Tangent options

By default, MotionBuilder automatically manages the slope, or interpolation, of function curves. The Auto, Linear, and Step tangent options allow you to manually change the interpolation values between keyframes in order to modify the animation. These three tangent options appear in the FCurves pane normal mode, as shown in the following figure:

Tangent options in the FCurves pane normal modeA. Auto B. Linear C. Step

The FCurves pane in Classic Mode displays all five (Auto, TCB, Stepped, Linear, and Fixed) tangent options, as shown in the following figure:

To change interpolation, select one or more keyframes, then select an interpolation option. Depending on the option you choose, the interpolation between the selected keyframes changes.

When you change the interpolation of a keyframe region, you affect the shape of the curve between the keys in the region and the next set of keys that follow the region.

For information on changing the mathematical algorithm that calculates the extrapolation of a curve, see Extrapolation.

Auto option

Auto interpolation mode automatically adjusts the tangents of an Auto key type, based upon the interpolation of the previous and next keys. How this tangent will be affected is based upon one of the Auto types, which you can choose from the FCurve Preferences window.

Auto tangent default types in the Preferences window

The following table describes the default types os Auto tangent:

Auto tangent default type	Description
Progressive Clamp	This is the default Auto type. As an all-encompassing Auto key type, Progressive Clamp is a combination of spline, clamp, and flat behavior. Characteristics of this key type: Time independent Clamp with ease-in and ease-out behavior Flat overshooting and undershooting interpolation
Spline	Characteristics of this key type: Time independent Clamp (optional)
Smooth	Characteristics of this key type: Time dependent Clamp (optional)

There are two options in the FCurves context menu that affect your use of Auto and Fixed modes. See Switch to Fixed on Edit and Reset Tangents on Switch to Auto for more information.

TCB option

Joins each keyframe using a Hermite-type curve. The slope on either side of a keyframe is changed using the TCB (Tension/Continuity/Bias) values.

Stepped option

Maintains the same keyframe value between keyframes. With Stepped interpolation mode active, there is no slope between keyframes. This option replaces the behavior of Constant in previous versions of MotionBuilder.

While working in Classic Mode, clicking the Stepped option on the FCurves window toolbar displays the Set Keys to Constant Normal and Set Keys to Constant Next options (B and C in the figure).

Tangent options A. Stepped option is active B. Set Keys to Constant Normal option C. Set Keys to Constant Next option

Set Keys to Constant Normal option

When this option is active, Stepped interpolation maintains the value of the selected keyframe until the next keyframe.

Step interpolation A. Selected keyframe B. Next keyframe

Set Keys to Constant Next option

When this option active, Stepped interpolation maintains the value of the next keyframe between the selected and next keyframes.

Step Next interpolation A. Selected keyframe B. Next keyframe

Tip: The Set Keys to Constant Next interpolation is useful for marking the end of a referential when using the Multi-Referential constraint.

Linear (Lin) option

Joins the selected keyframe and the next keyframe with a straight line. The slope between keyframes, while this mode is active, is constant.

Fixed option

Produces a cubic curve, whose slopes are not influenced by neighboring keyframes. When you edit neighboring keyframes, the tangents of a Fixed keyframe do not change. In Fixed mode, moving one tangent handle moves the other tangent equally.