From BlenderWiki

Hooks

Hooks are similar to shape keys in that they deform a mesh over time (frames). The difference is that hooks make it look like the mesh is snagged with a fish hook. Moving the hook moves selected vertices under the influence of the hook (which is usually just an empty – but might be any object), and you make the hook move by animating the motion of the empty through Ipo keys or any other animation method. As the hook moves, it weighted-pulls vertices from the mesh with it. If you have used proportional editing, you can think of it as animated proportional editing. While hooks do not give you the fine control over vertices movement that shape keys do, they are much simpler to use.

Object Hooks

Description

Hooks give access at object level to the underlying geometry of meshes, curves, surfaces or lattices. A hook is an object feature and it is like a parent to an object, but for vertices. You can create as many hooks to an object as you like, and assign for each hook vertices that will be affected. Overlapping hooks is also possible, here a weighting factor per hook is provided that determines the amount each hook will affect the overlapping vertices.

All object level options and transformations are possible on the hook object, including using hierarchies, constraints, Ipo and path animations. You can also make the hook-parent a child of the original object if you don’t want object transformations to deform the hooks.

Examples

A typical example of using hooks is to animate vertices or groups of vertices. For example, you may want to animate the vertices associated with a “Mouth” on a character’s face.

In (Animated face frame 1) and (Animated face frame 10) a face made from Bézier curves has two hooks applied. One applied to a control-point on the mouth labelled “A” and one applied to the eyebrow labelled “B”. The animation has 10 frames over which hook A moves up and hook B moves down.

Animated face frame 1.

Animated face frame 10.

Adding Hooks

Description

Since hooks relate to vertices or control points, most editing options are available in Edit mode for meshes, curves, surfaces and lattices (not texts, as you might have guessed!).

Options

Hooks menu.

Add, New Empty

Adds a new hook and creates a new empty object, which will be the hook controller, at the center of the selection.

Add, To Selected Object

When another object is selected (you can do this in Edit mode with CtrlRMB ), the new Hook modifier is created with that selected object as hook controller.

Editing Hooks

Description

Once hooks are available in an object, the Hooks menu will give additional options:

Options

Extended Hooks menu.

Remove...

This will give a new menu with a list of hooks to remove.

Reassign...

Use this if you want to assign new vertices to a hook.

Select...

Select the vertices attached to a specific hook

Clear Offset...

Neutralize the current transformation of a hook parent.

Hook Modifier

Description

Hooks are modifiers, that are added to the modifier stack. For each Hook modifier, you can give a hook a new name (the default name is the hook controller name, prefixed with “Hook-”), give it a new controller, or assign it a Force weighting factor.

Options

In the Editing context (F9), Modifiers panel, when a hook is created, you can control it via this panel.

Ob

The parent object name for the hook. Changing this name also recalculates and clears offset.

Reset

Recalculate and clear the offset transform of hook.

The same as the Clear Offset... option from Hooks menu (CtrlH).

Recenter

Set hook center to cursor position.

Select – Edit mode only

Select affected vertices on mesh.

The same as the Select... option from Hooks menu (CtrlH).

Reassign – Edit mode only

Reassigns selected vertices to this hook.

The same as the Reassign... option from Hooks menu (CtrlH).

Force

Since multiple hooks can work on the same vertices, you can weight the influence of a hook this way. Weighting rules are:

• If the total of all forces is smaller than 1.0, the remainder (1.0 - (forces sum), will be the factor the original position have as force.
• If the total of all forces is larger than 1.0, it only uses the hook transformations, averaged by their weights.

Falloff

If not zero, the falloff is the distance where the influence of a hook goes to zero. It currently uses a smooth interpolation, comparable to the proportional editing tool.

Examples

Hook modifier on lattices