How to do Reflecting rays?

#1

Hi , I have been trying to reflect rays but so far no progress . I am using concepts from unity tutorials but their approach are different hard to try in defold.

Any Suggestions.

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#2

Here is an example project with 1 reflection (which I believe was done right but if I did it wrong please anyone correct me).

ReflectRays.zip (4.7 KB)

The important part is in the reflect script

local max_reflection_length = 1000
local max_reflections = 3
local original_color = vmath.vector4(1,1,1,0.25)
local reflection_color = vmath.vector4(1,0,0,0.5)
local raycast_groups = {hash("default")}

local function draw_line(start_point, end_point, color)
	-- start, end = vmath.vector3()
	-- color = vmath.vector4()
	msg.post("@render:", "draw_line", { start_point = start_point, end_point = end_point, color = color } )
end

local function draw_reflections(self)
	local direction = self.target - self.position
	if vmath.length(direction) == 0 then
		direction = vmath.vector3(0,1,0)
	end
	direction = vmath.normalize(direction)
	--print(direction)

	local target_position = self.position + direction * max_reflection_length

	draw_line(self.position, target_position, original_color)

	local result = physics.raycast(self.position, target_position, raycast_groups)
	if result ~= nil then
		 pprint(result)
		-- sample result 
		-- 	{ --[[000001B088ACE200]]
		-- 	normal = vmath.vector3(0, -1, 0),
		-- 	fraction = 0.089520141482353,
		-- 	position = vmath.vector3(75.563575744629, 48, 0),
		-- 	group = hash: [default],
		-- 	id = hash: [/wall]
		-- }

		-- rvec = ivec - (2 * dot(normal, ivec) * normal)

		local reflected_vector = direction - (2 * vmath.dot(result.normal, direction) * result.normal)

		local reflected_target = result.position + reflected_vector * max_reflection_length
		
		draw_line(result.position, reflected_target, reflection_color)

		-- todo do multiple reflections
	end
	
end

function init(self)
	msg.post(".", "acquire_input_focus")
	self.position = vmath.vector3()
	self.target = vmath.vector3()
end

function final(self)
	-- Add finalization code here
	-- Learn more: https://defold.com/manuals/script/
	-- Remove this function if not needed
end

function update(self, dt)
	--pprint(self.position, self.target)

	go.set_position(self.position)

	draw_reflections(self)
end

function on_message(self, message_id, message, sender)
	-- Add message-handling code here
	-- Learn more: https://defold.com/manuals/message-passing/
	-- Remove this function if not needed
end

function on_input(self, action_id, action)
	if action.x then
		self.target.x = action.x
		self.target.y = action.y
	end

	if action.released then
		self.position.x = action.x
		self.position.y = action.y
	end
end

function on_reload(self)
	-- Add reload-handling code here
	-- Learn more: https://defold.com/manuals/hot-reload/
	-- Remove this function if not needed
end

Sort of related if you want to do lasers :slight_smile:

LaserExample.zip (4.0 KB)

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#3

In this picture, we have an incoming ray with a direction v relative to the plane we want to reflect with.
Then the formula is r = v - 2 * dot(v, n) * n, where the n is the normalized normal (unit length equals 1) of the plane.

In Defold:

local r = v - (2 * vmath.dot(v, normal)) * normal

Sometimes you want to use actual positions, not only directions. Then you need to convert your coordinates into plane space, by picking an (arbitrary) point on the plane.

local v = plane_pos - world_pos -- v now points towards the plane

and once you’re done calculating r, you’ll want to convert back into world space again:

local world_reflection = plane_pos + r

this works in both 2D and 3D.
I hope this helps!

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#4

Awesome, Thanks for the help. This will really help me.

2 Likes

#5

ReflectRaysMultiple.zip (4.7 KB)

Updated version supporting as many reflections as you want. :sunglasses:

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