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Version Date Comments
0.1 21/05/2024 Initial version

Matrix Rain Simulation

This is a literate program to simulate the famous Matrix Rain simulation. It is a straightforward implementation written in Love2d. The rain visualization is implemented using classes which represent the objects displayed on the screen.

The source code for this program is on github at


Here is a demo of the final program output. Note that it is in low resolution to reduce the file size.

Matrix Rain Demo (low quality)


A screenshot of the final program on windows shows how one frame looks like.

Screenshot of the program on Windows

Literate Programming using litpd

This program is written using my own literate programming tool named litpd. litpd is a command-line tool that takes a markdown document in pandoc format, and creates two outputs. The first output is a human readable document in a format like html/pdf. The second output is the source code files for building and running the program.

Building and Running the Program

See the Makefile in the current directory to see how to build and run the program.

Design of the Program

  • The Matrix Rain program tries to simulate each of the objects in the scene as classes.
  • Each individual character dropping down the scene is modelled as a RainDrop class.
  • Each column of falling RainDrops is modelled as a RainColumn class.
  • Finally the whole rain is modelled as a RainSheet.
  • The following class diagram shows the relationships of the three classes.
  • The main.lua program instantiates the RainSheet and calls the update and draw functions of the object at appropriate times.
  • The RainSheet in turn calls update and draw on each RainColumn which in turn does the same for each RainDrop.
  • Various input parameters to the RainSheet decide the number of columns, number of drops per column and also the maximum speed of the rain.

Class Diagram

In the subsequent sections we implement the classes first and then we use the classes in the main.lua program.

RainDrop Class

  • The RainDrop class represents the smallest unit/atom of the program. It represents one falling letter on the screen.
  • Each RainDrop is constructed with a configuration, some initialization parameters.
  • The update(dt) and draw() methods correspond to the lifecycle methods of love2d and are supposed to be called every frame. They update the state of the instance and draw the instance respectively.
  • There are a couple of utility methods resetPosition and setAlphabet provided to help reuse the instance as another drop after the current one has gone past the screen.
  • The inFrame method helps figure out if the RainDrop is beyond the screen dimenstions.

The program uses the middleclass library for implementing classes.

file: raindrop.lua

local Class = require 'middleclass'
local utf8 = require("utf8")


-- fonts for the raindrop

local GREEN_HSV = {1/3, 1, 1}

local RainDrop = Class('RainDrop')







return RainDrop

HSV to RGB Utility

The colours used do draw the RainDrop are stored in HSV and are only converted to RGB when drawing. This way the value part of the colour can be changed on every update to give a flickering look to the letter and the scene as a whole.

The following utility method from the wiki site helps convert from HSV values to RGB values.

id: hsvrgb

--- copied from
-- Converts HSV to RGB. (input and output range: 0 - 1)
local function HSV(h, s, v)
    if s <= 0 then return v,v,v end
    h = h*6
    local c = v*s
    local x = (1-math.abs((h%%2)-1))*c
    local m,r,g,b = (v-c), 0, 0, 0
    if h < 1 then
        r, g, b = c, x, 0
    elseif h < 2 then
        r, g, b = x, c, 0
    elseif h < 3 then
        r, g, b = 0, c, x
    elseif h < 4 then
        r, g, b = 0, x, c
    elseif h < 5 then
        r, g, b = x, 0, c
        r, g, b = c, 0, x
    return r+m, g+m, b+m

RainDrop Constructor

  • The RainDrop is constructed with some initialization parameters. These parameters specify:
    • A position (config.x, config.y)
    • The dimensions of the drop (config.w, config.h)
    • The speed of the drop (config.vx, config.vy). The vx is always 0 in the current program as the rain only drops down and does not move horizontally.
    • The optional config.color can set the colour of the text to the specified HSV value.
  • The constructor generates a random alphabet from a few supported codepoint ranges in unicode and also loads the appropriate fonts.
  • A love2d text object representing the alphabet to be drawn is created and stored to avoid creating it in every draw call.

