Controlling the Raspberry Pi 3 Model B Sense HAT 8×8 LED matrix from Lua 5.3

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


local text = "HELLO"
local pics = letter_pics()
clear_pixels()
for i = 1, #text do
  sleep(2)
  print(text:sub(i, i))
  local c = pixel_from_picture(pics[text:sub(i, i)], default_colors())
  set_pixels(c)
end
sleep(2)
--clear_pixels()

colortable = pixel_from_picture([[
        
 00  00 
 00  00 
        
00    00
 00  00 
  0000  
        
]], default_colors())

set_pixels(colortable)
clear_pixels()