What is interpolation

The frames that are between the keys are called interpolation.
These frames automatically calculate the intermediate poses between the keys. The intermediate poses can be calculated in different ways, depending on the type of interpolation.

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STEP definition

STEP - a type of interpolation where intermediate frames are not calculated at all. Poses in frames with step interpolation are the same as in the key in front of them.

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Exercise 9.1: Step
Chibi_jump_02_keyframes.casc

Open the Chibi_jump_02_keyframes scene to complete the following exercises.

  • Select all the animation frames. (Put the cursor in frame 0 and drag it while holding LMB)
  • Click on the center of mass to view its trajectory. (The center of mass is a large blue dot located approximately in the middle of the character)

  • Enable Show all trajectories fixed and Set trajectory interval to keep the trajectory visible all the time. 

  • Hover over the space between any two keys on the timeline. The tooltip will show the STEP interpolation type.
  • Try to play the animation.

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Bezier definition

Bezier - a type of interpolation where intermediate poses are calculated so that the trajectories of the character’s points form arcs. In this case, the speed is uneven and strongly depends on neighboring gaps.

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Exercise 9.2: Bezier
Chibi_jump_02_keyframes.casc

  • Select all animation frames.
  • Open the interpolation selection menu and select Bezier. The gaps between the keys should turn blue.

  • Play the animation to see how the trajectory of the center of mass has changed.

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Exercise 9.3: Trajectory changes with Bezier
Chibi_jump_02_keyframes.casc

  • Go to frame 28.
  • Select the character's center of mass and move it with the help of the movement manipulator.
  • Observe how the trajectory changes in the gap between the edited key and those adjacent to it.
  • Return the character back with Ctrl + Z

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Linear definition

Linear - a type of interpolation where the intermediate poses are calculated in such a way that the trajectories between the character’s points follow straight lines with equal speed.

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Exercise 9.4 Linear
Chibi_jump_02_keyframes.casc

  • Select all animation frames.
  • Open the interpolation selection menu and select Linear.
  • Play the animation to see how the trajectory of the center of mass has changed.

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What is a fixed interpolation?

Fixed interpolation is an interpolation where the pose in each frame is set by the user. It is not recalculated automatically.

Fixed interpolation works as if all frames in the interval were keys. Interpolation is automatically fixed if you change the character’s pose outside of the keyframe, in a frame with linear or bezier interpolation.

This way, fixed interpolation allows you to save your changes without creating extra keys.

 

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Exercise 9.5: Fixed interpolation
Chibi_jump_02_keyframes.casc

  • Go to any non-key frame with Linear or Bezier interpolation
  • Change the character’s pose - select all the point controllers and turn the character slightly
  • Pay attention to the timeline - the interpolation interval between adjacent keys has turned green

*If the interpolation interval turns red, then the pose change did not affect all the layers. We will talk more about layers in the next tutorial.

  • You can return the previous type of interpolation - however, the changes made will be lost and the character’s pose will be reset.

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Different types of interpolation for different intervals

Interpolation is applied to the frame interval between two keys. At different intervals, there may be different types of interpolation. This way, different types of interpolation can be combined to create the desired movement.

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What is IK and FK interpolation?

With IK interpolation of the arms or legs, the palms and feet will move to a point in the next key along the shortest path — a straight line. This way, the arms will bend in the elbows, and the legs in the knees. With FK interpolation, the entire limb is moved, so the palms and feet make a semicircle.
 

FK interpolation is useful for sweeping circular motions and movements in the mid-air.

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Exercise 9.6: Swinging arms
Standard_model.casc

  • Open the Standard Model.casc scene
  • Select all the arm’s points starting from the shoulder using the rotation manipulator.
  • Do the same with the second hand.
  • Create a new key in frame 25.
  • In turn, select the points of the arms and raise them using the rotation manipulator.
  • Set the bezier interpolation on frames 0 to 25.
  • Select a frame interval from 0 to 25 and a palm point to track the trajectory of the hand.
  • Set the FK interpolation on the interval using the button above the timeline on the right.
  • See how the trajectory and animation have changed.

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