Core tracing: depicting connections between features in electron density.

Core tracing is a threshold-independent method of determining connectivity (long chains of high-density values) in electron-density maps. It gives visually sparse pictures of large volumes which are useful for initial fitting and for molecular-boundary determination. New methods for visual presentation of the traces are suggested by the way that the connectivity is parameterized in terms of local connections between maxima and the saddle (lowest) points along the connecting paths. The algorithm also partitions the density into small compact volumes containing the maxima. These volumes are useful for localization and statistical analysis.