How to maintain relative CFrame of a part

[Blunce]

When I set the CFrame of a certain part A I want to set the CFrame of a different part B on the surface of part A such that it maintains the same relative orientation, how would I do this?

[VegetationBush (FartedWayTooHard)]

You can weld both of them together with WeldConstraints.

[Blunce]

This is clever, but when I set the CFrame using a script it doesn’t work.

[goldenstein64 (Shoop)]

I made a post a while ago on how to use CFrame:Inverse, there’s a section at the end that explains how to make a relative CFrame offset:

@goldenstein64

So, if you have two part CFrames, you can multiply the inverse of one part’s CFrame by the other to get a relative CFrame between the two:

relativeCFrame = part.CFrame:Inverse() * part2.CFrame

You can use this relativeCFrame to apply a constant relative position from one part to the other, if you so wanted:

-- in a loop:
part.CFrame = -- whatever you want
part2.CFrame = part.CFrame * relativeCFrame

e.g:

The code I used:

local relativeCFrame = part.CFrame:Inverse() * part2.CFrame

local i = 0
while true do
	local dt = wait()
	i = (i + dt/3)%1 -- loops between [0, 1) in 3 seconds

	-- changes the position using sine curves
	part.CFrame = CFrame.new(math.sin( math.rad(360*i) ), math.sin( math.rad(720*i) ), 0)

		-- changes the angle
		* CFrame.Angles(0, math.rad(360*i), 0)

	-- gives `part2` a position relative to `part`
	part2.CFrame = part.CFrame * relativeCFrame
end

You can pseudo-decompose the relativeCFrame into its original components, and you’ll notice two of the CFrames cancel out, leaving the original CFrame from part2 :

part2.CFrame = part.CFrame * relativeCFrame

-- decomposed: [   these two would cancel out!   ]
part2.CFrame = part.CFrame * part.CFrame:Inverse() * part2.CFrame

However, these don’t truly cancel out because the part's CFrame changed after the relative CFrame was created, which is what creates the static relative offset from part without keeping part2 in the same position.

Hopefully that answers your question.

[Blunce]

Thank you so much, if you don’t mind could you run through the linear algebra quickly?

[goldenstein64 (Shoop)]

Personally, I don’t know anything about the calculations behind CFrame operations, I only know how to use them.

I know EgoMoose made some resources on how CFrames work, if that helps:

Articles/CFrames at master · EgoMoose/Articles

I recommend you search the devforums for other resources on CFrames.