Knife projection is a non-interactive tool where you can use objects to cookie-cut into the mesh rather than hand drawing the line. The outline of selected objects that are not in Edit Mode are used to cut the mesh along the view axis of objects that are in Edit Mode. Afterwards the resulting geometry inside the cutters outline will be selected. Outlines can be a wireframe or boundary edges (i.e. the unconnected edges of a mesh).
The primitives, being manifold objects, do not have wireframe or boundary edges. In the case of the cube, deleting the top face will result in cutting edges. Select Non-Manifold (Wire, Boundaries) will highlight the cutting edges of mesh objects.
To use Knife Project, in Edit Mode, select the cutting object (Ctrl-LMB) and choose.
3D Viewport Alignment to adjust the projection axis.
- Cut Through
Projects the cut through the entire mesh.