Change only a CFrame upVector

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#1

Can anyone tell me how to change the UpVector of a CFrame without changing the relative Y axis rotation?

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#2

Can you elaborate on your question? It’s not making sense right now.

#3

I’m not really sure how to ask it any differently.

Consider a block that spins like a top left/right. The upvector points to the sky. Now change the upvector to point toward world forward. But not change the degree it was already rotated relative.

Hard to articulate this.

#4

You have a start and target upvector:

local start = part.CFrame.UpVector
local target = Vector3.new(1, 1, 1).Unit

You can do this in a single rotation about an axis. The axis is perpendicular to both:

local axis = start:Cross(target)

And figure out the angle between the two

local angle = math.acos(start:Dot(target))

Then just rotate the CFrame by that amount

part.CFrame = CFrame.fromAxisAngle(axis, angle) * part.CFrame.Rotation + part.CFrame.Position

I’ll do some testing to check, but I’m pretty sure that would work. Ignoring cases like full 180 degree rotations or 0 degree rotations, for which you’ll have to write special cases.

Edit: left out one part, gimme a sec :slight_smile: Fixed. Final function:

local function AlignUpVector(cframe: CFrame, up: Vector3)
	local start = cframe.UpVector
	local angle = math.acos(start:Dot(up))
	local axis = nil
	if angle < 0.00001 or angle > math.pi - 0.00001 then
		axis = Vector3.xAxis
	else
		axis = start:Cross(up)
	end
	return CFrame.fromAxisAngle(axis, angle) * cframe.Rotation + cframe.Position
end

Example use:

local target = Vector3.new(1, 1, 1).Unit
part.CFrame = AlignUpVector(part.CFrame, target)

Edit: Modified function to take into account special cases

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Move relative to camera
#5

It might help too if I explain what I’m trying to accomplish.

Basically, I’m trying to detect how much the block spins on the relative Yaxis. I can do this accurately while the upvector is world up. However the upvector is constantly changing. So I was thinking I could translate to the world up vector then grab the current relative Yaxis orientation to see what the change in orientation was.

#6

Ty I think this may work. I’m going to do some testing and verify I can accomplish my goal with it. Looks promising.

#7

Yeah, I think you’d be able to align the previous CFrame’s upvector with the current CFrame’s upVector and then compare the angle between their right vectors?

local function GetSpin(prev: CFrame, current: CFrame)
  local aligned = AlignUpVector(prev, current.UpVector)
  return math.acos(aligned.RightVector:Dot(current.RightVector))
end
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#8

Tyvm and kudos your trig game is def better than mine. :grin:
I should be able to accomplish the end goal with this.

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#9

Fixing it:

local function AlignUpVector(cframe: CFrame, up: Vector3)
	local start = cframe.UpVector
	local angle = math.acos(start:Dot(up))
	local axis = nil
	if angle < 0.00001 or angle > math.pi - 0.00001 or angle ~= angle then
		axis = Vector3.xAxis
	else
		axis = start:Cross(up)
	end
	return CFrame.fromAxisAngle(axis, angle) * cframe.Rotation + cframe.Position
end

added the angle~= angle cause it sometimes returns nan which glitches it.

#10

Angle should never be NaN

You may be giving the function a non-Unit vector, which can cause NaN

You probably want to call .Unit on your Vector before passing it to this function

Otherwise the angle will be wrong or NaN

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