Sorting sprite children...

SinisterSoft

Is there a way in gideros to sort a sprites children/class instances by one of the fields?

eg

-- a load of baddie children, all having different y's

scene.baddies:sort(y,true) -- the 1st param says what field to sort by, the 2nd true or false for accending or decending sort

zvardin

It probably wouldn't be too bad to write one in lua. You can call addChild passing a sprite that is already a child to change its position, so if you kept a table of them sorting them in order then you could just loop through the table calling addChild passing in each sprite again.

SinisterSoft

Yes, but that would be slow. I'd like to do it without having to add children every game frame. Can you swap the order of two children?

SinisterSoft

This is the same problem I'm trying to solve (they are using Corona):

http://developer.coronalabs.com/forum/2010/11/08/z-order-graphic-objects

Basically I want to be able to reorder the objects based upon a z-order.

zvardin

You could swap the order using addChildAt. So if you had something at index 2 and something at 5 and needed to swap you could do:

scene:addChildAt(scene.baddies[2],5)
scene:addChildAt(scene.baddies[5],2)

You would just need to make you swap the lower to greater index first, so you didn't throw off later swaps.

atilim

Although it's possible to swap 2 children with @zvardin's 2nd post I think sorting with swapping can be much more slower than @zvardin's 1st post

SinisterSoft

You mean using Lua's table sort?

A common c lib to do this (possibly popular function) would be better though...

atilim

Lua's table sort (table.sort) uses his own quick sort implementation in C. Therefore, it's fast enough. Here the bottleneck will be removing and adding all children.

SinisterSoft

I might just remap the existing children data from a table (only add, no remove) - hiding any not used... Will that be faster?

atilim

Most probably will be faster. Adding to the same parent (remapping/repositioning) have the advantage of getting rid of ADDED_TO_STAGE and REMOVED_FROM_STAGE events.

SinisterSoft

@atilim Ok, I've converted it all to use tables and remap and it's fast enough.

But how do I sort (using the lua sort) something like:

objects={}
.
.
.
noObjects=noObjects+1
objects[noObjects]={}
objects[noObjects].depth=255
objects[noObjects].type=0


based upon the depth part of the table?

zvardin

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)

SinisterSoft

@zvardin Thanks, that works great. Verified with 300 objects depth sorted and no slowdown.

zvardin

Awesome to hear it works so well performance wise. Looking forward to see how you use it (asteroid game? )

SinisterSoft

@zvardin - yes it will be used in that game.

I create a table of currently visible objects (on screen) with a depth field. Some matter, (real properly 3D tracked things like baddies) some don't (asteroids, plasma weapons, smoke trails). I sort them so that baddies will move in front of or behind objects visibly (when they do properly code wise).

Lua and garbage collection are the bottle necks for me to keep the speed up. That's why I was so concerned about adding and removing things. I like to do that as little as possible.

I'm only spending an hour or two every couple of days or so on the game as I have a Windows program I wrote that I need to support/market during the day. It's coming along though.

I'll post another video soon...

SinisterSoft

I've put an updated video on the other post.