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## Setting the seed

Figure 41

The OnionPeel operator uses a seed cell or a seed node as the seed to which all cells from other layers are added. When a layer is added around the seed, the new cells are those immediately connected to the seed. You specify the seed as a cell index or a node index by typing a new seed value into the Seed# or ij[k] text field. VisIt interprets the seed as a cell index by default. If you want to start growing cell layers around a given node, click on the Node radio button before entering a new seed value. The form of the seed index depends on how the underlying mesh is organized. Unstructured meshes, which are a collection of independent cells, require only a single integer value for the seed while structured meshes are indexed with i,j or i,j,k indices depending on the dimension of the mesh. To set the seed using i,j,k indices, type the i and j and k indices, separated by spaces, into the Seed# or ij[k] text field.

Some meshes that have been decomposed into multiple smaller meshes known blocks or domains have an auxiliary set of cell indices and node indices that allow cells and nodes from any of the domains to be addressed as though each domain was part of a single, larger whole. If you have such a mesh and want to specify seed indices in terms of global cell indices or global node indices, be sure to turn on the Seed# is Global check box.

The OnionPeel operator can only operate on one domain at a time and when the operator grows layers, they do not cross domain boundaries. The seed cell index is always relative to the active domain. To make a cell in a different domain the new seed cell, change the domain number by selecting a new domain from the Set drop down list.