# Preparing for Analysis: Selecting Boundaries of an FE Mesh - FE Boundary by Flood Fill

This article describes what the FE Boundary by Flood Fill block does and how you can utilize it to select boundaries of your FE Mesh.

Click here to jump down to the brake pedal at the end of the article.

## Overview

FE Boundary by Flood Fill is used to select surface entities on an FE Mesh. The entities available to be selected are nodes, faces, or edges.

When a direction is defined Flood Fill shoots a ray at the mesh and finds the intersection. The first intersecting face (face0) becomes the basis for Flood Fill. The Flood Fill compares the normal of face0 with all the neighboring faces which are below the tolerance angle provided by the user and selects it. When a direction is not provided it finds the closest face to the origin and that becomes the face0.

## No Directional Input:

Angle: Default
Direction: Undefined (Optional input)

Depending on where the origin point is, the Flood Fill operation will choose the closest entity to the origin as the seed entity (face0) on the surface when no direction is defined. Watch below to see this behavior in action.

## With Directional Input:

Angle: Default
Direction: Defined

When you define the direction with a vector, the Flood fill operation will shoot a ray from the origin in that direction to choose the seed entity (face0).

For example, if you choose the vector as 0,0,1 (x,y,z) then the closest intersecting surface mesh entity only in the positive Z-direction will be chosen.

If the origin point doesn't intersect with an entity in that direction, you will receive an error that says "No mesh entities were found". Watch below to see how Flood Fill with a defined direction works.

## Angle

The angle defines how many entities to select by comparing the normal of face0 (seed entity) with all neighboring faces. If the faces are below the tolerance angle they will be selected. It is very useful for selecting entities on curved surfaces.

The default unit for the Angle input is in Radians. To use degrees, simply add 'deg' to the angle input.

If the angle is x degrees, the Flood Fill will choose all entities that have an angle of up to but not including, x degrees between them and the normal of face0. Watch below to see this behavior.

## Brake Pedal Example

Let's use the FE Boundary by Flood Fill to select the hole in the Brake Pedal. This will choose the area for the Displacement Restraint Boundary Condition in further articles.

• Add an FE Boundary by Flood Fill block
• Input the existing FE Mesh into the Mesh input
• Choose 'Nodes' as the entity
• Set the origin to (0,0,0)

Move on to the next article in this series and learn how to create Boundary Conditions

Labels: Mesh FE select analysis face simulation surface Flood Fill Boundary entity node edge selection region prepare