Blender's Select Boundary Loop operator allows us to isolate the boundary edges of the currently selected geometry.
Video Transcript
Blender's Select Boundary Loop operator allows us to isolate the boundary edges of the currently selected geometry.
This will select not only the outer edges, but also all the edges of the holes in the selected portion.
Let us look at a couple of examples of its use in combination with the Individual Origins transformation mode.
To use this operator, we first select only the portion of the mesh on which we are interested in using it. In this example, for clarity, I am hiding the parts of the object that we are not interested in.
Let's switch to Edges selection mode, select all the geometries present, and use the Select Boundary Loop operator.
To remove the boundary from the selection, we do Shift, Alt and right-click on an edge of the boundary.
We now switch to Individual Origins transformation mode.
Now we can operate individually on the perimeters of the holes in this geometry. For example, we can make them more circular with the Circle operator in the Loop Tools group.
Or we can resize them by Scaling, which in this case will be done for each hole around its circle.
Don't forget that you can add these vertices to a vertex group to quickly select them later and perform any operations you can think of.