Added overworld battles

lib/BattlePics.lua

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+-- Overworld battles: giving a battle pic its paper back.
+--
+-- Gen 1 battle pics are two-bit art whose lightest shade is WHITE, and the
+-- engine's decoded PNGs key that shade to alpha 0 -- which was free, because
+-- the field behind them was white too. A transparent belly on a white page
+-- is a white belly.
+--
+-- Put a route behind it and the belly is grass. Charizard's chest, the whites
+-- of every eye, the highlight down a Pikachu's cheek: all of it turns into a
+-- hole with the world showing through, and the mon reads as a stencil.
+--
+-- So the paper is put back, and only where the paper was: the pic is read
+-- back once, the transparent region OUTSIDE the figure is flood-filled from
+-- the border, and every transparent pixel the flood could not reach -- every
+-- hole enclosed by the artwork -- is filled opaque white. The silhouette is
+-- untouched, so the mon still cuts cleanly against the world; only its
+-- insides stop being see-through.
+--
+-- Read back off the GPU rather than off the asset, deliberately. What comes
+-- back is the pic the engine actually decided to draw -- species palette,
+-- forced-mono rebuild, shiny recolour, a mod's replacement art -- so this
+-- needs to know nothing about how any of that was arrived at. Once per pic
+-- per session, cached on the image itself.
+
+-- the mod namespace (see main.lua): V.require loads a sibling module
+local V = ...
+
+local BattlePics = {}
+
+-- Cached by the image the engine handed over. Weak keys, so a pic that goes
+-- out of scope takes its filled twin with it rather than pinning a texture
+-- for the session.
+local cache = setmetatable({}, { __mode = "k" })
+
+-- What an enclosed hole is filled with. White, because white is what the
+-- battle field was: this restores the pixel the artist drew and the engine
+-- then keyed away, it does not invent a new one.
+BattlePics.FILL = { 1, 1, 1, 1 }
+
+-- Anything at or under this alpha counts as keyed-out rather than drawn.
+local CUT = 0.5
+
+-- Read the pixels the engine would actually blit. A LOVE Image does not hand
+-- its data back, so it is drawn into a canvas of its own size and the canvas
+-- is read -- which is also what makes this work for every path that produces
+-- a pic, without knowing which one produced this one.
+local function readBack(img)
+  local w, h = img:getDimensions()
+  if w <= 0 or h <= 0 then return nil end
+  local prevCanvas = love.graphics.getCanvas()
+  local prevBlend, prevAlpha = love.graphics.getBlendMode()
+  local prevR, prevG, prevB, prevA = love.graphics.getColor()
+  local data = nil
+  local ok = pcall(function()
+    local canvas = love.graphics.newCanvas(w, h)
+    love.graphics.setCanvas(canvas)
+    love.graphics.clear(0, 0, 0, 0)
+    love.graphics.setBlendMode("replace", "premultiplied")
+    love.graphics.setColor(1, 1, 1, 1)
+    love.graphics.draw(img, 0, 0)
+    love.graphics.setCanvas()
+    data = canvas:newImageData()
+    if canvas.release then pcall(canvas.release, canvas) end
+  end)
+  if prevCanvas then
+    love.graphics.setCanvas(prevCanvas)
+  else
+    love.graphics.setCanvas()
+  end
+  love.graphics.setBlendMode(prevBlend or "alpha", prevAlpha)
+  love.graphics.setColor(prevR or 1, prevG or 1, prevB or 1, prevA or 1)
+  return ok and data or nil
+end
+
+-- Mark every transparent pixel reachable from the border. That set is the
+-- OUTSIDE; everything transparent it does not reach is an enclosed hole.
+--
+-- An explicit stack rather than recursion: a 56x56 pic is three thousand
+-- pixels and a keyed-out background is most of them, which is a deeper call
+-- chain than is worth risking for no gain.
+local function markOutside(data, w, h)
+  local outside = {}
+  local stack, top = {}, 0
+  local function push(x, y)
+    if x < 0 or y < 0 or x >= w or y >= h then return end
+    local key = y * w + x
+    if outside[key] then return end
+    local _, _, _, a = data:getPixel(x, y)
+    if a > CUT then return end
+    outside[key] = true
+    top = top + 1
+    stack[top] = key
+  end
+  for x = 0, w - 1 do
+    push(x, 0)
+    push(x, h - 1)
+  end
+  for y = 0, h - 1 do
+    push(0, y)
+    push(w - 1, y)
+  end
+  while top > 0 do
+    local key = stack[top]
+    top = top - 1
+    local x, y = key % w, math.floor(key / w)
+    push(x - 1, y)
+    push(x + 1, y)
+    push(x, y - 1)
+    push(x, y + 1)
+  end
+  return outside
+end
+
+-- The pic with its enclosed holes filled, or the pic itself when that could
+-- not be done (no pixel access, a driver that refused the readback). Never
+-- nil for a non-nil argument: a caller must always have something to draw.
+function BattlePics.filled(img)
+  if not img then return img end
+  local hit = cache[img]
+  if hit ~= nil then return hit or img end
+
+  local made = nil
+  local ok = pcall(function()
+    local data = readBack(img)
+    if not data then return end
+    local w, h = data:getDimensions()
+    local outside = markOutside(data, w, h)
+    local fill = BattlePics.FILL
+    local changed = false
+    for y = 0, h - 1 do
+      local row = y * w
+      for x = 0, w - 1 do
+        if not outside[row + x] then
+          local _, _, _, a = data:getPixel(x, y)
+          if a <= CUT then
+            data:setPixel(x, y, fill[1], fill[2], fill[3], fill[4])
+            changed = true
+          end
+        end
+      end
+    end
+    -- nothing enclosed: hand the original back rather than a copy of it
+    if not changed then return end
+    local out = love.graphics.newImage(data)
+    out:setFilter("nearest", "nearest")
+    made = out
+  end)
+
+  cache[img] = (ok and made) or false
+  return made or img
+end
+
+function BattlePics.invalidate()
+  cache = setmetatable({}, { __mode = "k" })
+end
+
+return BattlePics