Help with a dragging system

Help and Feedback Scripting Support
#1

Hello! I am looking to make a X and Y dragging system for my game using the players mouse when I ran into a problem.

I can have the player move the part on the X, Y, and Z axis perfectly fine
xyz.wmv (153.8 KB)

But when I try to lock the axis to only X and Y, this happens
xy.wmv (247.5 KB)

RunService.RenderStepped:Connect(function()
	if Clicking == true then
		if Target ~= nil and CollectionService:HasTag(Target, "FoodItem") then
			
			Target.Position = Vector3.new(Mouse.Hit.x, Mouse.Hit.y, Target.Position.z)
			-- The code above is what has the weird offset with the mouse.
			-- Target is just a pre determined part in workspace.
			
		end
	end
end)

If anybody can help me figure out a solution to fix the weird offset when you drag around a part it would be greatly appriceated! Sorry if this is a stupid question, I have no idea what I’m doing, lol.

3 Likes
#2

It seems like the mouse isn’t hitting where you expect. Try printing the Z coordinate of mouse.Hit, and compare it to what it should be.

I’d guess the FoodItem part isn’t at the same Z coordinate as the part, so you’re getting a kind of parallax effect that is clearly visible in the video.

2 Likes
#3

Thanks for responding! Mouse.Hit.z’s position is indeed behind where the parts z position is. So how exactly would I get rid of this parallax effect to have it exactly where the mouse is on the X and Y axis?

1 Like
#4

You do it by figuring out where the mouse hits the specific plane you’re interested in, by doing ray-plane collision detection. The code for this is available on one of my favorite sites: https://geomalgorithms.com/code.html Specifically we want the function intersect3D_SegmentPlane() - find the 3D intersection of a segment and a plane.

Here's the license, make sure you read and comply with it 
// Copyright 2001, 2012, 2021 Dan Sunday
// This code may be freely used and modified for any purpose
// providing that this copyright notice is included with it.
// There is no warranty for this code, and the author of it cannot
// be held liable for any real or imagined damage from its use.
// Users of this code must verify correctness for their application.

If you want to see the original C++ source, it’s in the file C05_Line_Plane_Intersection.cpp.

Here's the ported Lua code, ready to plop into any roblox game: 
--[[
// Copyright 2001, 2012, 2021 Dan Sunday
// This code may be freely used and modified for any purpose
// providing that this copyright notice is included with it.
// There is no warranty for this code, and the author of it cannot
// be held liable for any real or imagined damage from its use.
// Users of this code must verify correctness for their application.
]]
-- intersect3D_SegmentPlane(): find the 3D intersection of a segment and a plane
--  Input:  
--      segment = a segment = {Vector3 P1, Vector3 P2}, and 
--      plane = a plane = {Vector3 P, Vector N}
--  Output: *I0 = the intersect point (when it exists)
--  Return: 
--      nil,           0 = disjoint (no intersection)
--      Vector3 point, 1 = intersection in the unique point point
--      nil,		   2 = the segment lies in the plane (counted as no intersectio)
local SMALL_NUM = 10e-5
function intersect3D_SegmentPlane(segment, plane)
	local u = segment.P2 - segment.P1
	local w = segment.P1 - plane.P

	local D = plane.N:Dot(u)
	local N = -plane.N:Dot(w)

	if math.abs(D) < SMALL_NUM then     -- segment is parallel to plane
		if N == 0 then                  -- segment lies in plane
			return nil, 2
		else                            -- no intersection
			return nil, 0 
		end
	end

	-- they are not parallel
	-- compute intersect param
	local sI = N/D
	if sI < 0 or sI > 1 then -- no intersection
		return nil, 0
	end

	local intersection = segment.P1 + sI * u -- compute segment intersect point
	return intersection, 1
end

And here’s a test place showing how it can be used to get the intersection of the mouse ray with an any plane, in this case defined by a part.

RayPlaneIntersectTest.rbxl (28.9 KB)

And here's the code for the usage example 
function getMouseRaySegment()
	return {
		P1 = mouse.UnitRay.Origin,
		P2 = mouse.UnitRay.Origin + mouse.UnitRay.Direction * 1000
	}
end

function getPartFrontPlane(part)
	return { -- A plane, represented as a point P in the plane and the normal N of the plane
		P = part.CFrame:PointToWorldSpace(Vector3.new(0, 0, -part.Size.Z/2)),
		N = part.CFrame.LookVector
	}
end

while wait() do
	local s = getMouseRaySegment()
	local p = getPartFrontPlane(game.Workspace.Plane)
	local i, err = intersect3D_SegmentPlane(s, p)
	if i then
		game.Workspace.Cursor.CFrame = CFrame.new() + i
	else
		print(err) --either s is in the plane so there is no UNIQUE intersect, or they don't interesect at all
	end
end

Let me know if you need help adapting it to your specific situation

#5

I’ve almost got it working, newcode.wmv (271.0 KB) , my only issue now, as you can see in the video, is that it’s not preserving the parts original Z position. Sorry for asking so many questions, but how would I make it so that the Z position of the part never changes?

#6

You’d have to place the plane at the original Z position

1 Like