Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:The ray tracer uses a hierarchical grid acceleration structure and three level bricking (three-dimensional tiling) for volumes, as discussed in further detail in [Parker et al. 1999b] and [Parker et al. 1998].
Category: