6. Selecting Data¶
A typical visualization process has two components: setting up the visualization scene and performing the analysis of the results to gain insight. It is not uncommon for this process to be iterative. Often, what you are looking for drives from what filters you should use to extract the relevant information from the input datasets and what views will best represent that data. One of the ways of evaluating the results is inspecting the data or probing into it by identifying elements of interest. ParaView data selection mechanisms are designed specifically for such use-cases. In this chapter, we take a closer look at various ways of selecting data in ParaView and making use of these selections for data analysis.
6.1. Understanding selection¶
Broadly speaking, selection refers to selecting elements (either cells, points, rows (in case of tabular datasets), etc.) from datasets. Since data is ingested into ParaView using readers or sources and transformed using filters, when you create a selection, you are selecting elements from the dataset produced as the output of source, filter, or any such pipeline module.
There are many ways to create selections. Several views provide means to
create specific selections. For example, in the
SpreadSheet View , which
shows the data attributes as a spreadsheet, you can simply click on any row to
select that row. You can, of course, use the ⇧ and CTRL (or
⌘) keys to select multiple rows, as in typical spreadsheet-based
While this seems like an exercise in futility, you are hardly achieving anything by highlighting rows in a spreadsheet. What transforms this into a key tool is the fact that selections are linked among views (whenever possible). Linked selection means that you select an element from a dataset in a specific view. All other views that are showing the same dataset will also highlight the selected elements.
To make this easier, let’s try a quick demo:
Starting with a fresh
paraview session, create a sample dataset
using the Sources > Wavelet menu, and then click the
Apply button. If you
paraview with a default setup, that should result in a
dataset outline being shown in the default
Render View . Next, let’s split
the view and create
SpreadSheet View . The
SpreadSheet View will
automatically show the data produced by the
Wavelet source. Upon closer
inspection of the header in the
SpreadSheet View , we see that the view
is showing the
Point Data or point attributes associated with the dataset.
Now we have the same dataset, the data produced by the
shown in two views. Now, highlight a few rows in the
SpreadSheet View by clicking on them. As soon as you start selecting rows, the
Render View will start highlighting some points in space as tiny magenta specks
( Fig. 6.1). That’s linked selection in
action! What is happening is that, as you highlight rows in the
SpreadSheet View ,
you are creating a selection for selecting points (since the view is
Point Data ) corresponding to the rows. Due to the linking of
selections between views, any other view that is showing the dataset (in this
Render View ) will also highlight the selected points.
Of course, if you want to select cells instead of points, switch the
SpreadSheet View to show cells by flipping the
Attribute combo-box to
Cell Data and then highlight rows. The
Render View will show the
selected cells as a wireframe, rather than points.
Conversely, you could have created the selection in the
Render View , and the
SpreadSheet View will also highlight the selected elements. We will see how
to create such selection later in this chapter.
The first thing to note is that, when you create a new selection, the existing selection is cleared. Thus, there is at most one active selection in the application at any given time. As we shall see, certain views provide ways of expanding on the existing selection.
The second thing to note is that selections are transient, i.e., they cannot
be undone/redone or saved in state files and loaded back. Nor can you apply
filters or other transformation to the selections themselves. There are cases,
however, where you may want to subset your dataset using the selection defined
interactively and then apply filters and other analysis to that extracted
subset. For that, there are filters available, namely
Extract Selection and
Plot Selection Over Time , that can capture the active selection in as
the filter parameters and then produce a new dataset that is comprised of the
The third thing to note is that there are different types of selections, e.g., id-based selections, where the selected elements are identified by their indices; frustum-based selections, where the selected elements are those that intersect a frustum defined in 3D space; query-based selections, where the selected elements are those that match the specified query string; and so on.
Did you know?
While, technically, selection is possible using a Python-based API, that API is currently cumbersome and requires understanding of some of ParaView internals. There are plans to simplify that API in future releases. As a result, we do not cover Python API for selection in this chapter. Once this new API is available, it will be included in this chapter.
6.2. Creating selections using views¶
Views provide a convenient mechanism for creating selections interactively.
Render View can create multiple types of selection (id- or
frustum-based selections for selecting points and cells), while others like the
SpreadSheet View and
Line Chart View only support one type (id-based
selections for points or cells).
6.2.1. Selecting in Render View¶
To create a selection in the
Render View , you use the toolbar at
the top of the view frame. There are two ways of selecting cells,
points or blocks in ParaView: interactive and non-interactive.
ParaView enters a non-interactive selection mode when the user clicks one of the non-interactive selection buttons: The type of selection you are creating will depend on the button you clicked. Once in non-interactive selection mode, the cursor will switch to cross-hair and you can click and drag to create a selection region. Once you release the mouse, ParaView will attempt to create a selection for any elements in the selection region and will go back to default interaction mode.
