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## The role of expressions

Database comparison is accomplished by creating new expressions that involve the contents of multiple databases using VisIt's expression language. Database comparisons use a special expression called conn_cmfe, which is capable of mapping a field from one mesh to another mesh using a mesh connectivity-based approach. The name "conn_cmfe" means connectivity-based common mesh field evaluation (CMFE) and as the name implies, the expression takes fields from one mesh and maps the field onto another mesh by taking the cell or node-centered values on the donor mesh and mapping them onto the cells or nodes having the same indices in the new mesh. Mismatches in mesh sizes result in not all data values being used or, alternatively, VisIt can pad the remapped field with zeroes. The conn_cmfe expression can be used to map fields from one database onto a mesh in another database, which then allows you to create expressions involving the active database.

There are plans to add position-based CMFE, which will resample the field from one mesh onto another mesh by calculating the values of the field on the first mesh using the locations of the cells or nodes in a second mesh.