Algorithm 531: Contour Plotting [J6]
William V. Snyder · ACM Transactions on Mathematical Software · 1978
Given a two-dimensional array of samples of a surface, contour values, and a subroutine to draw fines, the subroutine GCONTR determines sequences of points in the plane which may be used to draw contours through equal values of the surface.A contour plotting algorithm may be constructed by following contours from some starting point until they either close or intersect a boundary, or by examining each cell of the grid in turn and drawing all contours found inside the cell.The advantages of contour following are that contour labeling is relatively easy and that the pen of an incremental plotter does not move about as much without writing anything.The advantages of methods which draw all contours found inside a cell are that less auxiliary storage is needed and that each cell can be completely processed before going on to the next, thereby allowing generation of contours over a much larger array than can be accommodated in main memory at one time.GCONTR is of the type which follows contours.On a representative problem, a program using GCONTR generated about 11,000 plotter commands with about 57,000 commands generated by a program using a cellular method.For this problem, grid lines, user identification, and a table of contour values required about 36,000 additional commands.