vtkStreamTracer

Section: Visualization Toolkit Graphics Classes

Usage

vtkStreamTracer is a filter that integrates a vector field to generate streamlines. The integration is performed using a specified integrator, by default Runge-Kutta2. vtkStreamTracer produces polylines as the output, with each cell (i.e., polyline) representing a streamline. The attribute values associated with each streamline are stored in the cell data, whereas those associated with streamline-points are stored in the point data.

vtkStreamTracer supports forward (the default), backward, and combined (i.e., BOTH) integration. The length of a streamline is governed by specifying a maximum value either in physical arc length or in (local) cell length. Otherwise, the integration terminates upon exiting the flow field domain, or if the particle speed is reduced to a value less than a specified terminal speed, or when a maximum number of steps is completed. The specific reason for the termination is stored in a cell array named ReasonForTermination.

Note that normalized vectors are adopted in streamline integration, which achieves high numerical accuracy/smoothness of flow lines that is particularly guranteed for Runge-Kutta45 with adaptive step size and error control). In support of this feature, the underlying step size is ALWAYS in arc length unit (LENGTH_UNIT) while the 'real' time interval (virtual for steady flows) that a particle actually takes to trave in a single step is obtained by dividing the arc length by the LOCAL speed. The overall elapsed time (i.e., the life span) of the particle is the sum of those individual step-wise time intervals.

The quality of streamline integration can be controlled by setting the initial integration step (InitialIntegrationStep), particularly for Runge-Kutta2 and Runge-Kutta4 (with a fixed step size), and in the case of Runge-Kutta45 (with an adaptive step size and error control) the minimum integration step, the maximum integration step, and the maximum error. These steps are in either LENGTH_UNIT or CELL_LENGTH_UNIT while the error is in physical arc length. For the former two integrators, there is a trade-off between integration speed and streamline quality.

The integration time, vorticity, rotation and angular velocity are stored in point data arrays named "IntegrationTime", "Vorticity", "Rotation" and "AngularVelocity", respectively (vorticity, rotation and angular velocity are computed only when ComputeVorticity is on). All point data attributes in the source dataset are interpolated on the new streamline points.

vtkStreamTracer supports integration through any type of dataset. Thus if the dataset contains 2D cells like polygons or triangles, the integration is constrained to lie on the surface defined by 2D cells.

The starting point, or the so-called 'seed', of a streamline may be set in two different ways. Starting from global x-y-z "position" allows you to start a single trace at a specified x-y-z coordinate. If you specify a source object, traces will be generated from each point in the source that is inside the dataset.

To create an instance of class vtkStreamTracer, simply invoke its constructor as follows

  obj = vtkStreamTracer

Methods

The class vtkStreamTracer has several methods that can be used. They are listed below. Note that the documentation is translated automatically from the VTK sources, and may not be completely intelligible. When in doubt, consult the VTK website. In the methods listed below, obj is an instance of the vtkStreamTracer class.