particles.lua

local gfx = (...) or _G.gfx
local META = prototype.CreateTemplate("particle_2d")
META:GetSet("Speed", 1)
META:GetSet("Rate", 0.1)
META:GetSet("EmitCount", 1)
META:GetSet("Position", Vec2(0, 0))
META:GetSet("Additive", true)
META:GetSet("ThinkTime", 0.1)
META:GetSet("CenterAttractionForce", 0)
META:GetSet("PosAttractionForce", 0)
META:GetSet("MoveResolution", 0)
META:GetSet("Texture", NULL)
META:GetSet("ScreenRect", Rect())

function gfx.CreateParticleEmitter(max)
	max = max or 1000
	local self = META:CreateObject()
	self.max = max
	self.particles = {}
	self.last_emit = 0
	self.next_think = 0
	self.poly = gfx.CreatePolygon2D(max * 6)
	return self
end

local list_insert = list.insert
local list_remove = list.remove
local math_deg = math.deg
local math_lerp = math.lerp
local math_ceil = math.ceil
local math_randomf = math.randomf

function META:Update(dt)
	local time = system.GetElapsedTime()

	if self.Rate == 0 then
		self:Emit()
	elseif self.Rate ~= -1 then
		if self.last_emit < time then
			self:Emit()
			self.last_emit = time + self.Rate
		end
	end

	local remove_these = {}
	local center = Vec3(0, 0, 0)
	dt = dt * self.Speed
	local cull = not self.ScreenRect:IsZero()

	for i, p in ipairs(self.particles) do
		if p.life_end < time or (not p.Jitter and p.life_mult < 0.001) then
			list_insert(remove_these, i)
		else
			if self.CenterAttractionForce ~= 0 and self.attraction_center then
				p.Velocity.x = p.Velocity.x + (
						self.attraction_center.x - p.Position.x
					) * self.CenterAttractionForce
				p.Velocity.y = p.Velocity.y + (
						self.attraction_center.y - p.Position.y
					) * self.CenterAttractionForce
			end

			if self.PosAttractionForce ~= 0 then
				p.Velocity.x = p.Velocity.x + (self.Position.x - p.Position.x) * self.PosAttractionForce
				p.Velocity.y = p.Velocity.y + (self.Position.y - p.Position.y) * self.PosAttractionForce
			end

			-- velocity
			if p.Velocity.x ~= 0 then
				p.Position.x = p.Position.x + (p.Velocity.x * dt)
				p.Velocity.x = p.Velocity.x * p.Drag
			end

			if p.Velocity.y ~= 0 then
				p.Position.y = p.Position.y + (p.Velocity.y * dt)
				p.Velocity.y = p.Velocity.y * p.Drag
			end

			p.life_mult = (p.life_end - time) / p.LifeTime

			if self.CenterAttractionForce ~= 0 then center = center + p.Position end

			if cull then
				if
					p.Position.x > self.ScreenRect.w or
					p.Position.y > self.ScreenRect.h or
					p.Position.x < self.ScreenRect.x or
					p.Position.y < self.ScreenRect.y
				then
					list_insert(remove_these, i)
				end
			end
		end
	end

	self.attraction_center = center / #self.particles
	list.multi_remove(self.particles, remove_these)
end

function META:Draw()
	render.SetPresetBlendMode(self.Additive and "additive" or "alpha")

	if self.Texture:IsValid() then
		render2d.SetTexture(self.Texture)
	else
		render2d.SetTexture()
	end

	render2d.SetColor(1, 1, 1, 1)

	for i, p in ipairs(self.particles) do
		local size = math_lerp(p.life_mult, p.EndSize, p.StartSize)
		local alpha = math_lerp(p.life_mult, p.EndAlpha, p.StartAlpha)
		local length_x = math_lerp(p.life_mult, p.EndLength.x, p.StartLength.x)
		local length_y = math_lerp(p.life_mult, p.EndLength.y, p.StartLength.y)
		local jitter = math_lerp(p.life_mult, p.EndJitter, p.StartJitter)

		if jitter ~= 0 then
			size = size + math_randomf(-jitter, jitter)
			alpha = alpha + math_randomf(-jitter, jitter)
		end

		local w = size * p.Size.x
		local h = size * p.Size.y
		local a = 0

		if not (length_x == 0 and length_y == 0) and self.Mode2D then
			a = math_deg(p.Velocity:GetAngles().y)

			if length_x ~= 0 then w = w * length_x end

			if length_y ~= 0 then h = h * length_y end
		end

		local ox, oy = w * 0.5, h * 0.5
		self.poly:SetColor(p.Color.r, p.Color.g, p.Color.b, p.Color.a * alpha)
		local x, y = p.Position.x, p.Position.y

		if self.MoveResolution ~= 0 then
			x = math_ceil(x * self.MoveResolution) / self.MoveResolution
			y = math_ceil(y * self.MoveResolution) / self.MoveResolution
		end

		self.poly:SetRect(i, x, y, w, h, p.Angle + a, ox, oy)
	end

	self.poly:Draw()
end

function META:GetParticles()
	return self.particles
end

local create_particle

do
	local META = prototype.CreateTemplate("particle")
	META:GetSet("Position", Vec3(0, 0, 0))
	META:GetSet("Velocity", Vec3(0, 0, 0))
	META:GetSet("Drag", 0.98)
	META:GetSet("Size", Vec2(1, 1))
	META:GetSet("Angle", 0)
	META:GetSet("StartJitter", 0)
	META:GetSet("EndJitter", 0)
	META:GetSet("StartSize", 10)
	META:GetSet("EndSize", 0)
	META:GetSet("StartLength", Vec2(0, 0))
	META:GetSet("EndLength", Vec2(0, 0))
	META:GetSet("StartAlpha", 1)
	META:GetSet("EndAlpha", 0)
	META:GetSet("LifeTime", 1)
	META:GetSet("Color", Color(1, 1, 1, 1))

	function META:SetLifeTime(n)
		self.LifeTime = n
		self.life_end = system.GetElapsedTime() + n
	end

	META:Register()
	META.__index = META
	create_particle = function()
		return setmetatable({Position = Vec2()}, META) -- META:CreateObject()
	end
end

function META:AddParticle()
	local p = create_particle()
	p.Position.x = self.Position.x
	p.Position.y = self.Position.y
	p.life_mult = 1
	p:SetLifeTime(1)

	if #self.particles >= self.max then list_remove(self.particles, 1) end

	list_insert(self.particles, p)
	return p
end

function META:Emit()
	for _ = 1, self.EmitCount do
		local p = self:AddParticle()

		if self.OnEmit then self:OnEmit(p) end
	end
end

META:Register()