Blender Knife Subdivide Tutorial

This is a tutorial for the new "knife subdivide" (KS) tool added in Blender in rev 2.30.

It is somewhat misnamed. Probably its most usefull application is not to simply cut a mesh into smaller pieces, but to selectively allow additions of geometry (subdivisions) to a mesh.

The story behind this tool is simple. I bought a book (Ken Brilliant's "Building a Digital Human"). It was constantly saying "add more detail by splitting this row or cut an extra layer of edges around the lips...

For a long time Blender has lacked a way to cleanly and selectively subdivide a mesh. But this is Open Source, so no more.

Currently, KS only works for Meshes.

KS works by subdividing edges if they both verts are selected and the edge is intersected by a user-drawn knife line. This mode was chosen, since the knife line exists only in the 2D space of a 3D window. There is no way to modify the selection before the cut is made. By requiring both you can modify your selection before you cut. For example, if you wished to cut a hole only in the front of a sphere, you could select only the front verts, and then invoke KS.

So add a mesh and go ahead and use "A" to select all the verts. With a large mesh, it will be quicker to select a smaller number of verts, since KS tests all selected verts for knife trail crossings.

The Hotkey to invoke KS directly is "Shift-K" in mesh editmode.

You are then prompted to choose the type of cut. "Exact" will divide the edges exactly where the knife line crosses them, "Centers" divides them at the edge's midpoint. For this cut, I chose "Centers".

Once the type is selected, the cursor will change to a pencil.*

When the cursor changes, Click the Left Mouse Button (LMB) to begin drawing a polyline knife trail.

*At the time of this writing it changes to an arrow under Windows. Improved cursor support for all platforms is in the works.

Move the mose and click again to draw a segment.

The polylines can be drawn with an arbitrary number of segments, but The intersection routines only detect one crossing per edge. Crossing back over an edge mulitple times does not perform additional cuts on it.

MMB constrains drawing to an axis as expected. Snap-to-Grid is not currently implimented, but is being looked at for future releases.

When you are done drawing your line, hit enter to confirm the cut. ESC at any time cancels the operation.

The "Centers" cut divides each of the intersected edges in the center.

Notice that the subdivide routines have been modified to produce fewer triangles. Under the old subdivition method, the corner face would have been subdivided into 4 tris. Under the new method, we get 3 quads.

Cutting an odd number of edges still yields tris, as this is mathematically impossible to avoid.

Both knife modes can also be invoked from the new "Loop/Cut menu" mapped to the K Key in Mesh Edit mode.

For the next example I will choose "Exact".

As the name of the option implies, it divides the mesh where the mouse trail crosses the edges.

Note hovever, that UV mapping information will not be faithfully preserved with "Exact" cuts. It is preserved for "Centers" cuts.

Holding the LMB down while moving the mouse will allow you to draw a freehand line. Freehand and polyline segments can be freely mixed, since a freehand segment is really just a bunch of 1 pixel long polylines.

Subdivision still only occurs at edges. If you want your slice to be smooth, you need to start with a relatively finely subdivided mesh.

While the tool can be used in combination with the PKey to cut odd shaped chunks out of a mesh, it was primarily designed to allow the adding of geometry to concisely cotrolable areas of a mesh.

It was used here twice to increase the detail around the eyes and mouth to allow further modeling of this crude head.