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Choosing a Function Curve Interpolation Type

Function curve interpolation is a way of estimating how animation progresses between keys. When you set two or more keys for a parameter, Softimage determines the interpolation between them.

By default, fcurves use a spline interpolation which creates a smooth ease-in and ease-out transition at each key. However, if you want to change the nature of the animation, you can switch to constant or linear interpolation.

To set the interpolation type

1. Select the keys (or region) between which you want the interpolation. If you select an fcurve, the interpolation is applied to the entire curve.

2. If you’re going to use Spline or Linear interpolation, open the Fcurve Editor Preferences property editor and select the Apply spline/linear interpolation to both sides of selected keys option on the Keys page. This makes both slopes of the selected keys use the same interpolation type.

This option doesn’t apply to Constant interpolation because you usually want it to hold the value on only one slope at a key.

3. Do either of the following:

- Choose an interpolation option from the Curves menu on the fcurve editor’s command bar.

or

- If you have selected keys, you can also choose Keys > Key Properties and select an Interpolation option in the Key Properties editor.

4. Select one of these interpolation options:

- Spline Interpolation is the default. It uses a curved spline interpolation that accelerates and decelerates into (ease in) and out of (ease out) each keyframe, resulting in a smooth transition.

The degree of acceleration and deceleration before and after the keyframe is determined by the slope handles associated with the key. For information on the slope, see Editing a Function Curve’s Slope.

You can also click the Spline icon on the fcurve editor toolbar to select this option.

- Linear Interpolation connects keys by straight line segments. The movement is characterized by constant speed with sudden changes at each key, creating a mechanical feel to the animation.

This may be useful for animating lights and colors, where you don’t need the ease-in and ease-out that a spline interpolation offers.

You can also click the Linear icon on the fcurve editor toolbar to select this option.

- Constant Interpolation repeats the value of a key until the next one. The movement is characterized by sudden changes at keys and static positions between keys.

While you may not want to use this interpolation for character animation, it might be useful for animating a cut from one camera to another.

Setting the Default Interpolation Type

You can determine how movement will be calculated in an fcurve between keyframes by default. This setting is used when fcurves are created.

To set the default interpolation type

1. Choose File > Preferences from the main menu.

2. In the explorer that appears, select Animation to open the Animation Preferences property editor.

3. Select one of the Interpolation for Inserted Keys options:

- Constant, Linear, and Spline are the same as described previously.

- Automatic (From Neighboring Keys) ensures that the slopes for the keys are automatically calculated from the neighboring keys. This is the default option and it uses splines to ease in and out of each key.

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