Editing a Function Curve’s Slope
The fcurve’s slope determines the rate of change in the animation. By modifying the slope, you can change the acceleration or deceleration in or out from a key, thus making the animation change rapidly or slowly, or even reversing it. The steeper the slope’s orientation, the faster the values change.
You can change the slope of any fcurve that uses spline interpolation by using the two handles (called slope handles or tangents) that extend out from a key. By modifying the handles’ length, direction, and orientation, you can define the way the curve moves to and from keys.
By default, the slope handles are tangent to the curve at their key. This keeps the acceleration and deceleration smooth, but you can also “break” the slope at a certain point to have a sudden animation acceleration or deceleration, or change of direction altogether (see Breaking and Unifying the Slope Angle).
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• Use the View > Slopes on Unselected Keys command to show or hide the slope handles of all unselected keys on a selected curve. If you choose this command, you can manipulate handles without having to first select the key. The default, however, is to hide these slope handles, which prevents you from accidentally manipulating them. • Many of the slope commands that are available from the Keys menu are also available by Alt+right-clicking on a slope handle’s end point. |
To select slope handles
1. Activate the Select tool (press Y).
2. Select one or more keys on an fcurve with spline interpolation—slope handles appear on each selected key.
3. Click anywhere on a slope handle to select it—the handle turns yellow.
Press Shift or Ctrl to select multiple handles on other selected keys.
If you select a key, its slope handle is deselected, and vice versa. However, if you select a different key, the selected slope handle remains selected; if you select a different slope handle, the selected key remains selected.
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If you are having trouble selecting a slope handle because it is in the same location or too close to a key, choose Edit > Select with Priority to Tangents or press the P key to give priority to the slope handle. |
Changing the Length and Angle of Slope Handles
The angle of a slope handle determines the shape of the curve that goes into and comes out of a key point. The steeper the slope, the faster the acceleration; the shallower the slope, the slower the acceleration.
The length of the slope handles determines the weight of the key’s influence on the fcurve. The longer the handle, the more the key influences the connecting curve segment. By having control over the handle length, you can also draw more complex curve shapes with fewer keys.
As you change the orientation and length of the slope handles, you can also set how the handles move: the left and right handles can be manipulated independently, together, or locked in place.
For more information, see Breaking and Unifying the Slope Angle and Controlling the Length of the Slope Handles.
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You can determine the level of control when moving slope handles by setting the Interaction Lag on the View page in the Fcurve Editor Preferences property editor (see Setting Fcurve Editor Preferences). The default is the mouse moving one pixel before interaction starts. |
Changing the Slope Handles by Dragging
To change the slope handles interactively
1. Select one or more keys on an fcurve with spline interpolation—slope handles appear on each selected key.
You can also select the slope handles themselves, but then you lose the current key selection.
2. Click anywhere on a slope handle and drag it to a new location to change either its angle (orientation) or length.
The handle that you are manipulating is selected (turns yellow) only for the duration of the movement. When you finish moving the handle, and release the mouse, the handle is deselected and your original key selection is retained.
• If there are multiple keys selected, all their handles change accordingly when you drag one of the key’s handles.
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If the Selection Tools > Translate on MMB option in the Animation Editor preferences is on, you don’t need to click directly on the handle to move it—you can drag anywhere in the graph. If the Selection Tools > Prevent Movement on LMB option is also on, you can use the left mouse button only for selection and the middle mouse button only for manipulation. See Animation Editor Preferences for more information on these options. |
To change only the angle of the slope handles interactively
1. Select one or more keys on an fcurve with spline interpolation.
You can also select the slope handles themselves, but then you lose the current key selection.
2. Click anywhere on a slope handle, then press Shift as you drag up or down (or in a direction perpendicular to the handle) to change only the handle angle.
If there are multiple keys selected, their handles all change accordingly when you drag one of the handles.
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If the Selection Tools > Translate on MMB option in the Animation Editor preferences is off, use the middle mouse button to change only the handle’s angle. |
To change only the orientation of the slope handles interactively
1. Select one or more keys on an fcurve with spline interpolation.
You can also select the slope handles themselves, but then you lose the current key selection.
