4x4 Bayer dithering

tsuki.lua

local RADIUS = 88
local GRAV_REACH = math.sqrt(usagi.GAME_H * usagi.GAME_H + usagi.GAME_W * usagi.GAME_W)
local G_MAX = 0.05
local GRAV_BRACKET = (GRAV_REACH - RADIUS)
    - ((GRAV_REACH * GRAV_REACH) - (RADIUS * RADIUS)) / (2 * GRAV_REACH)
local DAMAGE_FACTOR = 0.0006
local SUN_DIR = { x = 1, y = 0 }
local MAX_INTEGRITY = 100

local BAYER = {
  { 0,  8,  2,  10 },
  { 12, 4,  14, 6 },
  { 3,  11, 1,  9 },
  { 15, 7,  13, 5 },
}
local DITHER_W = 9

local function lit_dithered_at(px, py, ax, ay)
  local s = ((px + 0.5) * SUN_DIR.x) + ((py + 0.5) * SUN_DIR.y)
  local f = util.clamp((s + DITHER_W) / (2 * DITHER_W), 0, 1) -- 0 (dark) .. 1 (lit)
  local th = (BAYER[(ay % 4) + 1][(ax % 4) + 1] + 0.5) / 16   -- 0..1 threshold from pattern

  return f > th
end

local Tsuki = {}

local function die(tsuki)
  tsuki.dead = true
  tsuki.active = false
end

local function attract_entities(tsuki, entities, sign)
  for _, e in ipairs(entities) do
    local dx, dy = tsuki.cx - e.cx, tsuki.cy - e.cy
    local d = math.sqrt(dx * dx + dy * dy)
    if d > RADIUS and d <= tsuki.beam_reach
        and not e.latched_to then
      local strength = tsuki.strength * math.max(0, 1 - (d / tsuki.beam_reach))
      e.force.x += (dx / d) * -sign * strength
      e.force.y += (dy / d) * -sign * strength
    end
  end
end

function Tsuki.is_lit(t, px, py)
  return ((px - t.cx) * SUN_DIR.x + (py - t.cy) * SUN_DIR.y) > 0
end

function Tsuki.grav_ceiling(accel)
  return math.sqrt(2 * G_MAX * accel * GRAV_BRACKET)
end

function Tsuki.spawn()
  local t = {
    x = usagi.GAME_W / 2,
    y = usagi.GAME_H / 2,
    w = RADIUS * 2,
    h = RADIUS * 2,
    vel = { x = 0, y = 0 },
    mass = 1e9,
    beam_reach = GRAV_REACH,
    strength = G_MAX,
    static = true,
    max_integrity = MAX_INTEGRITY,
    integrity = MAX_INTEGRITY,
    dead = false,
    active = true,
  }

  local r = RADIUS
  local cx, cy = t.x, t.y
  t.color_mask = {}
  for py = -r + 1, r - 1 do
    for px = -r + 1, r - 1 do
      if px * px + py * py <= r * r then
        local ax, ay = cx + px, cy + py
        t.color_mask[#t.color_mask + 1] = {
          px, py, lit_dithered_at(px, py, ax, ay),
        }
      end
    end
  end

  return t
end

function Tsuki.affect(t, entities)
  if not t.active then return end

  t.cx, t.cy = t.x, t.y
  attract_entities(t, entities, -1)
end

function Tsuki.try_contact(a, b, E)
  local t, e
  if a.static then
    t, e = a, b
  elseif b.static then
    t, e = b, a
  else
    return false
  end

  if not e.enemy then return true end

  local hard = E > Collision.IMPACT_THRESHOLD

  if Tsuki.is_lit(t, e.cx, e.cy) then
    if hard then
      Enemy.die(e)
      Tsuki.damage(t, DAMAGE_FACTOR * (E - Collision.IMPACT_THRESHOLD))
    else
      Enemy.latch(e, t)
    end
  elseif hard then
    Enemy.die(e)
  end

  return true
end

function Tsuki.damage(t, amount)
  t.integrity = math.max(0, t.integrity - amount)

  if t.integrity <= 0 then
    die(t)
  end
end

function Tsuki.draw(t)
  if not t.active then return end

  for _, m in ipairs(t.color_mask) do
    local cx, cy = t.cx, t.cy
    local px, py = cx + m[1], cy + m[2]
    local lit = Colors.YELLOW
    local dark = Colors.SHADOW
    gfx.px(px, py, m[3] and lit or dark)
  end
end

return Tsuki