Drawing edge line of a group of tiles

pie · May 2016

Hi, I am trying to draw an outline from some tiles but I can't think of a "smart" way to do it.
I suppose that I could use a complex set of if..then..else, checking neighbours for every tile, but if someone has a better suggestion I am willing to listen..

I have example A, and I'd like to get to example B (see attachment)

Thanks in advance

antix · May 2016

Check this article out @pie - http://www.saltgames.com/article/awareTiles/ The article only has a 4-bit algorithm but can be expanded to 31-bit if required.

And there's this one that I just found again after losing the bookmark ages ago... http://www.angryfishstudios.com/2011/04/adventures-in-bitmasking/

n1cke · May 2016

This is my Tracer library. It's old and I used it with LuaJit only so I modified it for 'normal' Lua just now and don't know if it will work for you. Anyway you can check the algorythm.

require "media"
require "clipper"
 
-- usage:
-- local media = Media.new(path)
-- local polygon = media:getTracedPolygon()
 
local pixel = Media.new("pixel.png") -- "pixel.png" is 1x1 pixel image
 
function Media:getTracedPolygon(isPixelTransparent)
	isPixelTransparent = isPixelTransparent or function(x,y)
		local _,_,_,a = self:getPixel(x,y)
		return (a < 0.5)
	end
 
	self:drawImage(0,0,pixel,0) -- turbo
	local w,h = self:getWidth(), self:getHeight()
	for x = 1, w   do self:setPixel(x,1,0,0,0,0) end -- верх
	for x = 1, w   do self:setPixel(x,h,0,0,0,0) end -- низ
	for y = 2, h-1 do self:setPixel(1,y,0,0,0,0) end -- лево
	for y = 2, h-1 do self:setPixel(w,y,0,0,0,0) end -- право
	self:drawImage(0,0,pixel,0) -- turbo
 
	local poly = Polygon.new()
	local x0, y0
 
	local function findOrigin()
		for x = 2, w do
			for y = 2, h do
			  if not isPixelTransparent(x,y) then
				x0,y0 = x-1,y
				poly:addPoint(x0,y0)
				return true
			  end
			end
		end
		return false
	end
 
	if not findOrigin() then
		poly:addPoint(1,1)
		poly:addPoint(self:getWidth(),1)
		poly:addPoint(self:getWidth(),self:getHeight())
		poly:addPoint(1,self:getHeight())	
		return poly
	end
 
	local x, y, dir = x0+1, y0-1, 2
 
	-- 8 1 2
	-- 7 x 3
	-- 6 5 4
 
	local function getXY(x,y,dir)
		if     dir == 1 then return x,  y-1
		elseif dir == 2 then return x+1,y-1
		elseif dir == 3 then return x+1,y
		elseif dir == 4 then return x+1,y+1
		elseif dir == 5 then return x,  y+1
		elseif dir == 6 then return x-1,y+1
		elseif dir == 7 then return x-1,y
					    else return x-1,y-1
		end
	end
 
	local function getFirstDir()
		if dir < 7 then return dir+2 else return dir-6 end
	end
 
	local function getNextDir()
		if dir > 1 then return dir-1 else return 8 end
	end
 
	local function getPrevDir()
		if dir < 8 then return dir+1 else return 1 end
	end
 
	local function getNextPixelXY()
		return getXY(x,y,getNextDir())
	end
 
	local function getPrevPixelXY()
		return getXY(x,y,getPrevDir())
	end
 
	while x ~= x0 or y ~= y0 do
		dir = getFirstDir()
		for i = 1,8 do
			local nx,ny = getXY(x,y,dir)
			if isPixelTransparent(nx,ny)
				and (dir%2==1 or isPixelTransparent(getNextPixelXY())
				or isPixelTransparent(getPrevPixelXY()))
			then
				poly:addPoint(x,y)
				x,y = nx,ny
				break
			end
			dir = getNextDir()
		end
	end
 
	poly:clean(0.45)
	poly:clean(0.45)
	return poly
end

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