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## Variable centering

Variables in a database can be associated with a mesh in various ways. Databases supported by VisIt allow variables to be associated with a mesh's zones (cells) or its nodes. When a variable is associated with a mesh's zones, the variable field consists of one value for each zone and is said to be zone-centered. When a variable is associated with a mesh's nodes, there are values for each vertex making up the zone and the variable is said to be node-centered.

Figure 28

VisIt's Pseudocolor plot attributes window (shown in figure) allows you to specify how variables should be centered. There are three settings for variable centering: Natural, Nodal, and Zonal. Natural variable centering displays the data according to the way the variable was centered on the mesh. This means that node-centered data will be displayed at the nodes with colors being linearly interpolated between the nodes, and zone-centered data will be displayed as zonal values, giving a slightly "blocky" look to the picture. If Nodal centering is selected, all data is displayed at the nodes regardless of the variable's natural centering. This will produce a smoother picture, but for variables which are actually zone-centered, you will lose some data (local minima and maxima). If you select Zonal centering, all data is displayed as if they were zone-centered. This produces a blockier picture and, again, it blurs minima/maxima for node-centered data.