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## Plotting the difference between two databases

Simulations are often run as parts of parameter studies, meaning the initial conditions are changed slightly to see what observable effects the changes produce. The ability to do direct numerical comparison of multiple simulation databases is required in order to observe the often miniscule differences between the fields in the two databases. VisIt provides the conn_cmfe expression, which allows you to map a field from one simulation database onto a mesh from another simulation database. Once the mapping has been done, you can then perform difference operations using an expression like this:

<mesh/ireg> - conn_cmfe(</usr/local/visit/data/dbB00.pdb:mesh/ireg>, mesh)

The expression above is a simple difference operation of database A minus database B. The assumption made by this expression is that database A is the active database and we're trying to map database B onto it so we can subtract it from database A's mesh/ireg variable. Note that the conn_cmfe expression takes two arguments. The first argument encodes the name of the file and the field that we're mapping onto a mesh from the active database, where the mesh name is given by the second argument. In this example, we're mapping database B's mesh/ireg field onto database A's mesh. figure shows a picture that illustrates the database differencing operation.

Figure 17