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## Expression Window

VisIt provides an Expression Window, shown in figure, that allows you to create new variables that can be used in visualizations. You can open the Expression Window by clicking on the Expressions option in the Main Window's Controls menu. The Expression Window is divided vertically into two main areas: Expression list and Definitions. The Expression list contains the list of expressions. The Definitions area displays the definition of the expression that is highlighted in the Expression list and provides controls to edit the expression definition.

Figure 1

In addition to displaying expressions that you create yourself, the Expression Window displays expressions that were written to the database or expressions that were added by one of VisIt's database reader plugins. Expressions that came from a database change when you change open databases but expressions that you defined yourself remain in the window regardless which database is open. Expressions that came from a database are not usually shown in the Expression list by default but you can choose to show them by clicking on the Display expressions from database check box.

Figure 2

There is one special case when VisIt adds expressions to the expression list that were not in the database and were not added by a database reader plugin: mesh quality expressions. When the open database contains meshes that are unstructured or curvilinear, VisIt adds special mesh quality expressions to the expression list to let you more easily plot metrics of mesh quality such as the volume of cells or how much cells are skewed. The mesh quality expressions are primarily used to evaluate the fitness of computational meshes and VisIt adds them to the expression list to make them more accessible to users since they are so commonly used. figure shows the how the mesh quality expressions appear in the variable list for plots that accept scalar variables.