.. include:: ../macros.hrst .. include:: ../abbreviations.hrst .. _chapter:SelectingData: 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. .. _sec:UnderstandingSelection: 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 |SHIFT| and |CTRL| (or |CMD|) keys to select multiple rows, as in typical spreadsheet-based applications. 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. .. figure:: ../images/UnderstandingLinkedSelections.png :name: fig:UnderstandingLinkedSelections :width: 80% :align: center Linked selection between views allows you to select elements in a view and view them in all other views showing the selected data. In this demo, as we select rows in the ``SpreadSheet View`` , the corresponding points in the ``3D View`` get highlighted. To make this easier, let's try a quick demo: Starting with a fresh |paraview| session, create a sample dataset using the :guilabel:`Sources > Wavelet` menu, and then click the ``Apply`` button. If you are using |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 ``Wavelet`` source, 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 ( :numref:`fig:UnderstandingLinkedSelections`). 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 showing ``Point Data`` ) corresponding to the rows. Due to the linking of selections between views, any other view that is showing the dataset (in this case, the ``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 selected elements. 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. .. admonition:: **Did you know?** :class: tip 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. Creating selections using views =============================== Views provide a convenient mechanism for creating selections interactively. Views like ``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). .. _sec:SelectingInRenderView: 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: |pqSurfaceSelectionCell24| |pqSurfaceSelectionPoint24| |pqFrustumSelectionCell24| |pqFrustumSelectionPoint24| |pqPolygonSelectSurfaceCell24| |pqPolygonSelectSurfacePoint24| |pqSelectBlock24| 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. .. |pqSurfaceSelectionCell24| image:: ../images/pqSurfaceSelectionCell24.png :width: 0.5cm .. |pqSurfaceSelectionPoint24| image:: ../images/pqSurfaceSelectionPoint24.png :width: 0.5cm .. |pqSelectBlock24| image:: ../images/pqSelectBlock24.png :width: 0.5cm .. |pqFrustumSelectionCell24| image:: ../images/pqFrustumSelectionCell24.png :width: 0.5cm .. |pqFrustumSelectionPoint24| image:: ../images/pqFrustumSelectionPoint24.png :width: 0.5cm .. |pqPolygonSelectSurfaceCell24| image:: ../images/pqPolygonSelectSurfaceCell24.png :width: 0.5cm .. |pqPolygonSelectSurfacePoint24| image:: ../images/pqPolygonSelectSurfacePoint24.png :width: 0.5cm To create a selection for cells visible in the view, use the |pqSurfaceSelectionCell24| button. For selecting visible points, use the |pqSurfaceSelectionPoint24| 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 |pqFrustumSelectionCell24| button (or |pqFrustumSelectionPoint24| for points). In this case, all elements, visible or otherwise, that are within the 3D space defined by the selection frustum are selected. To create a selection for blocks visible in the view use the |pqSelectBlock24| button. Note that there is no frustum selection for blocks. While most selection modes allow you to define the selection region as a rectangle, |pqPolygonSelectSurfaceCell24| (and |pqPolygonSelectSurfacePoint24| 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: |pqSurfaceSelectionCellInteractive| |pqSurfaceSelectionPointInteractive|. 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 |pqSurfaceSelectionCellInteractive| button to interactively select cells of the dataset and use |pqSurfaceSelectionPointInteractive| button to interactively select points. .. |pqSurfaceSelectionCellInteractive| image:: ../images/pqSurfaceSelectionCellInteractive.png :width: 0.5cm .. |pqSurfaceSelectionPointInteractive| image:: ../images/pqSurfaceSelectionPointInteractive.png :width: 0.5cm .. |ClearSelection| image:: ../images/ClearSelection.png :width: 0.5cm When there are selected elements, the |ClearSelection| button can be used to clear the selection. Several of these buttons have hotkeys too, such as :kbd:`S` for visible cell selection, :kbd:`D` for visible points selection, :kbd:`F` for frustum-based cell selection, and :kbd:`G` for frustum-based point selection. If you notice, these are keys are right next to each other on the keyboard, starting with :kbd:`S`, and are in the same order as the toolbar buttons themselves. .. figure:: ../images/FrustumCellSelection.png :name: fig:FrustumCellSelection :width: 80% :align: center Result of a frustum cell selection on disk_out_ref.ex2 dataset showing the frustum used to identify selected cells. All cells that fall in that frustum or that intersect it are selected, irrespective of whether they were visible from the view angle when the selection was made. .. admonition:: **Did you know?** :class: tip For id-based visible cell (or point) selection, you can expand the current selection by keeping the |CTRL| (or |CMD|) 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 |SHIFT|, or even toggle using |CTRL| (or |CMD|) + |SHIFT|. Selection modifier buttons, in the toolbar, can be used for the same effect. Add : |pqSelectChartPlus16| , remove : |pqSelectChartMinus16|, toggle : |pqSelectChartToggle16|. 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. 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 |CMD|) and |SHIFT| keys to add to the selection. Based on which data attribute the view is currently showing, i.e., ``Point Data`` , ``Cell Data`` , or ``Row Data`` , the selection will select points, cells, or rows, respectively. 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 |pqSelectChart16| or a polygonal selection |pqSelectChartPolygon16|. 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 |pqSelectChartPlus16|, removes points from the selected elements |pqSelectChartMinus16|, or toggles it |pqSelectChartToggle16|. 