To create a selection for cells visible in the view, use the button. For selecting visible points, use the button instead. Visible cells (and points) are only those cells (or points) that are currently rendered on the screen. Thus, elements that are occluded or are too small to be rendered on the screen will not be selected. If you want to select all data elements that intersect the view frustum formed by the selection rectangle you drew on the screen, use the button (or for points). In this case, all elements, visible or otherwise, that are within the 3D space defined by the selection frustum are selected.
While most selection modes allow you to define the selection region as a rectangle, (and for points) enables you to define the selection region as a closed polygon. However, this is limited to surface elements (i.e., no frustum-based selection).
ParaView enters an interactive selection mode when the user clicks on one of the interactive selection buttons: . In interactive selection mode, the user acts on visible elements (cells or points). ParaView highlights elements of the dataset as a user moves the cursor over them. An element can be selected by clicking on it. Clicking repeatedly on different elements adds them to the selection. A user ends the interactive selection mode by clicking on the interactive selection button pushed in or by pressing the Esc key. This mode is also ended when the user enters a non-interactive selection mode. You can use button to interactively select cells of the dataset and use button to interactively select points.
Several of these buttons have hotkeys too, such as S for visible cell selection, D for visible points selection, F for frustum-based cell selection, and G for frustum-based point selection. If you notice, these are keys are right next to each other on the keyboard, starting with S, and are in the same order as the toolbar buttons themselves.
Did you know?
For id-based visible cell (or point) selection, you can expand the
current selection by keeping the CTRL (or ⌘) key pressed when
clicking and dragging in selection mode.
paraview will then try
to add to the current selection by adding the newly selected elements to the
existing one. You can also subtract from the current selection using the
⇧, or even toggle using CTRL (or ⌘) + ⇧.
Selection modifier buttons, in the toolbar, can be used for the same effect.
Add : , remove : , toggle : .
These modifiers do not work, however, if the selection type (or the selected
data) is different from the current selection. If so, the current selection
will be cleared (as is the norm) and then the new selection will be created.
6.2.2. Selecting in SpreadSheet View¶
To create a selection in the
SpreadSheet View , you simply click on the
corresponding rows in the spreadsheet. You can use the CTRL (or
⌘) and ⇧ keys to add to the selection.
Based on which data attribute the view is currently showing, i.e.,
Cell Data , or
Row Data , the selection will select points,
cells, or rows, respectively.
6.2.3. Selecting in Line Chart View¶
Line Chart View enables you to select the elements corresponding to the
plotted data values. The selection interaction is similar to
Render View .
By default, you are in the interaction mode. You enter selection mode to create a
selection by using the buttons in the view toolbar for creating a rectangular
selection or a polygonal selection
. Once in selection mode, you
can click and drag to define the selection region. The selection is created once
you release the mouse press.
When a new selection is created, by default, it will clear any existing selection in the view. The selection modifier buttons in the view toolbar can be used to control whether a new selection adds to selected elements , removes points from the selected elements , or toggles it . These modifier buttons are mutually exclusive and modal, i.e., they remain pressed until you click to unpress them or until you press another modifier button. CTRL (or ⌘) and ⇧ can also be used to add to/subtract from the selection.
6.3. Creating selections using the Find Data dialog¶
Views provide mechanisms to create selections interactively. Selections in chart
SpreadSheet View can be used to select elements with certain data
properties, rather than spatial locations
( Fig. 6.3). For a richer data-based selection
for selecting elements matching certain criteria, you can use the
Find Data mechanism in
Find Data dialog can be split into three components,
reflecting how you would use this dialog. The
Create Selection component
includes UI elements to help you define the selection criteria or the query for
defining the data elements to find. The
Current Selection component shows the
results from the most recent selection results in a spreadsheet. Finally, the
Selection Display Properties component allows you to control how the
selected elements are shown in the active view.
You can create selections or define queries in the
Find Data dialog using
the widgets under the
Create Selection group. The UI is designed to be read
left-to-right. Thus, you start by specifying which data attributes you are
interested in selecting: cells, points, or rows. Then select which pipeline
module is producing the dataset of interest. Once that’s done, the next
step is to specify the selection criteria. The left-most combo-box is used to
select the array of interest. The available options reflect the data array
currently available on the dataset. The next combo-box is used to select the
operator. Options include the following:
ismatches a single value
is betweenmatches a range of values specified by min and max
is one ofmatches a list of comma-separated values
is >=matches all values greater than or equal to the specified value
is <=matches all values lesser than or equal to the specified value
is minmatches the minimum value for the array for the current time step
is maxmatches the maximum value for the array for the current time step
is less than meanmatches values lesser than or equal to the mean
is greater than meanmatches values greater than or equal to the mean
is equal to mean with tolerancematches values equal to the mean within the specified tolerance
Based on your selection of the operator, input widgets will be shown next to
this combo-box, where you enter the corresponding values. For example, for
between , you enter the min and max values for defining the range in the two
text entry widgets.