id: raindropconstructor

function RainDrop:initialize(config)
    self.config = config
    self.x = config.x
    self.y = config.y
    self.w = config.w
    self.h = config.h
    self.vx = config.vx
    self.vy = config.vy
    self.color = config.color or GREEN_HSV
    self.glowColor = self.color

    local lang = math.random(1, 3)
    if lang == 1 then
        self.alphabet = utf8.char(utf8.codepoint('अ') + math.random(0, 50))
    elseif lang == 2 then
        self.alphabet = utf8.char(utf8.codepoint('a') + math.random(0, 25))
    elseif lang == 3 then
        self.alphabet = utf8.char(utf8.codepoint('ಅ') + math.random(0, 30))

    if not NORMAL_FONT then
        NORMAL_FONT = {
  'NotoSans_Condensed-Regular.ttf', math.min(self.w, self.h)),
  'NotoSans_Condensed-Regular.ttf', math.min(self.w, self.h)),
  'NotoSansKannada-Regular.ttf', math.min(self.w, self.h))
        GLOW_FONT = {
  'NotoSans_Condensed-Regular.ttf', 0.95 * math.min(self.w, self.h)),
  'NotoSans_Condensed-Regular.ttf', 0.95 * math.min(self.w, self.h)),
  'NotoSansKannada-Regular.ttf', 0.95 * math.min(self.w, self.h))

    -- create love2d text for the alphabet
    self.text =[lang], self.alphabet)
    self.glowText =[lang], self.alphabet)

    self.timer = 0

RainDrop Update

  • The update(dt) method updates the position of the drop.
  • In this method we also use inbuilt Simplex noise to change the Value part of the drawing colour thus giving a flickering look to the text.
  • The timer used to get the time-based parameter for the noise lookup is also reset if it crosses a hard-coded threshold of 100 seconds.

id: raindropupdate

function RainDrop:update(dt)
    self.x = self.x + (self.vx * dt)
    self.y = self.y + (self.vy * dt)

    local timeSlot = self.timer %% 17
    local ySlot = self.y %% 50
    local xSlot = self.x %% 100
    self.color[3] = love.math.noise(ySlot, xSlot, timeSlot)
    self.glowColor[3] = love.math.noise(ySlot, xSlot, timeSlot)

    self.timer = self.timer + dt
    if self.timer > 100 then
        self.timer = 0

RainDrop Draw

  • We simply use the colour, position and text of the drop to draw it.

id: raindropdraw

function RainDrop:draw()
    local color_rgb = {HSV(unpack(self.color))}
    color_rgb[4] = 1
    local glowColor_rgb = {HSV(unpack(self.glowColor))}
    glowColor_rgb[4] = 0.8, self.x + self.w/2 - self.text:getWidth()/2,
            self.y + self.h/2 - self.text:getHeight()/2),
            self.x + self.w/2 - self.glowText:getWidth()/2,
            self.y + self.h/2 - self.glowText:getHeight()/2)

RainDrop In Frame?

  • This method checks if the current postion of the drop is beyond the bounds of the canvas.

id: raindropinframe

function RainDrop:inFrame(cw, ch)
    return self.x <= cw and self.y <= ch

RainDrop Reset Position

  • Updates the position of the drop back to its initial settings.

id: raindropresetposition

function RainDrop:resetPosition(x, y)
    self.x = self.config.x
    self.y = self.config.y

RainDrop Set Alphabet

  • Change the alphabet to the given alphabet.
  • This method is not used at the moment.

id: raindropsetalphabet

function RainDrop:setAlphabet(alpha)
    self.alphabet = alpha

RainColumn Class

  • The RainColumn class contains a sequence or an array of RainDrop instances.
  • In the simulation it represents one column of display in the matrix rain simulation.
  • The RainColumn is initialized with some configuration parameters including its location, size, velocity, and number of rows/drops.
  • The class has the standard lifecycle methods of update(dt) and draw() methods to be called at the appropriate time.
  • Some utility methods like initDrops, inFrame and resetDrops provide the ability to reuse the same instance once the entire column is past the screen. This helps us reduce the number of objects created by simply resetting the column to its original position.

file: raincolumn.lua

local Class = require 'middleclass'
local RainDrop = require 'raindrop'

local RainColumn = Class('RainColumn')


return RainColumn

RainColumn Constructor

  • The constructor takes a config table which contains initialization parameters for the column of rain drops.
    • The location of the column on the x-axis is given by config.x.
    • The size of column is given by (config.w, config.h).
    • The vertical velocity of the column is given by config.vy. The horizontal velocity is 0.
    • The number of drops in the column are given by config.numRows.
  • After initializing the state variables, the constructor calls the utility method initDrops to initialize the RainDrop instances.

