Compute position of a ring in front of MT player around cursor.

init.lua

vector.cross = vector.cross or function(a, b)
	return {
		x = a.y * b.z - a.z * b.y,
		y = a.z * b.x - a.x * b.z,
		z = a.x * b.y - a.y * b.x
	}
end

local function project(player)
	local camera_z = player:get_look_dir()
	local camera_x = minetest.yaw_to_dir(player:get_look_horizontal() + math.pi / 2)
	local camera_y = vector.cross(camera_x, camera_z)
	for theta = 0, math.pi * 2 - 0.1, 0.1 do
		local cv = {x = math.cos(theta) * 2, y = math.sin(theta) * 2, z = 2}
		local wv = player:get_pos()
		wv.y = wv.y + player:get_properties().eye_height
		wv = vector.add(wv, vector.multiply(camera_x, cv.x))
		wv = vector.add(wv, vector.multiply(camera_y, cv.y))
		wv = vector.add(wv, vector.multiply(camera_z, cv.z))
		minetest.add_particle({
				pos = wv,
				size = 0.25,
				texture = "solid_generated_color.png"
			})
	end
end

local function tick()
	for _, v in pairs(minetest.get_connected_players()) do
		project(v)
	end
	minetest.after(0.25, tick)
end
tick()