Cutting a mesh in half I have a mesh which I have imported from an OBJ file. I would like to apply Material A to the section of the mesh which is on one side of a given plane, and apply Material B to the section of the mesh which is on the other side of that plane. A potential plane is indicated here as a blue line. Note that it does not necessarily intersect the centre.



To do this, I need to split the any faces which intersect this plane, and select all faces on one side of the plane so that I can apply a material to them. How can I do this in the current versions of Blender?



I've tried using the K knife tool in Z cut-through mode, but the cuts haven't been reliable. Even when they were, after the cut was finished I  couldn't figure out how to select only the faces on one side of the cut. (The details were too fine to be reliably selected with any tools I know, such as the C circle select tool.) I tried using the B selection tool, but it only selected the faces on the near-side of the mesh.

We used to have a procedure to cut a mesh using a plane in Blender 2.4, but it involved using mesh boolean tools which appear to have been replaced, and I can't figure out how to adapt it.
I would recommend using the bisect tool (new in Blender 2.69):


Enter edit mode (Tab), then press Space and type in Bisect, or select Bisect from the toolshelf (under Add).
Click and drag, to bisect the object.

Press F6 to bring up the options for the tool, and select clear inner, or clear outer depending on which half you don't want.



If you don't want to delete half, but just want to select it:


Instead of step 3, press H to hide the loop:

Hover over the half you want and press L to select linked vertices under the cursor:

Unhide everything (AltH), leaving you with one half of the object selected: