editableImageOutlines.luau

--!strict

local AssetService = game:GetService("AssetService")

local THICKNESS = 5
local RGBA = { 255, 255, 255, 255 }

local NEIGHBOR_OFFSETS: { { number } } = {
	{ -1, -1 },
	{ 0, -1 },
	{ 1, -1 },
	{ -1, 0 },
	{ 1, 0 },
	{ -1, 1 },
	{ 0, 1 },
	{ 1, 1 },
}

local function outlineBuffer(imgBuffer: buffer, width: number, height: number, thickness: number, rgba: {number})
	local result = buffer.create(width * height * 4)
	buffer.copy(result, 0, imgBuffer)
	
	local function readRGBA(index: number)
		local r = buffer.readu8(result, index + 0)
		local g = buffer.readu8(result, index + 1)
		local b = buffer.readu8(result, index + 2)
		local a = buffer.readu8(result, index + 3)
		return r, g, b, a
	end
	
	local function writeRGBA(index: number, r: number, g: number, b: number, a: number)
		buffer.writeu8(result, index + 0, r)
		buffer.writeu8(result, index + 1, g)
		buffer.writeu8(result, index + 2, b)
		buffer.writeu8(result, index + 3, a)
	end
	
	local queue = {}
	local queueFront = 1
	local queueLength = 0
	
	local function pushBack(value: number)
		queue[queueFront + queueLength] = value
		queueLength = queueLength + 1
	end
	
	local function popFront()
		local value = queue[queueFront]
		queue[queueFront] = nil
		queueFront = queueFront + 1
		queueLength = queueLength - 1
		return value
	end
	
	local function getNeighbors(index: number)
		local j = index / 4
		local y = math.floor(j / width)
		local x = j - y * width
		
		local neighbors = {}
		for _, offset in NEIGHBOR_OFFSETS do
			local nx = x + offset[1] + 1
			local ny = y + offset[2] + 1
			
			if nx > 0 and nx <= width and ny > 0 and ny <= height then
				local k = ((nx - 1) + (ny - 1) * width) * 4
				table.insert(neighbors, k)
			end
		end
		
		return neighbors
	end
	
	local visited = {}
	
	for x = 1, width do
		for y = 1, height do
			local index = ((x - 1) + (y - 1) * width) * 4
			local _r, _g, _b, a = readRGBA(index)
			
			if a == 255 then
				visited[index] = true
				continue
			end
			
			local hasFullyOpaqueNeighbor = false
			for _, neighborIndex in getNeighbors(index) do
				local _nr, _ng, _nb, na = readRGBA(neighborIndex)
				if na == 255 then
					hasFullyOpaqueNeighbor = true
					break
				end
			end


			if hasFullyOpaqueNeighbor then
				visited[index] = true
				pushBack(index)
			end
		end
	end
	
	for _ = 1, thickness do
		for _ = 1, queueLength do
			local poppedIndex = popFront()
			
			writeRGBA(poppedIndex, rgba[1], rgba[2], rgba[3], rgba[4])
			
			for _, neighborIndex in getNeighbors(poppedIndex) do
				if not visited[neighborIndex] then
					pushBack(neighborIndex)
					visited[neighborIndex] = true
				end
			end
		end
	end
	
	return result
end

local function outlineEditImage(editImage: EditableImage)
	local imgSize = editImage.Size
	local imgBuffer = editImage:ReadPixelsBuffer(Vector2.zero, imgSize)
	
	local outlinedBuffer = outlineBuffer(imgBuffer, imgSize.X, imgSize.Y, THICKNESS, RGBA)
	local outlinedEditImage = AssetService:CreateEditableImage({ Size = imgSize })
	outlinedEditImage:WritePixelsBuffer(Vector2.zero, imgSize, outlinedBuffer)
	
	return outlinedEditImage
end

A few examples: