A Lua script for Aseprite that shows auto tiling preview for FangAutoTile ( https://assetstore.unity.com/packages/tools/sprite-management/fang-auto-tile-132602 ).

Fang Preview.lua

----------------------------------------------------------------------
-- Auto tiling preview for FangAutoTile (https://assetstore.unity.com/packages/tools/sprite-management/fang-auto-tile-132602).
--
-- It works for all color modes (RGB/GRAY/INDEXED).
----------------------------------------------------------------------

if app.apiVersion < 1 then
    return app.alert("This script requires Aseprite v1.2.10-beta3")
  end
  
  local cel = app.activeCel
  if not cel then
    return app.alert("There is no active image")
  end
  
  if cel.sprite.width % 2 ~= 0 then
      return app.alert("Invalid size. Width must be even.")
  end
  
  if cel.sprite.width * 5 ~= cel.sprite.height then
      return app.alert("Invalid size. Height must be 5 times the width.")
  end
  
  local tileSize = cel.sprite.width
  local partSize = tileSize / 2
  
  -- setup sprite pattern table
  local pattern = 
  {
      1, 1, 1, 1,  0, 0, 0, 0,  1, 3, 1, 3,  3, 1, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  1, 1, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 2, 4,  3, 3, 3, 3,  3, 1, 4, 2,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 2, 5,  3, 1, 5, 2,  0, 0, 0, 0, 
      0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 1, 3,  4, 4, 4, 4,  3, 1, 3, 1,  0, 0, 0, 0,  2, 2, 2, 2,  0, 0, 0, 0,  2, 2, 2, 2,  0, 0, 0, 0,  1, 3, 1, 3,  4, 5, 4, 5,  5, 4, 5, 4,  3, 1, 3, 1, 
      1, 1, 2, 2,  0, 0, 0, 0,  1, 3, 2, 5,  3, 1, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 2, 1, 1,  0, 0, 0, 0,  0, 0, 0, 0,  2, 4, 1, 3,  3, 3, 3, 3,  4, 2, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 1, 3,  5, 2, 3, 1,  0, 0, 0, 0,  
      2, 2, 1, 1,  0, 0, 0, 0,  2, 5, 1, 3,  5, 2, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  
      0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 1, 3,  3, 3, 4, 4,  3, 1, 3, 1,  0, 0, 0, 0,  1, 3, 2, 5,  3, 1, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  
      1, 3, 2, 4,  3, 3, 3, 3,  3, 1, 3, 1,  0, 0, 0, 0,  1, 1, 2, 2,  0, 0, 0, 0,  1, 1, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 2, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 1, 3,  5, 4, 4, 5,  3, 1, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,
      2, 2, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  2, 4, 2, 4,  3, 3, 3, 3,  4, 2, 4, 2,  0, 0, 0, 0,  1, 3, 1, 3,  4, 4, 3, 3,  3, 1, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 1, 3,  5, 2, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  
      2, 2, 1, 1,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  2, 2, 1, 1,  0, 0, 0, 0,  2, 2, 1, 1,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  
      0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  1, 1, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 2, 5,  3, 1, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  
      1, 3, 2, 5,  3, 3, 5, 5,  3, 3, 5, 5,  3, 1, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  1, 3, 2, 5,  3, 3, 5, 4,  3, 1, 3, 1,  0, 0, 0, 0,  2, 4, 2, 5,  3, 1, 5, 2,  0, 0, 0, 0,  2, 5, 1, 3,  5, 4, 4, 4,  3, 1, 3, 1,  0, 0, 0, 0,  
      2, 5, 2, 5,  5, 5, 5, 5,  5, 5, 5, 5,  5, 2, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 2, 5,  5, 2, 5, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 1, 3,  5, 2, 4, 2,  0, 0, 0, 0,  0, 0, 0, 0,  2, 2, 2, 2,  0, 0, 0, 0,  0, 0, 0, 0,  
      2, 5, 2, 5,  5, 5, 5, 5,  5, 5, 5, 4,  5, 2, 3, 1,  0, 0, 0, 0,  1, 3, 1, 3,  4, 5, 3, 3,  5, 2, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  2, 2, 1, 1,  0, 0, 0, 0,  1, 3, 1, 3,  4, 4, 4, 5,  3, 1, 5, 2,  0, 0, 0, 0,  
      2, 5, 1, 3,  5, 5, 3, 3,  5, 2, 3, 1,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  0, 0, 0, 0,  2, 5, 1, 3,  5, 2, 3, 1,  0, 0, 0, 0
  }
  
  
  local horizontalTilesNumber = 17
  local verticalTilesNumber = #pattern / (horizontalTilesNumber * 4)
  
  
  -- prepare source sprite
  
  local sourceFrameNumber = cel.frameNumber
  local sourceSprite = Sprite(cel.sprite)
  
  sourceSprite:flatten()
  
  local sourceCel = sourceSprite.layers[1]:cel(sourceFrameNumber)
  local sourceImage = sourceCel.image
  local sourceCelBounds = sourceCel.bounds
  
  
  -- prepare destination sprite
  
  local destinationSprite = Sprite(tileSize * horizontalTilesNumber, tileSize * verticalTilesNumber, sourceSprite.colorMode)
  
  local destinationCel = destinationSprite.cels[1]
  
  local destination = destinationCel.image
  
  
  -- copy palette
  
  local palette = Palette(sourceSprite.palettes[1])
  destinationSprite:setPalette(palette)
  
  
  -- draw
  
  -- tile loop
  
  for tileY=1, verticalTilesNumber do
      for tileX=1, horizontalTilesNumber do
          
          -- inner tile loop
  
          local i = 0
          for innerY=1, 2 do
              for innerX=1, 2 do
  
                  -- index of pattern table
                  local index = (tileY - 1) * horizontalTilesNumber * 4 + (tileX - 1) * 4 + i
  
                  -- part id of target tile
                  local partIndex = pattern[index + 1]
  
                  -- origin position of destination tile
                  local destX = (tileX - 1) * tileSize + (innerX - 1) * partSize
                  local destY = (tileY - 1) * tileSize + (innerY - 1) * partSize
  
                  -- origin position of source tile
                  local sourceX = (innerX - 1) * partSize - sourceCelBounds.x
                  local sourceY = (partIndex - 1) * tileSize + (innerY - 1) * partSize - sourceCelBounds.y
                  
                  -- copy from source sprite to destination
                  local pix = 0
                  for it in destination:pixels(Rectangle(destX, destY, partSize, partSize)) do
  
                      local sourcePixelX = sourceX + (pix % partSize)
                      local sourcePixelY = sourceY + math.floor(pix / partSize)
                      
                      if sourcePixelX >= 0 and sourcePixelY >= 0 and sourcePixelX < sourceCelBounds.width and sourcePixelY < sourceCelBounds.height then
                          it(sourceImage:getPixel(sourcePixelX, sourcePixelY))
                      end
                      pix = pix + 1
                  end
  
                  i = i + 1
              end
          end
  
      end
  end
  
  sourceSprite:close()
  
  app.refresh()