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 |CMD|) and |SHIFT| can also be used to add to/subtract from the selection. .. |pqSelectChart16| image:: ./../images/pqSelectChart16.png :width: 0.5cm .. |pqSelectChartPlus16| image:: ./../images/pqSelectChartPlus16.png :width: 0.5cm .. |pqSelectChartMinus16| image:: ./../images/pqSelectChartMinus16.png :width: 0.5cm .. |pqSelectChartToggle16| image:: ./../images/pqSelectChartToggle16.png :width: 0.5cm .. |pqSelectChartPolygon16| image:: ./../images/pqSelectChartPolygon16.png :width: 0.5cm .. figure:: ../images/SelectingInLineChartView.png :name: fig:SelectingInLineChartView :width: 80% :align: center Selection in ``Line Chart View`` can be used to locate elements matching attribute criteria. In this visualization, by generating a scatter plot plotting Pres against Temp in the disk_out_ref.ex2 dataset by selecting the top-left corner of the ``Line Chart View`` , we can easily locate elements in the high Pres, low Temp regions in the dataset. Creating selections using the *Find Data* dialog ================================================ Views provide mechanisms to create selections interactively. Selections in chart views and ``SpreadSheet View`` can be used to select elements with certain data properties, rather than spatial locations ( :numref:`fig:SelectingInLineChartView`). For a richer data-based selection for selecting elements matching certain criteria, you can use the ``Find Data`` mechanism in |paraview|. The ``Find Data`` dialog can be accessed from the :guilabel:`Edit` menu or by using the keyboard shortcut :kbd:`V` or the |pqFindData32| button on the ``Main Controls`` toolbar. .. |pqFindData32| image:: ./../images/pqFindData32.png :width: 0.5cm The ``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. .. figure:: ../images/FindDataDialog.png :name: fig:FindDataDialog :width: 80% :align: center The ``Find Data`` dialog in |paraview| can be used to find data elements matching specific conditions. In this example, we are selecting all *Points* in *disk_out_ref.ex2* dataset where *Pres* is :math:`>= 0.02`. 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: * ``is`` matches a single value * ``is between`` matches a range of values specified by min and max * ``is one of`` matches 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 min`` matches the minimum value for the array for the current time step * ``is max`` matches the maximum value for the array for the current time step * ``is less than mean`` matches values lesser than or equal to the mean * ``is greater than mean`` matches values greater than or equal to the mean * ``is equal to mean with tolerance`` matches 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 ``is 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 ``Run Selection 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 ``Show`` combo-box. 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 ``Selection Display 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 |pqAdvanced26|. That will pop up the ``Advanced Selection Display Properties`` dialog ( :numref:`fig:AdvancedSelectionDisplayPropertiesDialog`). .. |pqAdvanced26| image:: ./../images/pqAdvanced26.png :width: 0.5cm .. figure:: ../images/AdvancedSelectionDisplayPropertiesDialog.png :name: fig:AdvancedSelectionDisplayPropertiesDialog :width: 60% :align: center ``Advanced Selection Display Properties`` dialog for controlling selection labelling parameters. .. admonition:: **Did you know?** :class: tip 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 :numref:`sec:SelectingInRenderView`, the ``Current 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 :numref:`sec:ExtractingSelections`). 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. .. code-block:: python # 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 ``'SUBTRACT'`` , ``'TOGGLE'`` , and ``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 two selections. 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 ``ClearSelection`` function. Selections by point or cell ID numbers are also possible, as shown in this example: .. code-block:: python 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: .. code-block:: python 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., ``help(SelectSurfaceCells)`` . * ``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. Displaying selections ===================== The ``Find Data`` dialog provides easy access to changing the ``Selection 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 :guilabel:`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 appropriate. The ``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 |pqSelect32| button in the view toolbar to show only selected elements. .. |pqSelect32| image:: ./../images/pqSelect32.png :width: 0.5cm .. _sec:ExtractingSelections: 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. The ``Extract Selection`` and ``Plot Selection Over Time`` filters fall in this category of filters. .. %Using these filters not only enables further %processing for the extraction result, but also makes the selection concrete i.e. %the filter will get saved in state files. Extract selection ^^^^^^^^^^^^^^^^^ .. figure:: ../images/ExtractSelectionPanel.png :name: fig:ExtractSelectionPanel :width: 60% :align: center ``Properties`` panel showing the properties for the ``Extract Selection`` filter.} The ``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 :guilabel:`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 ``Extract Selection`` button in the ``Find Data`` dialog ( :numref:`fig:FindDataDialog`). The ``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 ``Properties`` panel. .. _sec:PlotSelectionOverTime: Plot selection over time ^^^^^^^^^^^^^^^^^^^^^^^^ .. figure:: ../images/PlotSelectionOverTime.png :name: fig:PlotSelectionOverTime :width: 80% :align: center ``Plot Selection Over Time`` in action in |paraview|. The filter provides a convenient way to plot changes in attributes over time for the selected set of cells or for points in a temporal dataset. ``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. :numref:`fig:PlotSelectionOverTime` 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 ``Render View`` using the view-based selection mechanism. The selected cells are the highlighted elements in the left view. After having selected the cells, we create the ``Plot Selection Over Time`` filter using the :guilabel:`Filters > Data Analysis` menu. (You could also use the |pqPlotSelectionOverTime24| from 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 ``Apply`` , |paraview| will show a visualization similar to the one shown here. .. |pqPlotSelectionOverTime24| image:: ./../images/pqPlotSelectionOverTime24.png :width: 0.5cm 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. 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 ``Freeze 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 for ``Extract Selection`` or ``Plot Selection Over Time`` .