Once you are satisfied with the selection criteria, hit the
Query button to execute the query. On success, the
Current Selection spreadsheet will update to show the selected elements. You switch which element
attributes you are viewing in the spreadsheet using the
Additionally, you can invert the selection by checking
Invert selection .
Similar to selecting in views, once you create a selection, any view showing the
selected data will also highlight the selected elements, if possible. For example, the
Render View will show a colored wireframe marking the selected elements,
SpreadSheet View will highlight the rows, etc. The
Properties group allows you to change the selection that is shown in the active
view. Currently, it is primarily designed for
Render View . In the
future, however, it could support changing selection attributes for other views as well.
The available options allow you select the color to use to show the selected
elements, as well as the data attributes to use to label the cells/points. For finer
control on the label formatting, color, font, etc., use the
. That will pop up the
Advanced Selection Display Properties dialog
( Fig. 6.5).
Did you know?
Besides creating new selections, the
Find Data dialog can also be used
to inspect the current selection made from outside the dialog. For
example, if you select elements in the
Render View using the options
described in Section Section 6.2.1, the
Selection component in the
Find Data dialog will indeed update to
reflect the newly selected elements. Furthermore, you can change its display
properties and extract this selection using the extraction buttons
(which we will cover in Section Section 6.6).
6.4. Creating selections in Python¶
Another way to create selections is through ParaView’s Python scripting interface.
Python functions exist that are analogous to the selection operations available in the ParaView
Render View and
Find Data dialog. Let’s take a look at an example.
# import the selection module from paraview.selection import * renderView1 = GetActiveView() # Create an initial rectangular selection in the render view SelectSurfacePoints(Rectangle=[200, 321, 600, 744], View=renderView1) # Add points within a polygon in the active view SelectSurfacePoints(Polygon=[180, 200, 190, 400, 322, 300], Modifier='ADD') # Subtract points with another rectangle SelectSurfacePoints(Rectangle=[300, 400, 500, 700], Modifier='SUBTRACT') # Now extract and show the selected points into another dataset ExtractSelection() Show() # Clear the selection ClearSelection()
The script starts out by importing functions from the
paraview.selection module. Next, it creates a reference to the active render view and
passes it into the selection functions. The first selection function selects points
visible in the render view within a rectangular region. The rectangle is defined
by bottom left and upper right points, (200, 321) and (600, 744), given in pixel coordinates.
The second selection is of visible points within a polygon defined by the points
(180, 200), (190, 400), and (322, 300). In this call, the selection function
modifies the existing selection so that newly selected points are added to the selection.
This is controlled with the
Modifier named function parameter.
Other options for the
Modifier parameter are
None . When the
Modifier is set to
None , the previous selection
gets replaced with the new selection. The last call to
SelectSurfacePoints subtracts points from the current selection, which is the combination of the first
The last lines in this example script extract the currently selected points from
the currently active source and shows them on the screen. Lastly, the selection
is cleared with the
Selections by point or cell ID numbers are also possible, as shown in this example:
from paraview.selection import * # Select cell 1 from all blocks in a multiblock data set on process 0 SelectIDs(IDs=[0, 1], FieldType='CELL') # Add cell 3 from block 4 on process 0 and cell 5 from block 6 on process 1 # to the selection SelectCompositeDataIDs(IDs=[4, 0, 3, 6, 1, 5], Modifier='ADD')
Finally, selections by query expressions are also possible via the Python selection API.
As an example, the following selects cells that have the maximum value for
a cell variable named
EQPS in the currently active source:
from paraview.selection import * QuerySelect(QueryString='EQPS == max(EQPS)', FieldType='CELL')
The complete list of selection functions are briefly
described below. For full documentation on these functions, you can invoke the
help function on any of the functions, e.g.,
SelectSurfacePoints- Select visible points within a rectangular or polygon region.
SelectSurfaceCells- Select visible cells within a rectangular or polygon region.
SelectSurfaceBlocks- Select visible blocks within a rectangular region.
SelectPointsThrough- Select all points within a rectangular region regardless of their visibility.
SelectCellsThrough- Select all cells within a rectangular region regardless of their visibility.
SelectGlobalIDs- Select attributes by global IDs.
