Skip list:Drawing Order

Z-ordering in LÖVE is tedious because the engine doesn't offer any support for it. You must draw the sprites in order by yourself, making it hard to handle a large number of sprites on different planes.

With the skip list data structure, you can do:

zlist = makeSkipList(3) -- 3 is the estimated size of the structure.
                        -- It's indicative, but a good guess improves the performance


Clown = makeSprite (100, 100, 1, "clown.png") -- x, y, z, image
Frogs = makeSprite (100, 110, 1, "frogs.png") -- on the same plane as the Clown, but drawn on top because it's been created after.
Tools = makeSprite (90,  120, 0, "tools.png") -- under the clown


zlist:insert( Frogs )
zlist:insert( Tools )
zlist:insert( Clown ) -- You can insert them in any order

for _, sprite in zlist:iter() do
    -- draw the sprite in Z order :-)
    print( sprite.x, sprite.y, sprite.z, sprite.image )
end

Output:

90      120     0       tools.png
100     100     1       clown.png
100     110     1       frogs.png


If you want to change the position of an item on the Z axis, you remove it and insert it back.

Tools.z = 2
zlist:delete(Tools)
zlist:insert(Tools) -- now Tools is on top of the pile

for _,sprite in zlist:iter() do
    print(sprite.x, sprite.y, sprite.z, sprite.image) -- draw(sprite)
end

Here's the output:

100     100     1       clown.png
100     110     1       frogs.png
90      120     2       tools.png

Toy implementation of makesprite:

do
local count = 0
function counter()
    count = count + 1
    return count
end
end


do
local lt = function(a,b)
    if a.z < b.z then return true
    elseif a.z==b.z and a.id < b.id then return true
    else return false
    end
end

local mt={
    __lt=lt,
    __le=lt
}
function makeSprite (x,y,z,image)
    local s={x = x, y = y, z = z, image=image, id = counter()}
-- the id field is necessary because, otherwise, items on the same plane would cause troubles.
    return setmetatable(s,mt)
end

end
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