2. Click anywhere on a slope handle, then press Shift as you drag to the left or right (or in a direction along the handle’s tangent) to change only the handle length.
If there are multiple keys selected, their handles all change accordingly when you drag one of the handles.
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If the Selection Tools > Translate on MMB option in the Animation Editor preferences is off, use the right mouse button to change only the handle’s length. |
Changing the Slope Handles with Precise Values
To change the slope handles precisely
• Select one or more slope handles and enter values for the slope handles’ length and angle in the slope text boxes in the command bar.
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If you select multiple slope handles from different keys, their values are displayed in the slope text boxes only if they are identical. |
- The slope handle length is in fcurve graph units, and you can set up to one decimal place for the value (such as 1.2, rounding off the numbers).
- The slope handle angle value can be between -90 and 90 degrees, with up to two decimal places (such as 45.23). However, when you select the Express slopes as ratio as in SI|3D option (see below), you can enter any value, not just one between -90 and 90.
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The display of the slope handle in the fcurve graph may not look exactly like the angle that you enter depending on the amount you’ve zoomed in to the graph. |
When you enter a value in the slope handle angle text boxes, the value is displayed as an angle. However, you can set the display to express the ratio as Y (value) over X (time) values (see below).
To switch the slope angle value display
1. Open the Fcurve Editor Preferences editor (see Setting Fcurve Editor Preferences).
2. Click the Keys tab and select Express slopes as ratio as in SI|3D.
When this option is active, the slope angle text boxes display the slope values as a ratio of Y values over X values.
You can snap slope handles in two different ways:
• Snap the handle’s length to a specific number of Softimage units (default is 1), from 0.01 to infinity.
• Snap the handle’s angle to a particular degree (45 is the default), from between 1 and 90.
To snap the slope handles in length or angle
1. Open the Fcurve Editor Preferences editor (see Setting Fcurve Editor Preferences).
2. On the Editor page, set the Snap Slope > Length in units and/or Angle in degrees.
3. Click in the appropriate Snap checkbox to activate Length and/or Angle.
4. Select one or more keys.
5. Choose Edit > Snap Slopes to activate slope snapping and then drag a slope handle.
Setting the Slope’s Orientation (Angle)
The Slope constraint options that control how slope handles are set for selected keys. Select keys whose slopes you want to edit, then choose one of the slope options in the Keys menu as described in this section.
Slopes Based on Neighborhood Keys (Automatic)
The Automatic Slope command computes the orientation and length of a key’s slope based on the interpolation of the fcurve between the previous key and the next key (neighboring keys) in a plateau-type manner (the slopes gradually flatten out as they approach their local maxima/minima).
When moving keys, the ratio of the slope handle to its neighboring keys is preserved: as you move a handle towards or away from its neighboring keys, the handle gradually shortens or lengthens. This keeps the fcurve shape roughly the same and avoids dips or peaks that can occur when a handle gets too close to a neighbouring key.
When pasting copied keys, the slope handles are lengthened or shortened in the same way to keep the fcurve shape the same.
To change the slopes of selected keys
Do one of the following:
• Choose Keys > Automatic Slope.
or
• Press Shift+A.
or
• Click the 
icon in the toolbar.
Creating Flat Slopes (Zero Orientation)
The Zero Slope Orientation (Flat Key) command constrains keys to a slope of zero. The slope handles remain horizontal so that only the length can be modified.
This is useful to prevent the automatic interpolations from going above or below your keyframed values as they approach keys, such as keeping curves from changing when you don’t want them to.
To change the slopes of selected keys
Do one of the following:
• Choose Keys > Zero Slope Orientation (Flat Key).
or
• Press Shift+0 (zero).
or
• Click the 
icon in the toolbar.
Creating Break Points in the Slope
The Zero Slope Length (Break Point) command sets both the left and right handle lengths to zero, effectively creating a break point. When this is selected, you cannot change the handle length or angle since it is locked to zero.
To change the slopes of selected keys
Do one of the following:
• Choose Keys > Zero Slope Length (Break Point).
or
• Press Shift+B.
or
• Click the 
icon in the toolbar.