SelectPedigreeIDs- Select attributes by Pedigree IDs.
SelectIDs- Select attributes by attribute IDs.
SelectCompositeDataIDs- Select attributes by composite attribute IDs.
SelectHierarchicalDataIDs- Select attributes by hierarchical data IDs.
SelectThresholds- Select attributes in a source by thresholding on values in an associated array.
SelectLocation- Select points by location.
QuerySelect- Selection by query expression.
ClearSelection- Clears the selection on the source passed in as a parameter.
6.5. Displaying selections¶
Find Data dialog provides easy access to changing the
Display Properties for the selection in the active view. The same functionality
can also be accessed using the
Selection Display Inspector panel, which is a
dockable panel that can be shown/hidden using the View > Selection Display Inspector
menu. Currently, selection display properties apply to
Render View s alone.
In the future, this will be expanded to other view types, as
Current Selection section in the
Find Data dialog shows the
selected elements in a spreadsheet view. You can also make a regular
SpreadSheet View do the same by checking the
button in the view toolbar to show only selected elements.
6.6. Extracting selections¶
All the types of selections created through mechanisms discussed so far are
transient and primarily used for highlighting data. If you want to do further
operations on the selected subset, such as extract the selected elements and
then save the result out as a new dataset or apply other filters only on the
selected elements, then you need to use one of the extract selection filters.
Extract Selection and
Plot Selection Over Time filters fall in
this category of filters.
6.6.1. Extract selection¶
Extract Selection filter is used to extract the selected elements as a
new dataset for further filtering. There are multiple ways of creating this
filter. You can use the conventional method for creating filters, i.e., using the
Filters menu. When the filter is created, if there is any active
selection, the filter will automatically copy that selection for convenience.
Another way to extract the active selection is using the
Selection button in the
Find Data dialog
( Fig. 6.4).
Properties panel shows what defines the selection. You can update the
selection by making a new active selection using any of the mechanisms described
earlier in this chapter and then clicking on the
Copy Active Selection button on the
Properties panel for the
Extract Selection filter.
By default, the filter is set up to extract the selected elements alone. This
filter also supports passing the entire input dataset through by simply
marking which elements are selected. For that, check the
Preserve Topology check box on the
6.6.2. Plot selection over time¶
Plot Selection Over Time is similar to
Extract Selection in the sense
that it too extracts the selected elements from the input dataset. However,
instead of simply extracting the result, the goal here is to plot attributes at the
selected elements over time.
Fig. 6.7 shows an example use of this filter. In
this case, we wanted to see how the strain (or EQPS) cell attribute
changes over time for two specific cells that we selected in the
View using the view-based selection mechanism. The selected cells are the
highlighted elements in the left view. After having selected the cells, we
Plot Selection Over Time filter using the Filters > Data Analysis
menu. (You could also use the
Data Analysis toolbar.) Similar to the
Extract Selection filter, when this filter is created, it copies the active selection. You can
change it afterwards using the
Copy Active Selection button on the filter’s
Properties panel. On hitting
paraview will show a
visualization similar to the one shown here.
Instead of using the view for defining the selection, you could have used the
Find Data dialog. In that case, instead of being able to plot each element
over time, you will be plotting summaries for the selected subset over time.
This is essential since the selected subset can have a varying number of
elements over time. The summaries include quantities like mininum, maximum, and
median of available variables. You can make the filter always produce these
statics alone (even when the selection is created by selecting specific elements
in a view) by checking the
Only Report Selection Statistics property on the
Properties panel for the
Plot Selection Over Time filter.
6.7. Freezing selections¶
When extracting selections, you can use views or the
Find Data dialog to
define the selection. Since the extraction filters are indeed like any other
filters in ParaView, they are re-executed any time the input dataset changes,
properties on the filter change, or the current time changes. Every time the
filter re-executes, it performs the selection and extraction
operations. Thus, if you created the selection using
Render View to create
an id-based selection, the filter will identify which of the elements are of the
requested ids and then pass those. For frustum-based selection, it will
determine what elements fall within the frustum and extract those. Similarly,
with query-based selections created using the
Find Data dialog, the query
is re-evaluated. This can result in the selection of different elements with changes
in timestep. For example, if you are selecting the cells where the strain is
maximum, the selected cell(s) will potentially be different for each time step. Suppose
you want to plot the changes in a cell that has maximum strain at the last
time step – how can we do that? The answer is using the
Selection button on the
Find Data dialog. What that does is convert any
type of selection (frustum, query-based) to an id-based selection matching the
currently selected element ids. Now you can use this frozen, id-based selection
Extract Selection or
Plot Selection Over Time .