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I'll tell you the whole deal so maybe you can help me with ideas at different points.

First. I generated a terrain using Sketchup and the Location tool. I had to use a bigger section than what it was possible at 1 time, so i used many (20 or so) terrains and "stitched" them together

Second. I imported everything to Blender as a collada file.

Now. In Blender, i need to make a section to one of the mountains to show (in a later animation) what's going on inside it.... and that's what have been impossible.

I've tried to simply "boolean" it with a plane. and the operation can't be executed... I admit i'm a total newbie... but i thought i should work....

screen-capture of the awful moment

If this is not the proper way to do something as simple as this, can anyone tell how would you do it?

Messy Mesh after trying to "Select Loop Inner Region" Messy Mesh after trying to "Select Boundary Loop"

So, after tryin to put in practice the solutions you kindly lend to me, i keep having a problem. My favorite solution is with the bisect tool... but i can't find a way to keep both halves in a clean way after the bisection.

Here's an useful question, but i couldn't put into practice, apparently because I'm using a mac and the latest version of Blender, so the commands were not the same... maybe you could help me "translating" those commands.

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  • $\begingroup$ Which command did you have problems with? $\endgroup$ – gandalf3 Dec 20 '13 at 0:27
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You need a manifold (watertight) mesh for the boolean operation to work. Try replacing the plane with a cube:

enter image description here enter image description here

Another option is the Bisect tool:

  1. In edit mode, press Bisect in Tool Shelf (N) > Add:

    enter image description here

  2. Then draw the bisection line with LMB and drag:

    enter image description here

  3. Enable Clear inner or Clear outer (in the Redo menu at the bottom of the Tool shelf, or the F6 menu) to remove one side of the mesh as defined by the bisection line (also enable Fill to fill the hole):

    enter image description here

If you want to keep both sides of the bisect cut as separate objects, you could try either of the following:

  • Manually separating the bisected parts of the model:

    1. Bisect the mesh without enabling Fill or either Clear option, so it only makes a cut down the bisection line (the same as if you had used the knife tool):

      enter image description here

    2. Select one side of the mesh with Select Loop Inner-Region in 3D view > Header > Select > Select Loop Inner-Region:

      enter image description here

    3. Press PSeparate > Selection:

      enter image description here

    This will give you two objects, one for each side of the bisect operation.

    1. To fill in the hole, you could select the edge loop on the very edge with AltRMB and fill (F), however if the bisect line goes across any interfering geometry this will not work. (This might also be too limited for meshes that have an internal structure)
  • Or you could duplicate the mesh and run two bisect operations, one on each.

    1. Bisect one mesh and enable Fill and Clear Inner.

    2. Pull down the info window and scroll to the bottom. You should see a line which begins like this:

       bpy.ops.mesh.bisect(
      
    3. Select it with RMB and press CtrlC to copy it:

      enter image description here

    4. Select your duplicate mesh and enter edit mode (Tab).

    5. Split a new area, set the editor type to Python Console and paste the selected line. Note that you may need to add use_fill=True, and clear_outer=True, in between the parenthesis ().

    6. Press Enter. The mesh should be automatically bisected identically, except the opposite side is cleared.

Note that you can also set the exact coordinates from the UI:

enter image description here

So if you wanted to manually copy each value from one operation to the other, that would be another way.

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  • $\begingroup$ Thanks!, I think part of the problem is that in my file, the mesh and the "walls" of the terrain are not the same object. This was set this way for mapping reasons... i need them to have different textures later. so i duplicated the mesh alone and the intersection (with a cube) started to work. but its messy. Now i'm trying with the bisect tool. Thanks, Your answer has been really insightful. $\endgroup$ – invicente Dec 16 '13 at 21:43
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    $\begingroup$ @invicente You can have different textures on the same object, see this post $\endgroup$ – gandalf3 Dec 16 '13 at 21:46
  • $\begingroup$ Cool!, one final question: best way to paste them together? I think that "join" may not be perfect.... THANKS AGAIN! $\endgroup$ – invicente Dec 16 '13 at 21:50
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    $\begingroup$ @invicente Ctrl+J> join, A> Select all, and then W> remove doubles $\endgroup$ – gandalf3 Dec 16 '13 at 21:59
  • $\begingroup$ I have a concern.... (yes i'ms still stuck in this problem) First of all, my mesh of the terrain is messy, not neat like yours... So what would be the best way to keep both parts (after using bisect, i.e), because both methods are difficult to maybe copy and redo... I' don't know if i'm being clear, maybe i'd post a picture to an answer... any ideas? $\endgroup$ – invicente Dec 19 '13 at 19:02

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