The Plateau Slope command automatically sets the slope of a key to zero if its neighboring (previous or next) key has the same value or if that key is a local minima or maxima.
• A local minima is a key where both previous and next keys have larger values.
• A local maxima is a key where the previous and next keys have smaller values.
The shape of the curve is retained, but a linear segment is used between the two adjacent points with the same value. This allows the curve to hold key values, preventing the automatic interpolation from going above or below keyframed values that define extremes.
Using this type of slope can be useful for keeping objects in one place at keyframes where they are supposed to be still, and moving smoothly at keyframes where they’re supposed to move (such as preventing the backsliding of feet between keyframes in a walk cycle and preventing jerky movements that would happen with a linear interpolation).
When Plateau Slope is active, it affects all keys on a selected curve.
To change the slopes on the selected curve
Do one of the following:
• Choose Keys > Plateau Slope.
or
• Press Shift+- (dash).
or
• Click the 
icon in the toolbar.
Pointing at the Neighboring Keys
The Point at Neighboring Keys Slope Orientation command constrains the selected keys’ handles so that they point at the neighboring (previous and next) keys. This usually creates sharp break points in the slope.
The handles stay pointed at the neighboring keys even when you move the key. If you move the handle itself, the constraint is removed.
To change the slopes of selected keys
Do one of the following:
• Choose Keys > Point at Neighboring Keys Slope Orientation.
or
• Click the 
icon in the toolbar.
Freezing the Slope Handle Orientation
When you’re using slope orientation constraints on selected keys, you may want to keep (freeze) the resulting slope handles’ orientation for these keys. Using the Keys > Manual Slope command, you can freeze the current slope handles for these keys, then move the keys as you like without having their slope handles change orientation.
For example, let’s say you select a key and choose the Point at Neighboring Keys slope orientation command. When you move that key, its slope handles change their orientation to keeping pointing at the neighboring keys. Now if choose the Manual Slope command and move the selected key (or the neighboring keys), you’ll notice that the slope handles keep their orientation: they do not try to keep pointing at the neighboring keys.
To freeze the slope’s constraint
1. Select one or more keys.
2. Select one of the slope orientation constraint types as described in this section (Automatic, Plateau, Point at Neighboring Keys, etc.).
3. To freeze the slope’s shape using this current constraint, choose the Keys > Manual Slope command or click the 
button in the fcurve editor toolbar.
The slope handles retain their current orientation when you move the selected keys.
Setting the Default Slope Type
You can set the default slope mode for any keys that are inserted on an fcurve. This information is set in the Animation Preferences property editor and is saved in your preferences file so that it’s active for more than the current session in Softimage.
To set the default slope
1. Choose File > Preferences from the main menu.
2. In the explorer that appears, select Animation or click its icon to open the Animation Preferences property editor.
3. Select one of the Slope Orientation Constraint options.
- See the previous pages for descriptions of Plateau and Zero Orientation.
- None means that the slope’s orientation is free to be modified by you, and/or it will get automatically calculated based on neighboring keys.
Breaking and Unifying the Slope Angle
When slopes are unified (default), any change you make to one of the slope handles at a key is mirrored on the other one. However, you can “break” a slope at a key so that you can change either slope handle independently.
You may want to break the slope to create a sudden change in motion at a key, such as an object hitting a wall or a bouncing ball hitting the ground and then bouncing up again.
To control the orientation of the slope
• Select keys and do one of the following:
- Choose the Keys > Unified Slope Orientation command.
or
- Press Shift+O.
or
- Click the Slope Orientation icon 
in the fcurve editor toolbar.
or
- Choose Keys > Key Properties to open the Key Properties editor and select either of the Slope Orientation options (Unified or Broken).
• When the Unified Slope Orientation command is active, it keeps the slope handles of the selected keys unified as you move them, keeping a constant angle between them. Unified slopes provide smooth, unbroken fcurve segments, as values ease into and out of keys.
