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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
-- 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
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
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
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
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
