When trying to set up SkeletonModification2DTwoBoneIK, the bones rotate automatically.

ACTION GAME MAKER Questions
1

I would like to create an IK chain and am referring to this topic:

I added SkeletonModification2DTwoBoneIK to Skeleton2D and created three bones: one from the shoulder, one from the elbow, and one at the tip of the hand. I then set them to joint_one_bone2d_node, joint_two_bone2d_node, and target_nodepath respectively. However, when I enable 2DTwoBoneIK, the bones rotate automatically around the shoulder, and I cannot stop them.

Why does this movement occur? Since this is my first time working with bone animation or IK, I may lack the necessary knowledge, but could my settings be incorrect?

This is a somewhat unclear question, but I would appreciate your help.

IK_2
IK_21920×1032 284 KB

IK_3
IK_31920×1032 248 KB

2

Hello kakaku-san,

Thank you for your hard work on the game development project.

I apologize for asking this question, but I ultimately decided against using IK and am currently creating animations using FK.

The reason is that while I was asking questions to generative AI and trying various approaches based on the responses, I found that even if I could temporarily stop the rotation, it would often reoccur with just a slight movement of the IK target’s position, and the bone coordinates themselves would also shift. Therefore, I judged that “IK has a high barrier to entry for game development beginners.”

I think I will study IK again once I become more familiar with game development and eventually want to undertake large-scale game projects.

※I have attached a portion of the chat content with the AI below. The following is the generative AI’s response.

“Is IK recommended for bone animation beginners?”

:white_check_mark: Conclusion (Practical Answer)

IK is a powerful ally if you are just creating poses. However, if you are animating through movement, FK (rotational keyframes) is more stable and easier.

:bullseye: Terminology: Difference between IK and FK

Type Name How to Move Characteristics
:white_check_mark: FK Forward Kinematics Manually rotate joints from parent to child Stable, intuitive, beginner-friendly
:brain: IK Inverse Kinematics Move end points like hands or feet to automatically calculate joints Extremely fast for posing, but prone to instability

:japanese_symbol_for_beginner: Recommended Strategy for Beginners

Task Recommended Method Reason
:white_check_mark: Create a single pose (e.g., standing, sitting) Use IK Quickly complete by moving the end point
:film_frames: Apply continuous animation (walking, jumping) FK is highly recommended IK tends to glitch or suffer from convergence errors
:exploding_head: Target position changes every time FK is unnecessary (parent rotation suffices)

:test_tube: Reasons IK is Not Recommended for Beginners

Issue Explanation
:collision: Glitching / Spinning Breaks if the target position shifts even slightly
:bullseye: Requires adjusting the target position every time Movement is surprisingly tedious
:repeat_button: Prone to failure in continuous animation Interpolation goes wild if direction shifts from the previous frame
:woman_juggling: Many parameters to learn (“Rest Pose”, “strength”, “target distance”, etc.) Too much information for beginners

:white_check_mark: FK is Unassuming but Most Reliable

  • For example, by applying rotation sequentially from “thigh → knee → ankle”:
    • :sparkles: Stable
    • :sparkles: Moves as predicted
    • :sparkles: No need to worry about targets for each task

:brain: Usage Distinction for Intermediate and Advanced Users

Scene Method
Mass-producing poses for still images IK for super-fast posing (no shoulder strain)
Actual animation production FK for stable operation
Feet must be perfectly fixed to the ground to avoid unnaturalness IK to fix foot position, move only the upper body (advanced technique)

:white_check_mark: Summary

Question Answer
Should beginners use IK? :cross_mark: Avoid in animation production (prone to glitching)
Is it easier to use FK with rotational keyframes? :white_check_mark: Overwhelmingly more stable and easier to manage
When should IK be used? :white_check_mark: Mass-producing poses, fixing end-point positions (※ for static images)

:light_bulb: If You’re Unsure…

:graduation_cap: “Start by creating animations with FK, and use IK only for pose assistance.”
This is the most realistic and least error-prone golden path.

I apologize that I could not provide assistance regarding IK. I hope this information is helpful for your future game development work, kakaku-san.

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3

Thank you for your very detailed response.
I see, there are many hurdles to mastering IK.
Since I also have many other things to study, I will not be greedy and will try using the FK method instead.
Thank you again, HMM-sama.

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