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Knife Subdivide

Description

Knife Subdivide subdivides selected edges intersected by a user-drawn “knife” line. For example, if you wish to cut a hole in the front of a sphere, you select only the front edges, and then draw a line over the selected edges with the mouse. The tool is interactive, and works on primary edges, selected either implicitly by selecting all, or explicitly by box-selecting or ⇧ ShiftRMB -clicking a few edges.

Options

When you press K (or ⇧ ShiftK), a pop-up menu appears where you select the type of cut you want to make:

• Knife (Exact) (or Exact Line) divides the edges exactly where the knife line crosses them.
• Knife (Midpoints) (or Midpoints) divides an intersected edge at its midpoint.
• Knife (Multicut) (or Multicut) makes multiple parallel cuts. An additional number input is presented, allowing you to select the number of cuts.

Drawing the cut line

When using Knife Subdivide, the cursor changes to an icon of a scalpel and the 3D View header shows (Knife Tool 3DWindow header). You can draw connected straight lines by clicking LMB and moving repeatedly or you can create freehand lines by pressing and holding LMB while dragging. Also, exact cuts on the vertices can be made by holding Ctrl while cutting (snapping). MMB constrains the cut line to a vertical or horizontal screen axis.

Confirming and selection

Knife Tool 3DWindow header.

Pressing Esc or RMB at any time cancels the tool, and pressing ↵ Enter confirms the cut, with the following options:

• ↵ Enter will leave selected every edge except the new edges created from the cut.
• Ctrl↵ Enter will select only the new edges created from the cut. Note: only edges that intersect the hand-drawn selected edges will be selected.

Topology

Knife subdivide uses the same options as the other subdivide tools, located in the Editing context. If the Beauty option is toggled selected faces are only subdivided along the longest 2 sides. If both Beauty and Short options are toggled, selected faces are only subdivided along the shortest 2 sides.

Note: Using edge select mode to select only the edges you wish to subdivide creates a more accurate subdivision than using the Beauty toggle.

Examples

Exact Line Cut Type

(Exact Line before and after) is an example of using the Exact Line knife. The cut is determined from the hand-drawn line labeled “A” in the plane labeled “Drawing”.

The plane labeled “Enter” is the result of hitting the ↵ Enter key. The intersections on the edges of the plane are where the drawn line actually intersects, no matter how wiggly the line is. In addition, all the edges have been selected other than the newly created edges from the cut tool itself.

The plane labeled “Ctrl-Enter” is the result of hitting Ctrl↵ Enter. In this case only the newly created edges, “B” and “C”, are selected while edge “D” is not. “D” is a secondary edge added as a side effect of the cut tool.

Exact Line, before and after.

Midpoints Cut Type

(Midpoints before and after) is an example of using the Midpoints knife. The cut is determined from the hand-drawn line labeled “A” on the plane labeled “Drawing”. Notice how the line labeled “A” intersects the right edge twice; only the first intersection will be considered during the cut.

The plane labeled “Enter” is the result of hitting ↵ Enter. The intersections on the edges of the plane are the mid points of each edge, regardless of where the line was drawn. All the edges have been selected other than the newly created edges from the cut tool itself.

The plane labeled “Ctrl-Enter” is the result of hitting Ctrl↵ Enter. In this case only the newly created edges, “B” and “C”, are selected while edge “D” is not. “D” is a secondary edge as a result of the cut tool.

Midpoints before and after.

MultiCut Type

This cut type presents a popup dialog asking for the Number of Cuts, which defines how many equally spaced cuts the tool should make for each intersecting edge. For example, the default of 2 generates two intersections or three new edges for each intersection of the hand-drawn line.

(MultiCut before and after) is an example of using the MultiCut knife. The cut is determined from the hand-drawn line (“A”) on the plane labeled “Drawing”, while using the default 2 as the number of cuts. The line was drawn so that it deliberately intersected three edges.

The grid labeled “Enter” is the result of hitting ↵ Enter. There are two cuts equally spaced on each edge intersected by the hand-drawn line, labeled “A”, “B”, “C” and “D”. The cut tool do not produce any secondary edge here. The grid labeled “Ctrl-Enter” is the result of hitting Ctrl↵ Enter. In this case only the newly created edges are selected.

MultiCut before and after.

Limitations & Workarounds

The cut lines can be drawn with any number of segments, but only one intersection is detected per edge. Crossing back over an edge multiple times does not perform additional cuts on it. Snap to grid is not currently implemented, but is being looked at for future releases.

Optimizations

With a large mesh, it will be quicker to select a smaller number of vertices, those defining only the edges you plan to split since the Knife will save time in testing selected vertices for knife trail crossings.