# Closest Surface Position Using :GetPartBoundsInBox?

Hello, I am trying to figure out a way to get the closest surface Vector3 of a part inside a specific hitbox I have created. To further elaborate:

I have a starting CFrame, let’s label it as variable `origin`. From this `origin` CFrame, we cast a hitbox facing forwards that extends a certain amount with a specific size and width. Using this hitbox, we can use `workspace:GetPartBoundsInBox()` to get a list of all parts inside of this created hitbox, as we can set the first parameter using the `origin` CFrame and we can change the size as we please. From this, we gather some new variables:

• `hitboxSize` = Vector3 we can customize as we please
• `hitboxCenter` = `origin * CFrame.new(hitboxSize.X / 2, 0, 0)`

So what I am left with is these specified variables and an array of all the parts inside of the hitbox. What I am struggling with though is I want to get the closest surface POSITION, not the closest PART, to the `origin`. If I wanted to get the closest part, I could just iterate through all the bounding parts and using .Magnitude from the `origin` position to find the closest one, but in examples like these:

That simply wouldn’t work, as `part1`'s Position is way farther than `part2`, but we clearly see that in terms of the hitbox, `part1` is the closest. So if there was a way to find the closest surface position of `part1` relative to the hitbox, it would solve this issue of parts having varying sizes.

Ideally it would get the closest surface relative to the part that’s closest to the hitbox, like this:

Orange obviously being the Vector3 returned by this function. I could have some hacky solutions like firing rays around the hitbox, but I want this to work no matter the width of the hitbox, and in edge-case scenarios where the part is too thin and doesn’t completely cover the hitbox. I know there’s some mathmetical solution to this involving cross and dot products or something that I just can’t think of right now, so any help would be appreciated. You can use the variables I set above for your solution if you’d like. Thank you.

2 Likes

Unfortunately, there kinda isn’t. You’re trying to do something called an object sweep, which is pretty non-trivial.

If you wanted a mathematical solution, it’s theoretically possible but would involve coming up with signed distance functions for every piece of geometry you might encounter. Probably would be slow too.

Instead you could use Roblox’s built in collision tests and march an object along the direction.

Here’s a thread discussing the technique

Here’s a thread where I (untested) implement it for a single axis

Edit: actually this is only part of the solution I guess, since it doesn’t give you the actual intersection point but only the point where the collision-test-object stopped. Might give some inspiration though.

Edit2: a dense grid of raycasts seems simpler if you can get away with it

3 Likes

Yea I assumed so, guess I’ll just resort to lots of raycasts.

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