Creating a firework with the Raspberry Pi 3 Model B Sense HAT 8×8 LED matrix and Lua 5.3

rpi_sensehat_fireworks.lua

-- load pixels.lua (attached as part of this gist)
package.path = package.path .. ';?.lua'
local hat = require('rpi_sensehat_pixels')

-- a generic color table
local colortable = {
  [0] = '0000FF',
  [1] = '00FF00',
  [2] = 'FF0000',
  [3] = '1354AA',
  [4] = 'AAAAAA',
  [5] = 'FFFF00',
  [6] = '00FFFF',
  [7] = 'FFCC33',
}

-- set pixel at led_id to color
local set_pixel = function (led_id, color)
  local fd = io.open(hat.sense_hat_device_path(), "w")
  fd:seek("set", 2 * (led_id - 1))
  fd:write(hat.color_to_bin(color))
  fd:close()
end

-- Convert a (x, y) point on this 8×8 LED matrix
-- into a LED id. The x=0 y=0 is at the bottom left
-- and x=1 y=0 is one to the right.
local xy_to_id = function (x, y)
  return 57 + x - 8 * y
end

-- is the given point (x, y) on the circle with
-- center (cx, cy) and radius r?
local point_is_on_circle = function (cx, cy, r, x, y)
  local in_big_circle = (x - cx) * (x - cx) + (y - cy) * (y - cy) - r * r <= 1 
  local in_small_circle = (x - cx) * (x - cx) + (y - cy) * (y - cy) - (r-1) * (r-1) <= 1
  return in_big_circle and not in_small_circle
end

-- draw the explosion circle of a firework
-- with color and center (cx, cy) at time t ∈ {0, 1, …, 4}
local draw_explosion = function (color, cx, cy, t)
  if t == 0 then
    set_pixel(xy_to_id(cx, cy), color)
  end
  if t > 0 and t <= 4 then
    for x = 0, 7 do
      for y = 0, 7 do
        if point_is_on_circle(cx, cy, t, x, y) then
          set_pixel(xy_to_id(x, y), color)
        end
      end
    end
  end
end

-- draw a firework at some random pixel of Sense HAT
-- (actually, the rocket rises at least 2 pixels,
-- so it is not arbitrary)
local draw_firework = function ()
  hat.clear_pixels()

  local cx = math.random(8) - 1
  local cy = math.random(6) + 1
  local color = colortable[math.random(8) - 1]

  -- draw the rising rocket
  for t = 0, (cy - 1) do
    set_pixel(xy_to_id(cx, t), color)
    hat.sleep(0.1)
    hat.clear_pixels()
  end

  -- draw the explosion circle
  for t = 0, 4 do
    draw_explosion(color, cx, cy, t)
    hat.clear_pixels()
  end
end

for e = 0, 14 do 
  draw_firework() 
  hat.sleep(0.6)
end

rpi_sensehat_pixels.lua

local M = {_TYPE='module', _NAME='sensehat_pixels', _VERSION='0.0.1'}

-- Sleep this thread
function sleep(n)
  io.popen("sleep " .. tostring(n)):read()
end

-- Split a string into lines
function lines(str)
 local t = {}
 local function helper(line)
  table.insert(t, line)
  return ""
 end
 helper(str:gsub("(.-)\r?\n", helper))
 return t
end

-- Does file $path exist?
function file_exists(path)
  local fd = io.open(path, "r")
  if fd ~= nil then
    io.close(fd)
    return true
  else
    return false
  end
end

-- List files and folders in $path
function list_directory(path)
  return io.popen([[ls -1 ]] .. path):lines()
end

-- Find sense hat device
function sense_hat_device_path()
  local target_name = "RPi-Sense FB"

  for node in list_directory("/sys/class/graphics") do
    if string.sub(node, 1, 2) == "fb" then
      local name_path = "/sys/class/graphics/" .. node .. "/name"
      if file_exists(name_path) then
        local fd = io.open(name_path, "r")
        local content = fd:read()
        fd:close()

        if content == target_name then
          return "/dev/" .. node
        end
      end
    end
  end
end

