Compare sprites depth (z-index)

Rickyngk · March 2013

I try to compare sprites depth (z-index) by using the following code

function Sprite:CompareDepthTo(cmp)
	if cmp == self then return 0 end
 
	p1 = self:getParent()
	p2 = cmp:getParent()
	if p1 == cmp then --cmp is parent of self
		return 1 
 
	elseif self == p2 then --self is parent of cmp
		return -1 
 
	elseif p1 == p2 and p1 ~= nil then -- the same root
		return p1:getChildIndex(self) > p1:getChildIndex(cmp) and 1 or -1
 
	else --complicated relationship
		local p1 = self
		local p2 = cmp
		local pp1 = {}
		local pp2 = {}
		while p1 ~= nil do
			pp1[#pp1 + 1] = p1
			p1 = p1:getParent()
		end
		while p2 ~= nil do
			pp2[#pp2 + 1] = p2
			p2 = p2:getParent()
		end
 
		local m = math.min(#pp2, #pp1)
		local index = 0
 
		for i = 0, m - 1 do
			index = i
			if pp2[#pp2 - i] ~= pp1[#pp1 - i] then break end
		end
 
		if index == 0 then -- not the same root
			return nil
		elseif index == m - 1 then -- one of them are parent/root of other, for ex: [19 17 15 4 2 1] <-> [15 4 2 1]
			return #pp1 > #pp2 and 1 or -1
		else
			local common_p = pp1[#pp1 - index + 1]
			local child1 = pp1[#pp1 - index]
			local child2 = pp2[#pp2 - index]
			return common_p:getChildIndex(child1) > common_p:getChildIndex(child2) and 1 or -1
		end
	end
end

return :
+ (1): means self stays above
+ (-1): self stays below
+ (0): compare self itself
+ (nil): unknown, there's no relationship

I'm afraid of its performance, any better solution?

P/S: I also attach sample code.

john26 · March 2013

Your solution looks good. But it z-depth is so important why not put everything in a flat hierarchy? You could make every sprite a child of stage and then just compare index numbers.

Rickyngk · March 2013

@john26: I thought about it, but get problems in find put a way to number all sprites, especially real-time adding/removing child.

I also think about "label" sprites by hierarchy index. I'll post when finish
----
Edit: Oop,"label" idea failed because of real-tine adding/removing

loucsam · March 2013

n00b question:

What are the hashes(#) in the code for?

e.g.
pp2[#pp2 + 1] = p2

zvardin · March 2013

The # gets the length of a table.

Mells · March 2013

Just throwing an idea, did you check this post :
Sorting sprite children... and the solution of using a .depth variable and passing a sort function :

You can pass in a sort function, so you should be able to do something like:
-- descending order
table.sort(objects, function(a,b)
   return a.depth > b.depth
end)

Source

See if it helps, you never get too many options.

john26 · March 2013

@loucsam. The # operator returns the number of entries in the table with numerical keys

eg

a=["fox","goose"] -- keys are 1 and 2
print (#a) -- prints 2

non numerical entries are not accounted for

a.farm=true
print (#a) -- still prints 2

loucsam · March 2013

@loucsam. The # operator returns the number of entries in the table with numerical keys

Thanks john26.

I'm having display hierarchy(z-order) issues in my code at the moment I think. I might have to also think about not having certain sprites tied in with others, as in tying them to the stage instead.

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