id: raincolumnconstructor

function RainColumn:initialize(config)
    self.x = config.x
    self.w = config.w
    self.h = config.h
    self.vy = config.vy
    self.numRows = config.numRows
    self.rowHeight = self.h/self.numRows


RainColumn Initialize Drops

  • This method creates an array of RainDrop instances of the same size.
  • The size of each RainDrop is equal to (self.w, self.rowHeight). Where rowHeight is self.h/self.numRows.
  • The drops are placed one after the other in a vertical line and each of them is given the velocity (0, self.vy) thus making sure they all move at the same speed in tandem.
  • The bottom-most drop is given a colour white.

id: raincolumninitdrops

function RainColumn:initDrops()
    self.numDrops = math.random(1, 2 * self.numRows)
    local colHeight = self.numDrops * self.rowHeight

    self.drops = {}
    for i = 1, self.numDrops do
        local dropConfig = {
                x = self.x,
                y = colHeight/2 - ((i - 1) * self.rowHeight),
                w = self.w,
                h = self.rowHeight,
                vx = 0,
                vy = self.vy
        if i == 1 then
            dropConfig.color = {0, 0, 1}

RainColumn Reset Drops

  • This method iterates over all the drops in the array self.drops and resets their position by calling drop:resetPosition.

id: raincolumnresetdrops

function RainColumn:resetDrops()
    for _, drop in ipairs(self.drops) do

RainColumn In Frame?

  • This method checks if a sentinel drop in the column is past the frame by calling inFrame method on the drop.
  • Currently the sentinel is one-third of the way down the column.

id: raincolumninframe

function RainColumn:inFrame()
    local outIndex = math.floor(self.numDrops/3)
    if outIndex < 1 then
        outIndex = 1
    return self.drops[outIndex]:inFrame(

RainColumn Update

  • The update(dt) method first checks if the column is inFrame. If it is not in frame then it resets the drops, thereby resetting the column and reusing it for another run through the canvas.
  • After the check the method iterates over each drop and calls update(dt) on each of them individually.

id: raincolumnupdate

function RainColumn:update(dt)
    if not self:inFrame() then
    for i, drop in ipairs(self.drops) do

RainColumn Draw

  • This method iterates over each drop and calls draw() for each of them.

id: raincolumndraw

function RainColumn:draw()
    for i, drop in ipairs(self.drops) do

RainSheet Class

  • The RainSheet class as we discussed in the design represents the overall simulaton.
  • The RainSheet contains an array of RainColumn instances moving at different speeds. This is what creates the illusion of matrix rain.
  • Since the bulk of update and drawing takes place in the sub-ordinate classes RainColumn and RainDrop, this class is fairly simple. The bulk of the logic of the class is in setting up the initial parameters and starting up the simulation.
  • The constructor sets up the simulation parametrs and creates an array of columns to fit the width of the canvas.

file: rainsheet.lua

local Class = require 'middleclass'
local RainColumn = require 'raincolumn'

local RainSheet = Class('RainSheet')


return RainSheet

RainSheet Constructor

  • The RainSheet constructor accepts a config table with its initialization parameters. This helps the class setup the simulation.
    • The number of columns is given by config.numCols.
    • The maximum number of rows possible in a rain column is given by config.numRows.
    • The maximum possible vertical velocity of a rain column is given by config.maxVy.
    • The size of the canvas is given by (,
  • Once all the state is initialized, the array of columns are created with their postion one after the other on the x-axis. Each column is given a random vertical speed.

id: rainsheetconstructor

function RainSheet:initialize(config)
    self.numCols = config.numCols
    self.numRows = config.numRows
    self.maxVy = config.maxVy = =

    self.colWidth =

    self.columns = {}
    for i = 1, self.numCols do
        local column = RainColumn({
            x = (i - 1) * self.colWidth,
            w = self.colWidth,
            h =,
            vy = math.random(self.maxVy/8, self.maxVy),
            numRows = self.numRows
        table.insert(self.columns, column)

RainSheet Update

  • This method simply iterates over each RainColumn and calls the update(dt) method on each instance.

