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First of all, thanks to everyone for the effort needed to create this phenomenal app, maintenance it and answering questions.

How to Bisect a mesh into two parts but filling both halves?

I try to split Mesh Into two Parts but only one part get filled. I use Bisect tool with fill option checked then: select ->Select Loops ->Select Loop Inner-Region (select bigger), then Mesh -> Seperate -> Section.

In the end I get two parts of which only one is filled (bigger one).

I know that I can create a face manually, but the problem is that I have a very complex mesh and it will take me days to fill it in manually with my current knowledge in Blender (select edge->fill)

is there any simpler or automated way to do it in Blender 2.92.0?

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  • $\begingroup$ Have you considered selecting edge loops? $\endgroup$
    – TheLabCat
    Jun 18, 2021 at 23:50
  • $\begingroup$ You can duplicate your mesh and use a boolean modifier with a cube that intersects your object where you want to cut it. Then you set the boolean modifier to difference on one copy and intersection on another. $\endgroup$
    – Alexandre Marcati
    Jun 19, 2021 at 1:39
  • $\begingroup$ @ZargulTheWizard: In some cases this tool doesn't work as expected. $\endgroup$
    – Lukasz-40sth
    Jun 19, 2021 at 7:12
  • $\begingroup$ Drop us the screenshot of this mesh. $\endgroup$
    – Lukasz-40sth
    Jun 19, 2021 at 7:17
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    $\begingroup$ Related blender.stackexchange.com/a/133136/15543 $\endgroup$
    – batFINGER
    Jun 19, 2021 at 11:57

3 Answers 3

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Bisect

doesn't have this feature (hard to say why). The work around can be - duplicate object, do bisect with option Clear Inner, write Plane Point and Plane Normals values from Properties panel somewhere (or automate with python). Than to bisect duplicated object as Clear Outer with the same bisect's plane values.

Copy&paste Scripting

Instead of rewriting bisect's plane values manually you can copy&paste all values at once into text editor to automate this process, reuse again or use with another object.

  • First Duplicate your object and bisect original.

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  • Switch to Scripting Layout and in Text Editor create a New text file, type import bpy, it will call python.

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  • Than right-click Bisect Tool icon (in Tool Shelf) and choose Copy Python Command ...

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... and paste command on second line.

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Lastly go to Info Editor and copy bisect's plane values (whole line of setup) ...

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... and paste it on the third line. Result should look like this ... just change clear_inner to clear_outer in text. With all vertices selected of duplicated object, Run script Alt+P.

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Like that you have under one click action that can be repeated any time ... or you can duplicate text file and change some parameters, like to have one script for inner and second for outer bisect or to have a cut in horizontal direction and other one in vertical.

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    $\begingroup$ It's a mystery to me why this isn't an option. As far as the UI is concerned, it's not even an extra. A duped face would be the entirely appropriate behaviour for 'Fill' checked, without any clearance. Right-Click-Select post coming up, I think. $\endgroup$
    – Robin Betts ♦
    Jun 19, 2021 at 8:49
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    $\begingroup$ Right ... even more ... I just noticed bisect is not split into a two parts ... that is what I would expect with Fill option (splitted and filled on both sides) Who would wanted to have a filled face inside a mesh? $\endgroup$
    – vklidu
    Jun 19, 2021 at 9:21
  • $\begingroup$ ..which is redundancy, with the Knife tool... $\endgroup$
    – Robin Betts ♦
    Jun 19, 2021 at 9:32
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    $\begingroup$ Thank you @vklidu for this detailed explanation and on your time. Method Bisect – Scripting works perfectly. It is especially useful since my Mesh has to be cut into several pieces $\endgroup$
    – Sladjan Ilic
    Jun 20, 2021 at 12:25
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It's hard to say how complex your object is ... so if Loop selection Alt+Shift or Box/Circle selection is not working for you to easily Fill ... in some cases you can try ...

Boolean

... but it can fail for very complex meshes.

To split object in one go ... you can add Plane object with Solidify modifier, Thickness like 0.001 (1mm) as cutter ...

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For object to be cut add Boolean modifier, Object > Plane ...

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... Apply Ctrl+A modifier, select and Separate P half of the object.

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Of course if you don't care for this 1 mm gap between parts :)

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I'd like to add another way of doing this - vertex groups. I did this in Blender 3.1,it should however be possible to do it in 2.93 as well according the the manual.

After making the cut with the bisect tool, but before separating the meshes, add the still selected vertices to a vertex group. Don't forget to select fill in the bisect widget!

Adding the selected vertices to a vertex group

Afterwards, continue like in the question:

Select -> Select Loops -> Select Loop Inner-Region

Mesh -> Seperate -> Section

Now that you have two objects, select the one with the missing face. Go back into the vertex groups, select the group you just created and click "Select". The same vertices that were just cut should be selected and pressing F (New Face from Vertices) and you're done.

Selecting the cut vertices again from the vertex group. Don't forget to switch to Edit Mode

Filling the missing face with F after selecting the vertices via the vertex group

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