Change FastCast's projectile acceleration to achieve parabolic trajectory

[DudusJestem]

Hello I am using the FastCast API to manage projectiles in my tool wrapper module and I wanted to ask how could I change the acceleration of the projectile depending on the distance from the position I aimed at to make it hit/destroy itself there (like in the picture) with a parabolic trajectory (like basketball)

This is the code I use for firing function (some parts are from the example gun):

		-- Shortcuts
		local pointerRaycastResult = inputData.RaycastResult
		local projectile = self.Config.Projectile
		-- Shortcuts

		for i = 1, projectile.PerShot do
			-- If the input raycast didn't hit anything (likely when there was no object in front of the raycast)
			-- set the raycast hit position to farthest it could possibly go
			local raycastHitPosition = if pointerRaycastResult then pointerRaycastResult.Position else inputData.FarthestRaycastPosition

			-- Direction to the raycast hit position relative to tool's fire point position
			local inputDirection = (raycastHitPosition - self.FirePoint.WorldPosition).Unit

			local orientational = CFrame.fromOrientation(0, 0, Randomizer:NextNumber(0, Tau))
			local directional = CFrame.new(Vector3.new(), inputDirection)

			local angleSpread = CFrame.fromOrientation(math.rad(Randomizer:NextNumber(projectile.MinSpreadAngle, projectile.MaxSpreadAngle)), 0, 0)
			local direction = (directional * orientational * angleSpread).LookVector
			
			local distance = (self.FirePoint.WorldPosition - raycastHitPosition).Magnitude

			local calculatedAcceleration = self.Config.Projectile.Acceleration
			self.CastBehavior.Acceleration = calculatedAcceleration
			
			local simBullet = self.Caster:Fire(self.FirePoint.WorldPosition, direction, (direction * projectile.Speed), self.CastBehavior)
		end

[nicemike40]

@DudusJestem

change the acceleration of the projectile depending on the distance

The acceleration should always be the same no matter what the distance is. Acceleration means "how quickly the speed changes.

Seems to me like you just have to do

self.CastBehavior.Acceleration = Vector3.new(0, -196.2, 0)

[DudusJestem]

Well, the projectile still doesn’t land where I aimed at when I set it to the workspace gravity

[nicemike40]

Oh sorry, so it seems what you’re asking is how to change the launch angle of the projectile so that, with your acceleration, it lands at the right spot?

[DudusJestem]

Yes, sorry, english is not my main language, I didn’t know how to word it properly

[nicemike40]

I partially answered this before, it may be helpful to you:

Any way to calculate bullet drop through gravity

I made this desmos a while ago that does some of these calculations: Projectile Motion Calculation You can change the velocity/gravity with the sliders on the left, and you can drag the target point around. Maybe the angle calculations will be useful to you. tl;dr: [image] Where v is the launch velocity, g is gravity, and (x_0, y_0) is the target. I’ll leave it as an excercise for the reader to turn it into Lua

[nicemike40]

Here’s an example of that equation translated to Lua and moved to 3D. I put this in a LocalScript in StarterCharacterScripts for testing.

It just launches a part by setting it’s Velocity and assuming the acceleration is -workspace.Gravity, but the LaunchDirection code should be easy to adapt to your FastCast code.

You can see how there are two solutions, a higher arc and a lower arc. I toggle between them here, but you could just pick one.

There are some unsolved problems (like if you are too far away to make it with that launch angle, what should you do?) but it’s a good starting point.

Results:

local character = script.Parent
local root = character:WaitForChild("HumanoidRootPart")
local mouse = game.Players.LocalPlayer:GetMouse()

-- Compute 2D launch angle
-- v: launch velocity
-- g: gravity (positive) e.g. 196.2
-- d: horizontal distance
-- h: vertical distance
-- higherArc: if true, use the higher arc. If false, use the lower arc.
local function LaunchAngle(v: number, g: number, d: number, h: number, higherArc: boolean)
	local v2 = v * v
	local v4 = v2 * v2
	local root = math.sqrt(v4 - g*(g*d*d + 2*h*v2))
	if not higherArc then root = -root end
	return math.atan((v2 + root) / (g * d))
end

-- Compute 3D launch direction from
-- start: start position
-- target: target position
-- v: launch velocity
-- g: gravity (positive) e.g. 196.2
-- higherArc: if true, use the higher arc. If false, use the lower arc.
local function LaunchDirection(start, target, v, g, higherArc: boolean)
	-- get the direction flattened:
	local horizontal = Vector3.new(target.X - start.X, 0, target.Z - start.Z)
	
	local h = target.Y - start.Y
	local d = horizontal.Magnitude
	local a = LaunchAngle(v, g, d, h, higherArc)
	
	-- NaN ~= NaN, computation couldn't be done (e.g. because it's too far to launch)
	if a ~= a then return nil end
	
	-- speed if we were just launching at a flat angle:
	local vec = horizontal.Unit * v
	
