This is actually quite useful, however one issue I am having with it, is the hand touching the inner edge rather the centre.
How would I go about fixing this?
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It’s because the solve isn’t accounting for the outward offset the C1 of the shoulder has. Technically it’s correct since the line from the shoulder C0 to the point is still in line.
You can correct this by offsetting the originCF by the shoulder C1 offset outward (c1.X) or by rotating the final planeCF to compensate for the offset (think of the offset + arm lengths as a right angle triangle: tan(l1+l2 / c1.X)). Rotating means you’ll also need to adjust for the loss in distance by pushing the planeCF forwards.
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A = math.acos((-a^2 + b^2 - c^2) / (2 * b * c))
wouldn’t it be:
A = math.acos((-a^2 + b^2 + c^2) / (2 * b * c))
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This is a tutorial, not a project. This topic is intended to teach you the basics of IK, and it’s up to you to apply it yourself.
One case that needs to be adressed is when the axis is equal to Vector3.new()
A simple solution to this:
local axis = Vector3.new(0, 0, -1):Cross(localizedUnit)
local angle = math.acos(-localizedUnit.Z)
if axis == Vector3.new() then
axis = (angle == PI and Vector3.new(-1, 0, 0) or Vector3.new(0, 0, -1))
end
local planeCF = originCF * CFrame.fromAxisAngle(axis, angle)
This addresses two very specific cases where the math breaks 
Just wanted to add this on.
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No, the first equation is correct
So I am very unfamiliar with IK, however, I am getting a grasp. How would I go about adding another limb?
This is the only code I changed and it works fine except the goal part is at the joint I do not want it to be at. How do I fix this?
Code:
local workspace = game:GetService("Workspace")
local storage = game:GetService("ReplicatedStorage")
local runService = game:GetService("RunService")
local goalPart = workspace:WaitForChild("GoalPosition")
local shoulderPart = workspace:WaitForChild("ShoulderPosition")
local upperArm = workspace:WaitForChild("UpperArm")
local lowerArm = workspace:WaitForChild("LowerArm")
local grapple = workspace:WaitForChild("Grapple")
local solveModule = storage:WaitForChild("SolveIK")
local solveIK = require(solveModule)
----
local SHOULDER_OFFSET = CFrame.new(1.5, 0.5, 0)
local VECTOR3_UP = Vector3.new(0, 1, 0)
----
runService.Heartbeat:Connect(function()
local shoulderCFrame = shoulderPart.CFrame * SHOULDER_OFFSET
local goalPosition = goalPart.Position
local upperV3 = VECTOR3_UP * upperArm.Size
local lowerV3 = VECTOR3_UP * lowerArm.Size
local grappleV3 = VECTOR3_UP * grapple.Size
local planeCF, shoulderAngle, elbowAngle, otherelbow = solveIK(shoulderCFrame, goalPosition, upperV3.Y, lowerV3.Y, grappleV3.Y)
local shoulderAngleCFrame = CFrame.Angles(shoulderAngle, 0, 0)
local elbowAngleAngleCFrame = CFrame.Angles(elbowAngle, 0, 0)
upperArm.CFrame = planeCF * shoulderAngleCFrame * CFrame.new(-upperV3 * 0.5)
lowerArm.CFrame = upperArm.CFrame * CFrame.new(-upperV3 * 0.5) * elbowAngleAngleCFrame * CFrame.new(-lowerV3 * 0.5)
grapple.CFrame = lowerArm.CFrame * CFrame.new(-upperV3 * 0.5) * elbowAngleAngleCFrame * CFrame.new(-lowerV3 * 0.5)
end)
Thanks!
How would I make this R6 Because I Don’t Really See Many Posts?
It isn’t bad, it’s just older.
What is the issue, you gotta give context instead of just showing an image and saying “What am I supposed to do in this situation?”
Sorry, I’ve fixed the issue. I had a scripted animation playing.
Sorry for the bump!
Has anyone applied this to the module? I can’t seem to figure out where/how to apply this, testing this on a 2.0 rig is accurate enough as it is but when I test this using the woman package the arms are way off target (potentially also due to the elbow offset?)
This looks useful, and I was actually looking for inverse kinematic tutorials. But I’m still unsure about how to use it for walking in where you put the script wise or if this is even for walking. This kinda went over my head so I can’t really figure out how to use it with walking.
I might just change names or something, i’m also not sure if this is only for animations instead of actual walking (For an fps game a couple friends and i are working on).
How do I use this for walking? (Or if its already for walking just tell me I don’t really know how this works)
The function provided cannot be run as it errors at :pointToObjectSpace(), the error it provides is
For the originCF im passing through a cframe, for the targetPos im passing through a position.
I’m incredibly confused any help is appreciated.
Edit: my bad i was doing something wrong
Is there anyway I could apply this to 3 joints (upper arm, lower arm, hand)?
Youtube has got ya covered 
Sadly it doesn’t I’ve tried said method multiple times to no luck. I’ve covered the internet trying to find a solution yet cannot.
might be a little off topic but, why do make a variable for VECTOR3_UP instead of Vector.yAxis? Same goes for any other use you’ve given it
The release of vector.yAxis is on 2021 and the tutorial is on 2019.
Also localizing your variables is always preferred for minor optimization rather than indexing it from a table using the dot notation
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