Box2D Queries

-- Input action from the default input bindings for mouse or touch.
local TOUCH = hash("touch")

-- Static targets used by the example. Each entry stores the game object id,
-- its normal sprite color, and the AABB half-size used for the helper outline.
local TARGETS = {
	{ id = "box_target", color = vmath.vector4(0.4, 0.7, 1.0, 1.0), aabb_half_size = vmath.vector3(68.0, 68.0, 0.0) },
	{ id = "circle_target", color = vmath.vector4(0.9, 0.5, 0.2, 1.0), aabb_half_size = vmath.vector3(68.0, 68.0, 0.0) },
	{ id = "triangle_target", color = vmath.vector4(0.2, 0.5, 0.9, 1.0), aabb_half_size = vmath.vector3(32.0, 32.0, 0.0) },
	{ id = "diamond_target", color = vmath.vector4(0.9, 0.9, 0.4, 1.0), aabb_half_size = vmath.vector3(32.0, 32.0, 0.0) },
}

-- Query sizes used by the pointer-controlled AABB and circle queries.
local AABB_HALF_SIZE = vmath.vector3(66.0, 66.0, 0.0)
local CIRCLE_RADIUS = 64.0
local RAY_ORIGIN = vmath.vector3(360.0, 360.0, 0.0)

-- Colors used for helper lines, query overlays, and hit feedback.
local LINE_COLOR = vmath.vector4(1.0, 1.0, 1.0, 0.75)
local AABB_COLOR = vmath.vector4(1.0, 1.0, 1.0, 0.35)
local CIRCLE_COLOR = vmath.vector4(0.4, 0.9, 1.0, 0.35)
local HIT_COLOR = vmath.vector4(0.3, 1.0, 0.4, 1.0)

function init(self)
	-- Listen for mouse and touch input
	msg.post(".", "acquire_input_focus")

	-- Get the active Box2D world.
	self.world = b2d.get_world()

	-- Initialize the pointer position and targets
	self.pointer = vmath.vector3(360.0, 360.0, 0.0)
	self.targets = {}

	-- Fill in the targets data
	for index, target in ipairs(TARGETS) do
		local position = go.get_position(target.id)
		local aabb = { lower = position - target.aabb_half_size, upper = position + target.aabb_half_size }

		-- Cache target positions and defined AABBs at startup because the targets are static.
		self.targets[index] = {
			id = target.id,
			color = target.color,
			position = position,
			aabb = aabb,
		}
	end
end

function on_input(self, action_id, action)
	-- Handle touch/mouse input
	if action_id == TOUCH or action_id == nil then
		-- Store the pointer in world coordinates
		self.pointer = vmath.vector3(action.x, action.y, 0.0)
	end
end

-- Draw one temporary helper line through the render socket. They are sent every frame.
local function draw_line(from, to, color)
	msg.post("@render:", "draw_line", {
		start_point = from,
		end_point = to,
		color = color,
	})
end

-- Draw a rectangle outline from a Box2D AABB table with lower and upper fields.
local function draw_aabb(aabb, color)
	local lower = aabb.lower
	local upper = aabb.upper

	-- Create the corners of AABB to draw the lines connecting them
	local bottom_left = vmath.vector3(lower.x, lower.y, 0.0)
	local bottom_right = vmath.vector3(upper.x , lower.y, 0.0)
	local top_right = vmath.vector3(upper.x, upper.y, 0.0)
	local top_left = vmath.vector3(lower.x, upper.y, 0.0)

	-- Draw the 4 lines forming a rectangle representing the AABB
	draw_line(bottom_left, bottom_right, color)
	draw_line(bottom_right, top_right, color)
	draw_line(top_right, top_left, color)
	draw_line(top_left, bottom_left, color)
end

-- Return the Box2D body from a V2 or V3 overlap/raycast hit.
local function get_hit_body(hit)
	-- Box2D V2 overlap results expose the hit body directly.
	if hit.body then
		return hit.body
	elseif hit.fixture then
		return hit.fixture.body -- Box2D V2 raycast results expose the hit body through the fixture.
	elseif hit.shape then
		return b2d.shape.get_body(hit.shape.shape_id) -- Box2D V3 raycast results expose a shape info table with a shape id.
	end

	-- Box2D V3 overlap results expose a shape id, so the body is resolved from the shape.
	return b2d.shape.get_body(hit.shape_id) -- <7>
end

-- Convert overlap query results into a set of target ids.
local function collect_hit_targets(self, hits)
	local hit_targets = {}

	-- For each hit - read the hit body's position so it can be matched to one target game object.
	for _, hit in ipairs(hits) do
		local body_position = b2d.body.get_position(get_hit_body(hit))

		-- For each target check if position
		for _, target in ipairs(self.targets) do

			-- Check if shape position is almost the same as the given target position
			if vmath.length_sqr(body_position - target.position) < 0.1 then
				-- Save the data about hit in the table
				hit_targets[target.id] = true
				break
			end

		end
	end

	return hit_targets
end

function update(self, dt)
	-- Update the position of the queries
	local lower = self.pointer - AABB_HALF_SIZE
	local upper = self.pointer + AABB_HALF_SIZE
	local aabb = { lower = lower, upper = upper }
	local circle = { type = b2d.shape.SHAPE_TYPE_CIRCLE, center = self.pointer, radius = CIRCLE_RADIUS }
	local ray_delta = self.pointer - RAY_ORIGIN

	go.set_position(self.pointer, "/aabb")
	go.set_position(self.pointer, "/circle_query")

	-- Query all fixtures or shapes whose AABB overlaps the pointer-centered box
	local aabb_hits = b2d.world.overlap_aabb(self.world, aabb, nil, 16)

	-- Query all fixtures or shapes overlapping the pointer-centered circle
	local circle_hits = b2d.world.overlap_shape(self.world, circle, nil, 16)

	-- Cast a ray from the scene center using a translation vector.
	local ray_hits = b2d.world.cast_ray(self.world, RAY_ORIGIN, ray_delta, nil, 16)

	local aabb_targets = collect_hit_targets(self, aabb_hits)
	local circle_targets = collect_hit_targets(self, circle_hits)
	local ray_targets = collect_hit_targets(self, ray_hits)

	-- Change the circle shape tint to show if any overlap is detected
	go.set("/aabb#sprite", "tint", #aabb_hits > 0 and HIT_COLOR or AABB_COLOR)
	go.set("/circle_query#sprite", "tint", #circle_hits > 0 and HIT_COLOR or CIRCLE_COLOR)

	-- Draw the current AABB query visual shape to show if any overlap is detected
	draw_aabb(aabb, #aabb_hits > 0 and HIT_COLOR or LINE_COLOR)
	draw_line(RAY_ORIGIN, self.pointer, #ray_hits > 0 and HIT_COLOR or LINE_COLOR) -- Draw the ray in white, or green when it hits anything.

	-- Draw defined target AABBs outlines and highlight them when hit by the AABB query
	for _, target in ipairs(self.targets) do
		local shape_hit = circle_targets[target.id] or ray_targets[target.id]
		go.set("/" .. target.id .. "#sprite", "tint", shape_hit and HIT_COLOR or target.color) -- <12> Tint target sprites when the circle shape query or raycast hits the actual target shape.
		draw_aabb(target.aabb, aabb_targets[target.id] and HIT_COLOR or LINE_COLOR) -- <13> Highlight target AABBs only when the AABB query hits that target.
	end

	-- Show how many fixtures or shapes each query hit
	label.set_text("#counts", string.format("AABB: %d   Circle: %d   Ray: %d", #aabb_hits, #circle_hits, #ray_hits)) -- <14>
end