• When the Unified Slope Orientation command is not active, you can break the slope to control the slope’s shape on either side of the key individually. Depending on the slopes you define, the transition in the rate of the parameter’s change at the keyframe can be as smooth or as abrupt as you like.
When the slope is broken, you can drag either slope handle in any direction. You can also middle-click+drag to move only the handle’s angle, or right-click+drag to move only the handle’s length.
If you adjust one of the slope handles, then turn Unified Slope Orientation on again, you can see that the current angle between the slope handles is once again maintained when you drag one of the handles.
When the Unified Slope Orientation command is active, you can create a mirror constraint between a key’s tangent handles. This way, the changes that you make to one key’s handle are mirrored to its other handle. This also works well when the Unified Slope Length command is active.
To mirror the slope angle
1. Choose the Keys > Mirrored Slope Orientation command or click the 
icon in the toolbar.
2. Drag the handle of a selected key whose slope orientation is unified. The movement is mirrored on the other key’s handle.
Controlling the Length of the Slope Handles
You can set the length of the slope handles on both sides of the key to be equal or not.
To control the length of the handles
• Select keys and do one of the following:
- Activate the Keys > Unified Slope Length option.
or
- Press Shift+L.
or
- Click the Slope Length icon 
in the fcurve editor toolbar.
or
- Choose Keys > Key Properties to open the Key Properties editor and select either of the Slope Length options (Unified or Broken).
• If this option is active, both handles are kept the same length. This is the case even if the slope orientation is broken.
• If this option is not active (default), the length of each slope handle is independent.
Setting the Default Handle Length
You can determine the default slope handle length for when fcurves are created. This information is set in the Animation Preferences property editor and is saved in your preferences file so that it’s active for more than the current session in Softimage.
To set the default slope handle length
1. Choose File > Preferences from the main menu.
2. In the explorer that appears, select Animation or click its icon to open the Animation Preferences property editor.
3. Select one of the Slope Length Constraint options:
- None: The slope’s length is free to be modified by you, and/or it will get automatically calculated based on neighboring keys.
- Zero Slope Length: Creates sharp points (break points) on the curve, allowing you to manipulate the handles independently.
- Unified Length: Keeps the length for the slope handles equal on both sides of the key point.
Creating Fixed Length Slope Handles
You can have slope handles drawn with a fixed length, as is the case in SOFTIMAGE|3D. This length corresponds approximately to the horizontal length spanning five frames.
To convert fcurves to fixed length handles
1. Select the fcurves you want to convert.
2. Choose the Curves > SI|3D Curve command.
3. A warning message appears stating that the curves may be changed by doing this, and prompts you to continue or not.
If you continue, the handles of the selected curves change to be a fixed length and are locked. The shape of the curves may also change.
The slope length text boxes on the command bar (see Changing the Length and Angle of Slope Handles) become blank and are inaccessible after you have chosen this command for the selected curve. If several curves are selected and only some of them are using this command, the text boxes are still inaccessible.
4. To convert back to free length slopes (XSI-style handles), deactivate the Curves > SI|3D Curve by choosing it again.
To set fixed length as the default slope type
1. Choose File > Preferences from the main menu.
2. In the explorer that appears, select Animation or click its icon to open the Animation Preferences property editor.
3. Select SI3D (Fixed Length Slopes) as the Curve Style. XSI (Free Length Slopes) is the default.
If the fcurve uses a spline interpolation, you can freeze a key’s speed of transition and influence on the fcurve by locking the orientation and length of its slope handles:
1. On an fcurve, select the keys whose handles you want to freeze.
2. Do one or both of the following:
- Choose Keys > Lock Slope Orientation to lock the slope handles’ orientation. The slope handles turn blue.
and/or
- Choose Keys > Lock Slope Length to lock the length of each slope handle. The slope handles turn green.
The slope handles of the selected key are frozen in place and cannot be moved. You can still manipulate the position of the key itself, unless its position has also been locked (see Locking a Key’s Position).
3. Choose the same commands again to unlock the slope handles in that way, or choose Keys > Unlock All Slopes to unlock all slope handles that were previously locked in either length or orientation.
Autodesk Softimage v.7.5