id: rainsheetupdate

function RainSheet:update(dt)
    for _, column in ipairs(self.columns) do

RainSheet Draw

  • This method simply iterates over each RainColumn and calls the draw() method on each instance.

id: rainsheetdraw

function RainSheet:draw()
    for _, column in ipairs(self.columns) do

Matrix Rain Program

  • The main program in a love2d game is the main.lua file.
  • One must define love2d lifecycle functions to implement the simulation.
  • In the love.load() function we initialize the RainSheet with some configuration.
  • In love.update(dt) we update the RainSheet instance stored in sheet and in love.draw we draw the sheet.
  • There are a couple of shortcuts implemented:
    • ESC to quit the program
    • Ctrl+f to toggle the FPS of the simulation on the bottom right. By default the FPS display is turned off.

Module Imports

  • The only import needed is the RainSheet class in the rainsheet.lua file.

id: requiredeps

local RainSheet = require 'rainsheet'

File Globals

  • There are three sets of file global variables:
    • (cw, ch) store the size of the canvas. They are initialized in love.load().
    • fpsOn is a boolean which indicates if the FPS of the simulation is to be shown.
    • sheet represents the instance of the RainSheet class created in the love.load() function.

id: fileglobals

local cw, ch
local fpsOn
local sheet


  • The bulk of the code in this file is in the love.load() function where we setup the initialization parameters for the simulation.
  • Based on a fixed number of rows and the given canvas size (as decided in the conf.lua) we set up the number of columns and a maximum rain speed.
  • We then create a single instance of the RainSheet class with these parameters and store it in the sheet file-global variable.

id: loveload

function love.load()
    cw, ch =

    fpsOn = false

    local numRows = 40
    local numCols = cw / (ch/numRows)
    local maxRainSpeed = (ch/numRows) * 20

    -- create a font and set it as the active font
    -- with the default face, but size is equal to cw/numCols
    local font =

    sheet = RainSheet({
        numRows = numRows,
        numCols = numCols,
        maxVy = maxRainSpeed,
        cw = cw,
        ch = ch

Update the Simulation

  • This function calls sheet:update(dt).

id: loveupdate

function love.update(dt)

Draw the Simulation

  • This method calls sheet:draw().
  • It also checks the fpsOn variable to see if the FPS is to be displayed on the screen.

id: lovedraw

function love.draw()

    -- draw fps
    if fpsOn then, 1, 1, 1)"FPS: "..tostring(love.timer.getFPS()), cw - 100, ch - 25)

Handle Keyboard Events

  • The love.keypressed(key) lifecycle function is implemented to provide two shortcuts.
  • If the user presses ESC key then the application quits.
  • If the user presses Ctrl+f then the FPS display is toggled.

id: lovekeypressed

-- escape to exit
function love.keypressed(key)
    if key == "escape" then

    -- check for modifiers
    CTRL_KEY = ""
    SHIFT_KEY = ""
    ALT_KEY = ""
    if love.keyboard.isDown("lctrl") or love.keyboard.isDown("rctrl") then
        CTRL_KEY = "CTRL"

    if love.keyboard.isDown("lshift") or love.keyboard.isDown("rshift") then
        SHIFT_KEY = "SHIFT"

    if love.keyboard.isDown("lalt") or love.keyboard.isDown("ralt") then
        ALT_KEY = "ALT"

    if CTRL_KEY and key == "f" then
        fpsOn = not fpsOn

file: main.lua

--- main.lua: Matrix Rain Simulation in LÖVE
-- date: 16/05/2024
-- author: Abhishek Mishra







Simulation Configuration

  • In the conf.lua file we define some initialization parameters for the love2d simulation like size of the canvas, title of the window and we turn off some unused modules to make the simulation faster.

file: conf.lua

--- conf.lua: Config for the love2d game.
-- date: 4/3/2024
-- author: Abhishek Mishra

-- canvas size
local canvasWidth = 1024
local canvasHeight = 768

function love.conf(t)
    -- set the window title
    t.window.title = "Matrix Rain"

    -- set the window size
    t.window.width = canvasWidth
    t.window.height = canvasHeight

    -- disable unused modules for performance
    t.modules.joystick = false
    t.modules.physics = false
    t.modules.touch = false

    -- enable console
    -- TODO: turning on console crashes Love2D on Windows,
    -- so it's disabled for now
    -- t.console = true