Jitter effect when multiplying by delta time

[SkyLord_103 (Thy_Rizsler)]

I’m trying to make a weapon that has some basic movement with a spring module, based on recoil, character movement, and mouse movement.

When I add the mouse movement to the spring and multiply it by delta time in a RenderStepped function the product from the spring makes the weapon jitter but when I remove the * deltaTime and rely on my framerate the weapon moves smoothly.
Why is the delta time adding a jitter?

What I get the jitter
What I want the smoothness

Devforum just wouldn’t upload directly

This code is the jist of what happens for the mouse movement

local mouse_spring = Springs.Vector3Spring()

-- RenderStepped function
game.RunService.RenderStepped:Connect(function(deltaTime)
  local mouseDelta = UserInputService:GetMouseDelta()
  mouse_spring:shove(Vector3.new(mouseDelta.X, mouseDelta.Y))
  
  -- The jitter
  local sway_shove = (mouse_spring:update(deltaTime)* 24 * deltaTime) * RADIAN_CONV
  -- No jitter
  local sway_shove = (mouse_spring:update(deltaTime)) * RADIAN_CONV
  
  finalCFrame *= CFrame.Angles(sway_shove.Y, sway_shove.X / 5, sway_shove.X / 3)
end)

The one, the only, the Spring module!

local ITERATIONS = 16
local master = {}

export type Vector3Spring = {
	target: Vector3,
	position: Vector3,
	velocity: Vector3,
	cap: Vector3 | nil,
	
	mass: number,
	force: number,
	damping: number,
	speed: number,

	shove: (self: Spring, force: Vector3) -> (),
	update: (self: Spring, deltaTime: number) -> (Vector3)
}

function master.Vector3Spring(
	mass: number?,
	force: number?,
	damping: number?,
	speed: number?
)

	local spring: Vector3Spring = {
		target = Vector3.new();
		position = Vector3.new();
		velocity = Vector3.new();

		['mass'] = mass or 5;
		['force'] = force or 50;
		['damping'] = damping or 4;
		['speed'] = speed  or 4;

		shove = function(self: Spring, force: Vector3)
			local x, y, z	= force.X, force.Y, force.Z
			if x == math.huge or x == -math.huge then x = 0 end
			if y == math.huge or y == -math.huge then y = 0 end
			if z == math.huge or z == -math.huge then z = 0 end

			self.velocity = self.velocity + Vector3.new(x, y, z)
		end,

		update = function(self: Spring, deltaTime: number)
			local scaledDeltaTime = math.min(deltaTime, 1) * self.speed / ITERATIONS

			for i = 1, ITERATIONS do
				local iterationForce = self.target - self.position
				local acceleration = (iterationForce * self.force) / self.mass

				acceleration = acceleration - self.velocity * self.damping

				self.velocity = self.velocity + acceleration * scaledDeltaTime
				self.position = self.position + self.velocity * scaledDeltaTime
			end

			return self.position
		end
	}

	return spring
end

return master

Thanks!

• SkyLord_103

[azqjanna]

What is deltaTime coming from? You are using deltaTime twice in one of the formulas, which can’t be right.

[SkyLord_103 (Thy_Rizsler)]

It’s coming from the dt parameter in the RenderStepped function.

And holy guacamole what an oversight… I feel embarrassed to be a programmer rn.

I am multiplying by delta time in the wrong spot

  mouse_spring:shove(Vector3.new(mouseDelta.X, mouseDelta.Y) * deltaTime)
  local sway_shove = (mouse_spring:update(deltaTime)* 24) * RADIAN_CONV

Do I even need to multiply the mouseDelta by deltaTime