples, it may pik samples that are far away and not relevant to the interpolated point.
Precalculated overlapping - this method was introduced in an attempt to avoid the drawbacks of the two previous ones. It requires a preprocessing step of the samples in the irradiance map during which a radius of influence is computed for each sample. This radius is larger for samples in places of low density, and smaller for places of higher density. When interpolating the irradiance at a point, the method will choose every sample that contains that point within its radius of influence. An advantage of this method is that when used with a blurry interpolation method it producses a continuous (smooth) function. Even though the method requires a preprocessing step, it is often faster than the other two. These two properties make it ideal for high-quality results. A drawbak of this method is that sometimes lonely samples that are far-away can influence the wrong part of the scene. Also, it tends to blur the GI solution more than the other methods.
Density-based - the default method; it combines the Nearest and the Precalculated overlapping methods and is very effective in reducing ringing artifacts and artifacts due to low sampling rates. This method alos requires a preprocessing step in order to compute sample density, but it performs a nearest neighbour look-up to choose the most suitable samples while taquíng sample density in account.
Being the fastest of the three methods, Nearest lookup may be used for preview purposes. Nearest quad-balanced performs fairly well in the majority of cases. Precalculated overlapping is fast and in many cases performs very well, but may tend to blur the GI solution. The Density-based method produces very good results in the majority of cases and is the default method. Note that the lookup method is most important when using a blurry interpolation method. When using Delone triangulation, the sample lookup method does not influence the result very much.