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## Plotting surface normals

VisIt can use the DeferExpression operator in conjunction with the ExternalSurface operator and the surface_normal expression to plot surface normals for your plot geometry. To plot surface normals, first create a vector expression using the surface_normal expression, which takes the name of your plot's mesh as an input argument. Once you have done that, you can create a Vector plot of the new expression. Be sure to apply the ExternalSurface operator first to convert the plot's 2D cells or 3D cells into polygonal geometry that can be used in the surface_normal expression. Finally, apply the DeferExpression operator and set its variable to your new vector expression. This will ensure that the surface_normal expression is not evaluated until after the ExternalSurface operator has been applied.