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Asked: 2025-03-11T02:14:09+05:30

How can I reverse the direction of the slerp function to prevent incorrect mesh rotations during animation blending?

I’m dealing with an issue that’s been driving me up the wall while working on blending animations in my 3D model. So, here’s the situation: I’m using the slerp function to blend between animations, and for some reason, it’s causing my mesh to rotate in what seems to be the incorrect direction, making the animation look bizarre.

I have two animations where the character should smoothly transition from looking left to looking right. In theory, this should work well since both animations have the same amount of frames and the body positioning is identical, but what actually happens is that the model rotates “inside” instead of “outside” during the transition, creating awkward movement that makes it look like the character is twisting unnaturally. It’s really disconcerting to watch. I’ve attached some images to illustrate the problem, and they should give a better idea of what I’m dealing with.

I’ve been looking through the code trying to locate the issue. I’m using the following snippet to handle animation blending:

“`rust
let q = animacoes[&z.0].0[(((gtempo-timecut)*60.0)%animacoes[&z.0].2) as usize][n];
base.1 = base.1.slerp(q.1,animlist[&z.0]/temp);
“`

Here, I’m grabbing the rotation quaternion (q.1) from the animations and blending them using slerp. However, it appears that slerp is picking the shorter path between the two rotations, which results in that awkward inside rotation. I’ve tried some things to manipulate the rotation matrix before applying it, like flipping axes, but nothing seems to work effectively.

Is there a way to force the slerp function to take the longer rotation path instead, or some way to calculate the direction to ensure it turns the right way? Any input or ideas would be greatly appreciated. I really want to make this animation look natural and fluid instead of watching our poor character swivel in a funky way. Thanks in advance for any help!

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    2 Answers

    1. Animation Blending Issues

      It sounds like you’re running into a classic problem when using slerp for quaternion blending in animations. What you’ve described—your character rotating “inside” instead of “outside”—often happens when facing the shortest path between two rotations.

      One potential solution is to manually adjust the rotation quaternions to ensure you’re always taking the longer route. You can do this by checking the dot product of your quaternions. If the dot product is negative, it means you’re facing the opposite direction, so you may want to negate one of your quaternions before performing slerp.

      Here’s a basic idea:

      
if (q.1.dot(base.1) < 0.0) {
    q.1 = -q.1; 
}
base.1 = base.1.slerp(q.1, animlist[&z.0] / temp);

      This should help by flipping the quaternion to follow the longer rotation path, preventing that awkward twisting effect!

      Also, make sure that your animations are set up correctly and are in the same coordinate space, as any discrepancies could further complicate the blending.

      Good luck, and I hope this makes your animation look more natural!

    2. The issue you’re experiencing arises because quaternion spherical linear interpolation (slerp) naturally chooses the shortest rotation path between two orientations. When blending animations for a character rotation from one direction to another, this behavior can result in unnatural rotations like you’re seeing, as the shortest rotation isn’t always the visually intended rotation. To force slerp to use the longer, visually correct rotation path, you need to invert one of the quaternions before performing the interpolation. Specifically, compare the dot product of the two quaternions: if it’s negative, negate the target quaternion’s components prior to interpolation. This approach ensures that slerp will interpolate along the desired rotational path.

      In practice, your implementation can look something like this in Rust:

      let mut target_quat = q.1;
      if base.1.dot(target_quat) < 0.0 {
          target_quat = -target_quat;
      }
      base.1 = base.1.slerp(target_quat, animlist[&z.0] / temp);

      This adjustment ensures the interpolation always takes the intended rotation direction, resolving the unnatural twisting you’re encountering. By applying this quaternion-flipping method, your character’s animation transition from left-to-right rotation should now appear fluid and natural.