How to select part of a mesh by using a helper plane?

Question

I would like to select part of a mesh using a plane as a "boundary indicator".

In the screenshot below that would be the mesh part above the plane:

I can get the desired selection by using Bisect tool (Bisect, hide, select a vertex, select linked, unhide):

however, this is visually not as convenient as using a plane (this is only one step in a longer workflow which has to be simplified). Beside that, Bisect cuts trough the whole object, what is inconvenient (notice the bisect-line cutting another part of the mesh in lower right).

I know I can use Boolean modifier to get only the part above the plane, but I do not want to cut it off - I just want to select it for editing.

The "transition line" does not have to be straight (i.e. adding vertices and edges), it can use existing edges (resulting in a jagged "transition line").

Edit:
What I meant is to use the plane in a similar way it would be used in Boolean modifer, just not to "cut off" but to select that part of the mesh.
The goal is to automate the process, e.g. operator positions the plane and clicks on a button (to run a script that does the rest; I can write the script once I know the steps).

Answer 1

1. Lasso Select:

You can use Lasso Select with Ctrl + LMB.

Here a related Post for types of selection.

Limit selection to visible should be disabled to select all vertices in the Lasso area.

Image by iKlsR♦ answer

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2. Object as Camera:

This solution involves a 2nd Object which is perpendecular to the Plane and use it as a Camera. That makes it easier for a Border Selection and the 3D Cursor is used for the orientation.

• Position the Plane at the desired Location.
• Dublicate the Plane with Shift + D in Object Mode
• Rotate the duplicated Plane by 90° around the local X-axis. R + 2xX + 90

• Translate the rotated Plane along the local Z-axis with G + 2xZ.

  
  
  
  

• Select the first Plane and press Shift + S > Cursor to Selected.

• Select the second Plane and press Ctrl + Numpad 0 to make the Plane the active Camera.

• Select your Object, toggle to Edit Mode and Border Select B the desired Vertices. The 3D Cursor helps where to stop with the selection.

  Vertices selection successfully

Answer 2

Use Clipping Border to hide everything except for the desired part of mesh.

1. Select any face which is directed roughly under the same angle as you'd like your selection to be.
2. Enter front or side ortho view regarding that selected face via Shift+Numpad1 or Shift+Numpad3.
   This is done to ease the process of finding correct angle for selection.
3. Enable Clipping Border tool with Alt+B and draw desired area which will be the border of the selection.

4. Select with Border or Circle Select tools. Once selection is done disable Clipping Border with the same command, Alt+B.

   

Answer 3

I was able to do this using Boolean modifier to get two halves then joining them back.

This is the procedure (assuming a Plane is already in place):

1. Duplicate the original object
2. Add Boolean modifier to the dupe, Operation: Difference, Object: Plane, and Apply it (this leaves the part above the plane)
3. Switch to Edit Mode, select all
4. Add new Vertex Group, Assign it
5. Switch to Object mode, select original object
6. Add Boolean modifier to the original object, Operation: Intersect, Object: Plane, Apply it (this leaves the part below the plane)
7. Do Shift + select to add the top part to selection and join them (Object -> Join)
8. Switch to Edit Mode, select all
9. Do the Tools -> Remove Doubles (this "fuses" the halves but leaves one face between the two halves)
10. Do the Select -> Non Manifold to select the edge of the extra face
11. Do the Delete -> Face
12. Go to the Vertex Group assigned above and Select it

The result is this:

It may be a lot of steps but it is fully automatable.

Note that selecting non manifold edges and then deleting the faces may not be an option for automation if you have non manifold edges elsewhere.