Please make a Vector3.MagnitudeSquared to reduce expensive square roots

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1

Roblox should make a Vector3.MagnitudeSquared function because doing a sqroot is slow
This is a classic game dev optimization.

The Problem

– Vector3.Magnitude does this internally:
math.sqrt(xx + yy + z*z) – sqrt is expensive


## The Workaround Today

-- Roll your own
local function magnitudeSquared(v: Vector3): number
    return v.X*v.X + v.Y*v.Y + v.Z*v.Z
end

-- Compare distances without sqrt
local distSq = magnitudeSquared(a - b)
if distSq < 100*100 then  -- within 100 studs
    -- much faster than (a-b).Magnitude < 100
end

Why It Matters in Roblox

This pattern comes up constantly:

  • NPC aggro range checks (running every heartbeat for many NPCs)
  • Proximity detection
  • Particle/LOD culling

If you’re doing this in a loop with 100+ entities at 60hz, you’re calling sqrt 6000 times/second for no reason when you just need to compare distances.

The Ask Makes Sense

Roblox already has .Magnitude and .Unit — MagnitudeSquared (or .Magnitude2 / .LengthSquared like in Unity/Godot/Unreal) is a natural addition and a zero-cost one to implement.

5 Likes
2

What stops you from doing it manually and declaring the script/function as native?

@native
local function squaredMagnitude(v: Vector3): number
	return v.X*v.X + v.Y*v.Y + v.Z*v.Z
end

(After performing some tests I concluded that this runs 22 times faster than built-in v.Magnitude on my machine)

5 Likes
3

My theory is the engine written in c++ (probably) should have that MagnitudeSquared, because when it has to run a lanugage like lua in a script it is orders of magnitude slower than baked into the engine library.

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4

classic optimization every engine has (Unity’s sqrMagnitude, etc.).

Luau’s native .Magnitude is way faster now, but a built-in Vector3.MagnitudeSquared would still be cleaner + fastest for hot loops.

Roblox should add it. :+1:

2 Likes
5

I agree with your post but out of curiosity, have you ran into performance issues where doing this sqrt over and over was the cause?

3 Likes
6

supporting this too, micro optimization where ever possible is always good

7

This is true. It will be faster. I’m going to write an RFC for this.

8

dudes be saying micro optimize this micro-optimize that but dont micro-optimize their money :joy: micro opt culture is full of inbreds

9

You can dot the vector with itself as a shortcut to get this. Should be on the fast path too :slight_smile:

2 Likes
10

Sure, but this isn’t the appropriate category, move it to Engine Requests.

11

Good point, unfortunately it isn’t available on clients, probably won’t be for a while as they’d have to support so many different system architectures just to make it work on client devices.

12

This came up on Unity a while back. What you’re doing is faster. The conclusion was: On a average PC, you probably will not notice. On an average phone, it is up to six times faster. I filed this concept in my good stuff notes.. :rofl: I knew right away what you were talking about due to that.
Phones benefit the most, PCs barely notice.

1 Like