-- Represent a given color as RGB565 byte string
function color_to_bin(rgb)
  if rgb:len() ~= 6 then
    error("color " .. rgb .. " is not a 6-digit hex color")
  end

  local r = tonumber(rgb:sub(1, 2), 16)
  local g = tonumber(rgb:sub(3, 4), 16)
  local b = tonumber(rgb:sub(5, 6), 16)

  r = (r >> 3) & 0x1F
  g = (g >> 2) & 0x3F
  b = (b >> 3) & 0x1F
  local comb = (r << 11) + (g << 5) + b
  
  return string.pack("H", comb)
end

-- Given a string with 8 lines with length 8 each,
-- each byte represents a color. The byte to color
-- map is given by the second argument.
function pixel_from_picture(pic, map)
  local data = {}
  local lineno = 1

  for _, line in pairs(lines(pic)) do
    if line:len() == 0 then
      goto next
    end
    if line:len() ~= 8 then
      error("line must have length 8")
    end

    for i = 1, #line do
      local total_index = (lineno - 1) * 8 + i
      if map[line:sub(i, i)] == nil then
        error("character '" .. line:sub(i, i) .. "' could not be mapped to color")
      end
      data[total_index] = map[line:sub(i, i)]
    end

    lineno = lineno + 1
    ::next::
  end

  return data
end

function default_colors()
  local color_table = {
    ["white"] = "FFFFFF", ["silver"] = "C0C0C0", ["gray"] = "808080",
    ["black"] = "000000", ["red"] = "FF0000", ["maroon"] = "800000",
    ["yellow"] = "FFFF00", ["olive"] = "808000", ["lime"] = "00FF00",
    ["green"] = "008000", ["aqua"] = "00FFFF", ["teal"] = "008080",
    ["blue"] = "0000FF", ["navy"] = "000080", ["fuchsia"] = "FF00FF",
    ["purple"] = "800080"
  }
  color_table[" "] = color_table["black"]
  color_table["X"] = color_table["teal"]
  color_table["0"] = color_table["white"]
  color_table["1"] = color_table["blue"]

  return color_table
end

function letter_pics()
  return {
    ["E"] = [[
 000000 
 000000 
 00     
 000000 
 000000 
 00     
 000000 
 000000 
]],
    ["H"] = [[
 00  00 
 00  00 
 00  00 
 000000 
 000000 
 00  00 
 00  00 
 00  00 
]],
    ["I"] = [[
   00   
   00   
   00   
   00   
   00   
   00   
   00   
   00   
]],
    ["L"] = [[
 00     
 00     
 00     
 00     
 00     
 00     
 000000 
 000000 
]],
    ["O"] = [[
  0000  
 000000 
 00  00 
 00  00 
 00  00 
 00  00 
 000000 
  0000  
]],
  }
end

-- Given a map with indices 1..64, each index
-- is mapped to a color to be set on raspberry
-- pi's sense hat
function set_pixels(pixels)
  local fd = io.open(sense_hat_device_path(), "w")
  for idx, color in pairs(pixels) do
    fd:seek("set", 2 * (idx - 1))
    fd:write(color_to_bin(color))
  end
end

-- clear pixels on the RPi Sense hat
function clear_pixels()
  local color_table = {}
  for i = 1, 64 do
    color_table[i] = "000000"
  end

  set_pixels(color_table)
end

return {
  sleep = sleep,
  file_exists = file_exists,
  list_directory = list_directory,
  sense_hat_device_path = sense_hat_device_path,
  color_to_bin = color_to_bin,
  pixel_from_picture = pixel_from_picture,
  default_colors = default_colors,
  letter_pics = letter_pics,
  set_pixels = set_pixels,
  clear_pixels = clear_pixels
}