17.3.4.4 Polyline Interval Grid Cells

The polyline set is used to describe the geometry of lines in space. For streamlines specifically, these lines have been developed based on an underlying computational grid. The relationship from line to grid is described using the interval grid cells object. The grid indices element is the most important, because it indicates which grid, if any, corresponds to each interval of the streamline polyline set. Nulls are used to indicate intervals that do not lie on any grid. Non-null grid values indicate that that interval is a cell.

The indexable element type “cells” is used to attach properties or otherwise reference those intervals with non-null grid indices. The cell count is specified explicitly and must be equal to the total count of intervals with non-null grid indices. The remaining elements (cell indices array and local face pair per cell indices array) are used to further specify the spatial support of the streamlines on the cells of the grid(s).