	-- rotate around the axis perpendicular to that direction...
	local rotAxis = Vector3.new(-horizontal.Z, 0, horizontal.X)
	
	-- ...by the angle amount
	return CFrame.fromAxisAngle(rotAxis, a) * vec
end

higher = true
mouse.Button1Down:Connect(function()
	local dir = LaunchDirection(root.Position, mouse.Hit.Position, 50, workspace.Gravity, higher)
	higher = not higher -- toggle
	if not dir then
		print("nope!")
		return
	end
	local p = Instance.new("Part")
	p.Size = Vector3.new(1, 1, 1)
	p.CanCollide = false
	p.Position = root.Position
	p.Velocity = dir
	p.Parent = workspace
end)

[nicemike40]

Here’s some more detail on what to do if the distance is too far. This isn’t really part of your question, but I’ll go into it for posterity

Here’s an updated desmos with the max range stuff: Projectile Motion Calculation With Max

Explanation:

A bit of math to help.

The target is only within range when the part under the square root is positive, i.e.

We’re trying to solve for x here. But we have another unknown, y (which equates to “the height of the projectile when we launch it as far as we can”).

Instead, I will calculate the slope of the start → target line (m = y0/x0), then replace the y with mx.

So we’re finding the roots of this quadratic now:

Solving using the quadratic formula, simplifying, etc.:

This is “the furthest horizontal distance we can get along the start → target slope”.

Now we just need the launch angle to get to that distance:

(the same formula as we had originally, but we don’t need the square root because it’s 0)

Turning this into lua:

local character = script.Parent
local root = character:WaitForChild("HumanoidRootPart")
local mouse = game.Players.LocalPlayer:GetMouse()

local function MaxDistance(v, g, d, h)
	local m = h / d
	local v2 = v * v
	return (v2*math.sqrt(m*m + 1) - m*v2) / g
end

-- Compute 2D launch angle. First return value is true if within range, second is the angle.
-- v: launch velocity
-- g: gravity (positive) e.g. 196.2
-- d: horizontal distance
-- h: vertical distance
-- higherArc: if true, use the higher arc. If false, use the lower arc.
local function LaunchAngle(v: number, g: number, d: number, h: number, higherArc: boolean): (boolean, number)
	local max_x = MaxDistance(v, g, d, h)
	local v2 = v * v
	
	if d > max_x then
		return false, math.atan(v2 / (g * max_x))
	end
	
	local v4 = v2 * v2
	local root = math.sqrt(v4 - g*(g*d*d + 2*h*v2))
	if not higherArc then root = -root end
	return true, math.atan((v2 + root) / (g * d))
end

-- Compute 3D launch direction from. First return value is true if within range, second is the direction.
-- start: start position
-- target: target position
-- v: launch velocity
-- g: gravity (positive) e.g. 196.2
-- higherArc: if true, use the higher arc. If false, use the lower arc.
local function LaunchDirection(start, target, v, g, higherArc: boolean)
	-- get the direction flattened:
	local horizontal = Vector3.new(target.X - start.X, 0, target.Z - start.Z)
	
	local h = target.Y - start.Y
	local d = horizontal.Magnitude
	local inRange, a = LaunchAngle(v, g, d, h, higherArc)
	
	-- speed if we were just launching at a flat angle:
	local vec = horizontal.Unit * v
	
	-- rotate around the axis perpendicular to that direction...
	local rotAxis = Vector3.new(-horizontal.Z, 0, horizontal.X)
	
	-- ...by the angle amount
	return inRange, CFrame.fromAxisAngle(rotAxis, a) * vec
end

higher = true
mouse.Button1Down:Connect(function()
	local inRange, dir = LaunchDirection(root.Position, mouse.Hit.Position, 50, workspace.Gravity, higher)
	higher = not higher -- toggle
	local p = Instance.new("Part")
	p.Size = Vector3.new(1, 1, 1)
	p.CanCollide = false
	p.BrickColor = inRange and BrickColor.Green() or BrickColor.Red()
	p.Position = root.Position
	p.Velocity = dir
	p.Parent = workspace
end)

[DudusJestem]

Bro, you really are a math genius, thank you and have a nice new year! Btw how could I make the “higher” angle a bit flatter?

[nicemike40]

Your options are:

• Just use the lower arc (by passing false as the last argument, or leaving it out)
• Decrease the launch speed
• Increase gravity (or your bullet’s downwards acceleration)

Just so you know, this isn’t the only way to solve this problem. For example, you can fix the angle and determine the launch velocity.

You could also have some combination of the two ways depending on what exactly you want the result to look like.

A simple alternative is to calculate the launch angle for like three or four possible launch velocities, and pick the slowest one that works.

[nicemike40]

Oh I got the higherArc argument backwards. Replace this line:

	if higherArc then root = -root end

with

	if not higherArc then root = -root end