Any shape of collision object

Questions
#1

Hello everyone.
This is my first entry. I have been testing Defold for a few days now. I like it very much !

I have this question. Has anything changed recently when it comes to
possibility to create your own shapes of collision objects ?

I would like to create a semicircle like the letter “C” inside of which the “ball” will move. All with the help of dynamic collisions. I saw some old posts about a shape editor that someone prepared. But it seems to me that it is not developed.

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#2

Sounds like you’re thinking of the Defold Polygon Editor, which creates a .convexshape file that can be used as a convex hull shape for collisions.

However because those can only be convex, you couldn’t be able to make a “C” shape as it’s concave. What I think you’d want to do is use multiple primitive shapes in one collision object to make a rough arcing shape.

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#3

I wrote a small helper that can extract the convex shape from the alpha of an image. Maybe it can help

3 Likes
#4

There you go!
Note this is just my crappy test and it needs integration into your own workflow.
It is using the PNG extension here:

I could not find a decent png decoder (read working) in pure lua to create an editor extension and I’m not happy with a solution that needs an external app.

-- Convex hull
-- https://www.rosettacode.org/wiki/Convex_hull#Lua
-- MIT license (?)


local function ccw(a,b,c)
	return (b.x - a.x) * (c.y - a.y) > (b.y - a.y) * (c.x - a.x)
end

local function pop_back(ta)
	table.remove(ta,#ta)
	return ta
end

local function convexHull(pl)
	if #pl == 0 then
		return {}
	end
	table.sort(pl, function(left,right)
		return left.x < right.x
	end)

	local h = {}

	-- lower hull
	for i,pt in pairs(pl) do
		while #h >= 2 and not ccw(h[#h-1], h[#h], pt) do
			table.remove(h,#h)
		end
		table.insert(h,pt)
	end

	-- upper hull
	local t = #h + 1
	for i=#pl, 1, -1 do
		local pt = pl[i]
		while #h >= t and not ccw(h[#h-1], h[#h], pt) do
			table.remove(h,#h)
		end
		table.insert(h,pt)
	end

	table.remove(h,#h)
	return h
end

local function get_pixel_rgba(pixels, width, x, y)
	local index = y * width * 4 + x * 4 + 1
	local r = pixels[index + 0]
	local g = pixels[index + 1]
	local b = pixels[index + 2]
	local a = pixels[index + 3]
	return r,g,b,a
end

function init(self)
	local rel_path = "<your image here>"
	local abs_path = "<absolute path where to save>"
	local file = io.open(abs_path, "rb")
	local file = file:read("*a")
	local buf, w, h = png.decode_rgba(file, true)
	local pixels = buffer.get_stream(buf, hash("pixels"))

	local points = {}
	for x = 0, w - 1 do
		for y = 0, h - 1 do
			local r, g, b, a = get_pixel_rgba(pixels, w, x, y)	
			if a > 128 then
				table.insert(points, {x=x, y=y})
			end
		end
	end

	local ch = convexHull(points)

	local data = "shape_type: TYPE_HULL\n"
	for i = 1, #ch do
		data = data .. "data: " .. ch[i].x .. " data: " .. ch[i].y .. " data: 0\n"
	end
	
	-- write the resulting convex hull
	local save_path = "<save path>"
	local save_file = io.open(save_path, "w")
	save_file:write(data)
	save_file:close()

end
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