Center of mass

The center of mass is a point that depends on the mass and position of the body parts of the character.

For example, when a character tilts, his center of mass shifts towards the tilt. The projection of the center of mass is displayed on the ground.

It is convenient to monitor the balance of the character with its help. In stable poses, the center of mass should be between the fulcrum points.

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Exercise 12.1: Finding balance
Standard_model

  • Open the Standard_model scene
  • Raise a character’s leg
  • Click on the point of the center of mass and see the projection
  • Select the upper part of the character (hotkey - 1) and move the character's body until the projection of the center of mass is under the second leg.

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Ballistic trajectory

When a character is in the air during the jump or fall, his center of mass always moves strictly along a ballistic curve, regardless of what movements the character makes in flight.

Using the ballistic trajectory for the center of mass is already enough to make jumps and falls more realistic.

However, to maximize compliance with the laws of physics, it is important to control the character’s rotation in flight as well. The character’s rotation is affected by his pose. For example, the more the character is grouped, the faster he will rotate.

The Ballistic Trajectory tool also helps you calculate how long it will take to jump at a given height at a given gravity.

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Exercise 12.2: Creating ballistics
Chibi_jump02_before_ballistic.casc

  • Open the Chibi_jump02_before_ballistic.casc scene
  • Find the last key before the jump, where the character's legs have not yet taken off the ground. It should be frame 15.
  • Find the key after the jump, where the character’s legs first touch the ground again. It should be frame 34.
  • Select the interval that starts and ends with these frames.
  • Click the Ballistic trajectory button

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Exercise 12.3: Editing the ballistic curve
Chibi_jump02_before_ballistic.casc

  • Select the green dot on top of the ballistic curve.
  • Change the height of the curve by moving the point with the manipulators
  • Pay attention to the timeline - ballistics is shown above the timeline as a black line. The higher the trajectory, the longer is the line.
  • Choose a jump height so that the ballistics line above the timeline ends at frame 41.

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Exercise 12.4: Adjusting animation for ballistics
Chibi_jump02_before_ballistic.casc

So now the ballistics ends in frame 41. This is the key in which the character has already landed. But the ballistics should always end with the frame of the first contact with the ground.

  • Select frames 15 to 34 (these are the frames of the last and first contact with the ground).
  • Press Ctrl + T and extend the frame interval, so that the key with the landing pose is in frame 41.

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Ballistic ghosts

When a character is in the air, his poses do not affect the trajectory of the center of mass - it always moves according to ballistics. But poses affect the rotation of the character. For example, the tighter is the character’s pose, the faster he will rotate.

 

You can quickly adjust the rotation of the character using the ballistic ghost tool. Physically correct positions of the character are shown in red silhouettes.

When you snap it to physical ghosts, the character turns in each frame so that his position coincides with the silhouettes. At the same time, the tool does not change the poses set by the animator, but rotates the character entirely around the center of mass! Sometimes it happens that the rotation suggested by the program does not quite correspond to the animator's idea. In this case, it is necessary to mark the frames in which the character should take the position as close as possible to the one set by the animator.

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Exercise 12.5: Snapping to the ballistic curve
Chibi_jump02_before_ballistic.casc

  • Select an interval with a ballistic trajectory or one of the frames on this interval.
  • Open the ballistics trajectory menu and click the Snap center of mass to selected trajectory button - the character will move in space so that its center of mass in each frame will coincide with the points on the ballistic trajectory
  • Turn on the Ballistic ghosts button - the red silhouettes will indicate frames where the rotation of the character is not physically correct.
  • Go to frame 41 and press the set physic priority frame button  to leave the landing pose unchanged.
  • Click the Snap to ballistic ghosts button to fix the rotation of the character. Silhouettes should turn blue.

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