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I'm a 100% newbie when it comes to Blender, only needing it for a single task (for now, might return later to learn more).

I have a large scene involving many objects and I need to cut out a small cube of it. I've hit Alt-B and selected a cubic region. Now I want to export (obj at the moment, but I'm somewhat flexible as to format) only those visible faces, even if they were part of a larger object.

Alternatively, reversing the selection and deleting all the other faces would also seem like a possible solution, allowing me to export the remainder—my selection.

There's "Export Selection" in the export dialog, but that seems to still export the non-visible faces of the objects that intersect my chosen selection cuboid—which ends up being pretty much everything.

I don't need lights, cameras, or anything else except the geometry. I don't even need textures, though the texture names might still be of use to identify different parts of the data, so preserving the materials would help.

Any suggestions would be appreciated, especially if written for a newbie. Though I'll learn what I have to if given a push in the right direction. My Google searches have so far failed me in solving this problem.

EDIT: adding an image of the masked area I want to limit my export to: portion of terrain to export

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    $\begingroup$ Alt+B creates only a visibility mask so it not affects the export. You can separate the geometry by select faces what you want in Edit Mode then hit p and choose "By Selection". This will create a new object. Select it in Object Mode, then you can export only the selected. $\endgroup$
    – FFeller
    Commented Feb 5, 2020 at 19:04
  • $\begingroup$ I guess my problem then is going from a visibility mask to a selection of faces. This may be made more difficult by the fact that the forest which makes up a part of the desired area is procedurally generated with, if I'm getting my terminology correct, an Emitter. I'm not sure how to how to make a face selection over such a large number of objects. It seems that if I select the same visible area in Edit Mode, it is still selecting the non-visible faces associated with the currently visible faces—i.e. the whole forest and not just the portion I want to keep. $\endgroup$
    – Chris Innanen
    Commented Feb 5, 2020 at 20:12
  • $\begingroup$ Would be good to edit Q, add info about emitter there, someone can come with better idea. Also ... its better describe only what you have and what your desired result should looks like. there isn't necessary to describe all you tried or were you stacked. Q is than shorter = easier understand, more attractive for audience. $\endgroup$
    – vklidu
    Commented Feb 5, 2020 at 21:52

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If trees are particle instances you will have to make them real first...

enter image description here

than delete all unwanted meshes in object mode to make first clean up, than select all (the rest of objects you want to export and if there is a lot of unwanted faces Join all objects (Ctrl+J), because like now, trees objects shares same object data (that can't be edited separately). If you are able to simply go to edit mode, select faces and delete - great, you can stop here and export.

If you have some large faces out of your range you can cut them. Like ...

Go to Top View, create a new plane object on top of all objects, in size of area you want to export.

enter image description here

Select new plane first, than your joined object, switch to edit mode and search for Knife Project ...

enter image description here

in operator panel set Cut through

enter image description here

Select (B) the rest of vertices you want to keep ...

enter image description here

... invert selection (Ctrl+i) and delete.

enter image description here

Export result

enter image description here

but ... for a huge forest scene it can be killing process :)

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  • $\begingroup$ You weren't kidding about how slow converting a whole forest from particles to real objects is—I had to kill Blender after about an hour. I'm going to add a better video card and double my memory and try again later using your instructions. I don't mind if the edges of my cuboid are rough from trees hanging over the edge. It doesn't have to be clean, just much smaller than the original scene. (The output will go into a program of my own creation to generate simulated LIDAR data.) $\endgroup$
    – Chris Innanen
    Commented Feb 5, 2020 at 21:16
  • $\begingroup$ so only make real, don't join them, otherwise it become heavier ... tip - depend on situation, you can get calculation lighter if you delete all faces of emitter object, that are out of your area, so trees on these faces are gone = less trees to made real (but you will have to decrease number of particles as well and trees change their position) ... $\endgroup$
    – vklidu
    Commented Feb 5, 2020 at 21:42
  • $\begingroup$ That's actually what I did. :) I figured out how to adjust the area in which the trees were being generated and reduced it to my target area. I didn't need to use the Knife technique—deleting the remainder of the unwanted elements was enough. I now have a (smaller than depicted above) segment of the scene loaded into my own program. It's slow, so I might trim it more, but it's success. Thank you for your kind assistance! $\endgroup$
    – Chris Innanen
    Commented Feb 5, 2020 at 22:45

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