The Warp Tool is used to re-shape or deform a region of a mesh. Unlike many of our selection-based tools, currently Warp requires a single connected input selection (although it can be an arbitrarily-shaped region, including with interior "holes"). The way Warp works is that you drop control points and then move those control points, and this movement is transferred to the selection. If you have ever manipulated a NURBS patch via control points, this is very similar except that instead of a fixed grid of control points, you can put one anywhere.
The images below show an example. To drop a control point you left-double-click on the selection, and a red sphere will appear. You can only place control points at vertices. You can then move the control point by left-click-dragging. Currently we do not have our 3D transform widget in this tool, so movement happens in a screen-aligned plane. As a result you will likely need to rotate the camera to check and/or tweak the position. (We agree, this is not great...).
As you move a control point, the surface will be "pulled" with it, as in the rightmost image above. To delete a control point, left-double-click on it and it will disappear. If you remove a control point, the surface is no longer pinned at that location, and so it will likely move. Note that undo does work for these control points.
The power of Warp really comes from setting up sets of control points which allow you to create controllable deformations. In the example below I have added two more control points to the image above, and then moved all three to create a more complex shape.
Note that the effect of each control point is not strictly localized. In the leftmost image below I have added a single control point and then moved it upwards. Notice how the entire selection has moved upwards, including the tips of the ears, quite far from the nose. I can add more points to pin down the ears, limiting their movement. However, ultimately each control point affects the entire surface, so the entire selected area will change slightly.
Warp also works with open boundaries. In the example below we have a meshed hexagon on the left. By dropping a few constraint points, we can deform it into a different shape. This can be used to create quite interesting forms, as shown on the right (perhaps a roof?). Note that currently it is not possible to deform the interior while keeping the boundary fixed.
Snap to Pivots
Like in many other Tools, Pivots can be used to precisely define 3D locations of your control points. Simply create a control point and then drag it onto the pivot, and it will snap to that location, as shown in the example below.
Tips & Tricks
The Warp tool is one of the few tools in Meshmixer that does not do any remeshing internally. So, when you Warp a selection, you are moving the vertices but not changing their connectivity. This is useful in some cases where you wish to explicitly preserve an input mesh. Also, Warp will do a great job of preserving texture maps.
The Warp tool can be very sensitive to mesh quality. If your mesh contains tiny slivers or degenerate triangles, Warp can fail, in which case the selected area will disappear. In rare cases Meshmixer may even crash (unfortunately this is out of our control, the crash occurs in a third-party library). If this occurs, a Remesh will nearly always resolve the issue.