The use of precomputed subsamples in Volume Viewer decreases memory usage and time spent reading files.
The Step values (cell sizes) used to generate a subsample are inferred by comparing the numbers of data points in the full data set and in the subsample. For each dimension, valid cell sizes S for full data size N and subsample size M satisfy (M-1)S < N < (M+1)S. If there are no valid cell sizes, the subsample file will not be accepted, and a warning message will be issued. If there is more than one valid cell size, the user is queried. Examples:
The target Step values are shown in the Data and Step menus section of Volume Viewer. If a subsample is inferred to have been generated with these same values, it will be used. If not, then another subsample that includes the desired subsample will be sought and used, if possible. For example, if a (2,2,4) subsampling is the target but does not exist, an existing (2,2,2) subsampling can be used to generate it. A (2,2,3) subsampling could not be used in this situation. If no suitable subsample is available, the full data will be used.