How would I Use Raycasting for Particle Position?

[Viewported]

Hello! I am currently trying to replicate a dust effect caused by helicopters when hovering near terrain. I originally tried to use a prismatic constraint with a particle emitter, but the part would keep getting stuck on terrain and other objects. It seems as if I need to use raycasting to deal with this.
(Below is a screenshot of the hierarchy and setup.)

I do not have any knowledge of how to use raycasting. I need the helicopter to raycast downwards on the Y axis to locate what part or terrain is below the helicopter, and the attachment with the particle emmiter should be brought to the y location of the raycast.

[Manky94]

Here is how I would do it:

1. Cast a ray downwards from the helicopter.
2. Check to see if anything is hit.
3. If anything is hit, position a part there and add a particle emitter (in my example, I re-use the same part and disable/enable the particle emitter).
4. If something isn’t hit, disable the particle emitter.

Example (hide the part of course)
https://gyazo.com/9b0a51b4c7f91f7b779e810d979d817c

local MAX_DISTANCE = 20
local MAX_SMOKE_SIZE = 1

local Emitter = script.Parent.Emitter

local raycastParams = RaycastParams.new()
raycastParams.IgnoreWater = false -- Don't ignore water
raycastParams.FilterType = Enum.RaycastFilterType.Blacklist
raycastParams.FilterDescendantsInstances = {script.Parent.Emitter} -- Ignore the emitter and anything else we don't want here!

while (wait()) do
										-- Origin, Direction, RaycastParams
	local raycastResult = workspace:Raycast(script.Parent.Helicopter.Position, Vector3.new(0, -1, 0) * MAX_DISTANCE, raycastParams)
	
	if (raycastResult) then
		Emitter.Smoke.Enabled = true -- Enable smoke
		Emitter.Position = raycastResult.Position -- Set position of marker
		
		local Distance = (raycastResult.Position - script.Parent.Helicopter.Position).Magnitude -- Could use something like this to make smoke smaller when helicopter is higher up.
		Emitter.Smoke.Size = (Distance / MAX_DISTANCE) * MAX_SMOKE_SIZE
	else
		Emitter.Smoke.Enabled = false -- Disable the particle when we don't hit anything
	end
end

[CompilerError]

Here’s the documentation for the Raycast API in Roblox. WorldRoot | Roblox Creator Documentation.

So I think what you’d probably want to do here is create a RaycastParams | Roblox Creator Documentation object that uses the Enum.RaycastFilterType.Blacklist filter type and the helicopter is in the RaycastParams.FilterDescendantsInstances list so that the ray doesn’t hit the helicopter.

A quick example of how this could be done is:

local helicopter = script.Parent -- Assumes the script is within the helicopter model.
local floor_test_part = helicopter.FloorTestPart -- Some part that is placed where the ray test should start
-- and is oriented in the direction you want the ray to test in (top facing away from the ground.)
local particle_emitter_part = helicopter.SmokeEmitter

local maximum_ground_distance = 30 -- How far the ray will go to test for the floor

local ray_params = RaycastParams.new()
ray_params.FilterType = Enum.RaycastFilterType.Blacklist
ray_params.FilterDescendantsInstances = { helicopter }

--- Fires a ray directly downward from the floor_test_part CFrame.
-- @return RaycastResult for the ray or nil if the ray did not hit anything.
local function get_ground()
  local heli_position = floor_test_part.Position
  local floor_direction = (floor_test_part.CFrame * CFrame.new(0, -maximum_ground_distance, 0)).LookVector

  -- Perform the ray test to find the ground beneath the helicopter in the direction of the helicopter
  -- and not just straight down, since rotors don't blow air straight down on the Y-axis.
  return workspace:Raycast(heli_position, floor_direction, ray_params)
end

while wait(1) do
  local floor_ray_result = get_ground()
  if (floor_ray_result) then
    -- Position your particles here with floor_ray_result.Position
    particle_emitter_part.Position = floor_ray_result.Position;
    -- Could also angle it with the floor using floor_ray_result.Normal
  end
end

See RaycastResult | Roblox Creator Documentation for details on what a ray cast returns.

I haven’t tested this code but it should serve as a decent example of how you can do this.

[Viewported]

Neither of those solutions seem to work, nothing happens ingame.

[Manky94]

So in the first one, Helicopter is just the part where the ray should be sent from.

local raycastResult = workspace:Raycast(script.Parent.Helicopter.Position, Vector3.new(0, -1, 0) * MAX_DISTANCE, raycastParams)

Should be changed to

local raycastResult = workspace:Raycast(script.Parent.?????.Position, Vector3.new(0, -1, 0) * MAX_DISTANCE, raycastParams)

where ????? is the part that you want to emit the ray from.

[Viewported]

alright it works, the only issue I have now is it casts the ray once and then stops. Is there a way to keep casting the ray over and over?

[SummerEquinox]

You can link the cast to RunService so that it checks per frame. Provided you aren’t spawning 50 helicopters you’ll likely be fine.

See RunService. I would recommend using Heartbeat so that your calculation happens after each physics simulation.

You can write this connection to a variable and disconnect it when the helicopter becomes inactive.

local RunService = game:GetService("RunService")
local Helicopter = {}
Helicopter.__index = Helicopter

function Helicopter.new(helicopterObj)
	local t = {}
	setmetatable(t, Helicopter)
	
	t.Root = helicopterObj or false
	t.Connection = nil

	-- Set up various helicopter states & functions.

	return t
end

function Helicopter:RoutineCheck()
	return function()
		-- Raycast
	end
end

An example of using this to build the connection would be this:

local Example = Helicopter.new()

Example.Connection = RunService.Heartbeat:Connect(Example:RoutineCheck()) -- Connect to RunService
Example.Connection:Disconnect() -- Disconnect from RunService

Whether or not my example system is similar to yours doesn’t matter. You can run the Raycast on RunService and disconnect it when it becomes inactive.