ParaView/UsersGuide/Manipulating the Pipeline
Pipeline Basics
Data manipulation in ParaView is fairly unique because of the underlying pipeline architecture that it inherits from VTK. Each filter takes in some data and produces something new from it. Filters do not directly modify the data that is given to them and instead copy unmodified data through via reference (so as to conserve memory) and augment that with newly generated or changed data. The fact that input data is never altered in place means that unlike many visualization tools, you can apply several filtering operations in different combinations to your data during a single ParaView session. You see the result of each filter as it is applied, which helps to guide you data exploration work, and can easily display any or all intermediate pipeline outputs simultaneously.
The Pipeline Browser depicts ParaView's current visualization pipeline and allows you to easily navigate to the various readers, sources, and filters it contains. Connecting an initial data set loaded from a file or created from a ParaView source to a filter creates a two filter long visualization pipeline. The initial data set read in becomes the input to the filter, and if needed the output of that filter can be used as the input to a subsequent filter, etc.
For example, suppose you create a sphere in ParaView by selecting Sphere from the Sources menu. In this example, the sphere is the initial data set. Next create a Shrink filter from the Alphabetical submenu of the Filters menu. Because the sphere source was the active filter when the shrink filter was created, the shrink filter operates on the sphere source's output. Optionally, use the Properties tab of the Object Inspector to set initial parameters of the shrink filter and then hit Apply. Next create an Elevation filter to filter the output of the shrink filter and hit Apply again. You have just created a simple three element linear pipeline in ParaView. You will now see the following in the Pipeline Browser.
Within the Pipeline Browser, to the left of each entry is an eye icon indicating whether that data set is currently visible. If there is no eye icon, it means that the data produced by that filter is not compatible with the currently active View window. Otherwise, a dark eye icon indicates that the data set is visible; and when a data set is viewable but currently invisible, its icon is drawn in light gray. Clicking on the eye icon toggles the visibility of the corresponding data set. In the above example, all three filters are potentially visible, but only the ElevationFilter is actually being displayed. The ElevationFilter is also highlighted in blue, indicating that it is the currently "active" Filter. Since it is the "active" filter, the Object Inspector reflects its content and the next filter created will use it as the input.
Double-clicking the name of one of the filters causes the name to become editable, enabling you to change it to something more meaningful than the default chosen by ParaView.
You can always change parameters of any filter in the pipeline after it has been applied. Left-clicking the name of a filter in the Pipeline Browser causes it to become the "active" filter. The Properties, Display, and Information tabs are always reflect the "active" filter. When you make changes in the Properties tab and apply your changes, all filters beyond the changed one are automatically updated.
Right-clicking a data set's name displays a context menu from which you can do several things. For reader modules you can use this to load a new data file. For all modules you can additionally save it as a Custom Filter (see the last section of this chapter), or delete it (if it is at the end of the visualization pipeline). For filter modules you can also use this menu to change the input to the filter, and thus rearrange the visualization pipeline.
To rearrange the pipeline, right-clicking the filter's name in the Pipeline Browser, and y select Change Input from the context menu that appears. Only data sets that meet the qualifications for the input of this filter will be selectable in the dialog for changing inputs.
Note the input editor does not allow the creation of loops in the visualization pipeline, so in the diagram above Clip1 cannot be the input to itself. Left-clicking on one of the possible inputs in the Select Source window causes the Pipeline Preview window to show what the contents of the pipeline browser would be if the input was changed in this way. Clicking OK causes the input to change.
You can not create loops in the pipeline, but you can create branches. Select ShrinkFilter1 in the Pipeline Browser; then apply Extract Edges from the Alphabetical submenu of the Filters menu. Now the output of the shrink filter is being used as the input to both the elevation and extract edges filters. You will see the following in the Pipeline Browser.
Another way to introduce a branch into the Pipeline is to open a reader or filter that produces multiple distinct data sets. An example of this is the SLAC reader that produces both a polygonal output and a structured data field output. With either type of branch you can select one output or the other to make it active and this extend the pipeline from it alone.
Some filters that need to produce multiple data sets do so in a different way. Instead of producing several fundamentally distinct data sets, it is often more convenient to produce a single composite data set which contains some number of sub data sets. (See Chapter ?for a fuller explanation). In this case it is usually more convenient to treat the entire composite data set as one entity and you do not need to do anything in particular to do so. Sometimes though, you want to operate on a particular sub data sets as you can for a multi-output filter. Do so by first apply the Extract Datasets filter to your multi-group data set to extract the desired part(s), and then apply the filter only to the extracted part(s).
Pipelines are also allowed to contain filters that merge the pipeline, taking in more than one data set to produce their own output (or outputs). There are in fact two different varieties of merging filters. The Append Filter and Group Data Sets filter s are examples of the first kind. These filters take in any number of fundamentally similar data sets. An append filter for example takes in one or more polygonal datasets and combines them into a single large polygonal data set. To use this type of merge filter, select more than one filter within the Pipeline browser by pressing the <SHIFT?> key while you left click. Then instantiate the merging filter from the Filters menu as before. The pipeline in this case will look like the one in the following figure.
Other filters take in more than one, fundamentally different data sets. An example is the <?Probe With Arbitrary Data?> filter which takes in one data set the represents a field in space to sample values from and another data set which that to find the locations in space to sample the values onto. Begin this type of merge by simply choosing the filter from the Filters menu. When you do, ParaView creates a dialog that you use to identify which of the modules form the Pipeline to use for each input. The dialog is shown below:
