How can I get a list of parts that are inside a Frame?

Hello.

I'm in the process of making a RTS game in Roblox, but I've came up with an issue.

Making it short, I'm currently scripting a selection box UI. I've got it ready; it's like the Windows right click dragging that forms that blue rectangle box.

However, after detecting that the user has lifted their mouse left button and it is ready to detect units inside of the rectangle, I have came unable to be able to list these.

Does anybody have any idea how I could do this?

I've got this so far, but of course it doesn't work.

local xS = script.Parent.Selection.Position.X.Offset
			if script.Parent.Selection.Size.X.Offset < 0 then
				xS = xS - math.abs(script.Parent.Selection.Size.X.Offset)
			end
			
			local xE = script.Parent.Selection.Position.X.Offset
			if script.Parent.Selection.Size.X.Offset > 0 then
				xE = xE + script.Parent.Selection.Size.X.Offset
			end
			
			local yS = script.Parent.Selection.Position.Y.Offset
			if script.Parent.Selection.Size.Y.Offset < 0 then
				yS = yS - math.abs(script.Parent.Selection.Size.Y.Offset)
			end
			
			local yE = script.Parent.Selection.Position.Y.Offset
			if script.Parent.Selection.Size.Y.Offset > 0 then
				yE = yE + script.Parent.Selection.Size.Y.Offset
			end
			
			local castPoints = {}
			
			print(xS)
			print(xE)
			warn(yS)
			warn(yE)
			
			for x = xS, xE do for y = yS, yE do
				table.insert(castPoints, Vector3.new(x, y, 0))
			end end 
			local objs = workspace.CurrentCamera:GetPartsObscuringTarget(castPoints, { })
			
			for i, v in pairs(objs) do
				print(v.Name)
			end

I've got to the conclusion and answer about this problem.

Thanks @fredfishy for hinting me on ScreenPointToRay!

local dist = (workspace.CurrentCamera.CoordinateFrame.p - lp.Character.Head.Position).magnitude -- Calculates the camera's current distance

for x = xS, xE do
	for y = yS, yE do
		local ray = workspace.CurrentCamera:ScreenPointToRay(x, y, dist * 5) -- Sets maximum detection distance to 5 times the distance
		
		table.insert(castPoints, ray.Origin) -- Only using Origin to prevent camera rotation bugging the selection
	end
end

local objs = workspace.CurrentCamera:GetPartsObscuringTarget(castPoints, { })