# 3. Comparative visualization¶

Comparative visualization in ParaView refers to the ability to create side-by-side visualizations for comparing with one another. In its most basic form, you can indeed use ParaView’s ability to show multiple views side by side to set up simultaneous visualizations. But, that can get cumbersome too quickly. Let’s take a look at a simple example: Let’s say you want to do a parameter study where you want to compare isosurfaces generated by a set of isovalues in a dataset. To set up such a visualization, you’ll need to first create as many Render View s as isovalues. Then, create just as many Contour filters, setting each one up with a right isovalue for the contour to generate and display the result in one of the views. As the number of isovalues increases, you can see how this can get tedious. It is highly error prone, as you need to keep track of which view shows which isovalue. Now, imagine after having set up the entire visualization that you need to change the Representation type for all the of the isosurfaces to Wireframe !

Comparative View s were designed to handle such use-cases. Instead of creating separate views, you create a single view Render View (Comparative) . The view itself comprises of a configurable $$m\times n$$ Render View s. Any data that you show in this view gets shown in all the internal views simultaneously. Any display property changes, such as scalar coloring and representation type are also maintained consistently between these internal views. The interactions in the internal views are linked, so when you interact with one, all other views update as well. While this is all nice and well, the real power of Comparative View s becomes apparent when you set up a parameter to vary across the views. This parameter can be any of the properties on the pipeline modules such as filter properties and opacity, or it could be the data time. Each of these internal views will now render the result obtained by setting the parameter as per your selection. Going back to our original example, we will create a single Contour filter that we show in Render View (Comparative) with as many internal views as the isovalue to compare. Next, we will set up a parameter study for varying the Isosurfaces property on the Contour filter, and, viola! The view will generate the comparative visualization for us!

In this chapter, we look at how to configure this view and how to set these parameters to compare. We limit our discussion to Render View (Comparative) . However, the same principles are applicable to other comparative views, including Bar Chart View (Comparative) and Line Chart View (Comparative) .

## 3.1. Setting up a comparative view¶

To create Render View (Comparative) in paraview, split or close the active view and select Render View (Comparative) from the new view creation widget. paraview will immediately show four Render View s laid out in a $$2\times 2$$ grid. While you cannot resize these internal views, notice that you can still split the view frame and create other views if needed.

The Properties panel will show properties similar to those available on the Render View under the View properties section. If you change any of these properties, they will affect all these internal views, e.g., setting the Background color to Gradient will make all the views show a gradient background.

To configure the comparative view itself, you must to use the Comparative View Inspector ( Fig. 3.18) accessible from the View menu. The Comparative View Inspector is a dockable panel that is enabled when the active view is a comparative view.

To change how many internal views are used in the active comparative view and how they are laid out, use the Layout . The first value is the number of views in the horizontal direction and the second is the count in the vertical direction.

Besides doing a parameter study in side-by-side views, you can also show all the results in a single view. For that, simply check the Overlay all comparisons checkbox. When checked, instead of getting a grid of $$m\times n$$ views, you will only see one view with all visible data displayed $$m\times n$$ times.

To show data in this view is the same as any other view: Make this view active and use the Pipeline Browser to toggle the eyeball icons to show data produced by the selected pipeline module(s). Showing any dataset in this view will result in the data being shown in all the internal views simultaneously. As is true with View properties, with Display properties, changing Coloring , Styling , or any other properties will reflect in all internal views.

Since the cameras among the internal views are automatically linked, if you interact with one of the views, all views will also update simultaneously when you release the mouse button.

## 3.2. Setting up a parameter study¶

To understand how to setup a parameter study, let’s go back to our original example. Our visualization pipeline is simply Wavelet $$\rightarrow$$ Contour (assuming default property values), and we are showing the result from the Contour filter in the $$2 \times 2$$ Render View (Comparative) with Overlay all comparisons unchecked.

Now, we want to vary the Isosurfaces property value for the visualization in each of the internal views. That’s our parameter to study. Since that’s a property on the Contour filter, we select the Contour filter instance. Then, its property Isosurfaces is in the parameter selection combo-boxes. To add the parameter to the study, you must hit the button.

The parameter Contour1:Isosurfaces will then show up in the Parameter list above the combo-boxes. You can delete this parameter by using the button next to the parameter name.

Did you know?

Notice how this mechanism for setting up a parameter study is similar to how animations are set up. In fact, under the cover, the two mechanisms are not that different, and they share a lot of implementation code.

Simultaneously, the table widget below the combo-boxes is populated with a default set of values for the Isosurfaces . This is where you specify the parameter values for each of the views. The location of the views matches the location in the table. Hence, $$0-A$$ is the top-left view, $$0-B$$ is the second view from the left in the topmost row, and so on. The parameter value at a specific location in the table is the value used to generate the visualization in the corresponding view.

To change the parameter (in our case Isosurfaces ) value, you can double click on the cell in the table and change it one at a time. Also, to make it easier to fill the cells with a range of values, you can click and drag over multiple cells. When you release the mouse, a dialog will prompt you to enter the data value range ( Fig. 3.20). In case you selected a combination of rows and columns, the dialog will also let you select in which direction the parameter is varied using the range specified. You can choose to vary the parameter horizontally first, vertically first, or only along one of the directions while keeping the other constant. This is useful when doing a study with multiple parameters.

As soon as you change the parameter values, the view will update to reflect the change.

You add annotations like color legends, text, and cube-axes exactly as you would with a regular Render View . As with other properties, annotations will show up in all of the internal views.
You can use the Annotate Time source or filter to show the data time or the view time in each of the internal views. If Time is one of the parameters in your study, the text will reflect the time values for each of the individual views, as expected ( Fig. 3.19)!