Difference between revisions of "2D Space Shooter Part 3: Ships"

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m (Text replacement - "<source" to "<syntaxhighlight")
 
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The objective in a space shooter game is to destroy enemy spaceships, so lets work on that part.
+
__TOC__
 +
The objective in a space shooter game is to destroy enemy spaceships, so let's work on that part.
 +
 
 
There are two kind of ships in the game:
 
There are two kind of ships in the game:
* The player ship, which is controlled by the player and makes the game end when it is destroyed
+
* the player ship, which is controlled by the player and makes the game end when it is destroyed
* The enemy ships, which are computer controlled
+
* the enemy ships, which are computer controlled
  
However both kind of ships have common characteristics: armour, cannons, they can be hit by bullets, etc. Since we want to reuse code as much as possible, we'll create a Ship class to handle common features. But first lets define the actual ships we will have in our game.
+
However both kind of ships have common characteristics: armour, cannons, they can be hit by bullets, etc. Since we want to reuse code as much as possible, we will create a Ship class to handle common features. But first let's define the actual ships we will have in our game.
  
 
== Ships models ==
 
== Ships models ==
 
 
[[File:2D Spaceshooter Ship 3.png|thumb|A ship in our space shooter game]]
 
[[File:2D Spaceshooter Ship 3.png|thumb|A ship in our space shooter game]]
Our ships will have a texture, a few cannons, and a certain armour resistance.
+
Download the '''[[Media:2D_Spaceshooter_Ships.zip|ship images zip file]]''', unzip it and add each ship image to your project under the gfx folder.
We'll define them in a lua table. Create a ships.lua file and insert the ships description:
 
  
 +
Our ships will have a texture, a few cannons, and a certain armour resistance. We will define them in a lua table. Create a '''ships.lua''' file and insert the ships description:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
local SHIPS={
+
local SHIPS = {
A={ file="ship_1.png", armour=10, size=1,
+
A = { file="ship_1.png", armour=10, size=1, -- SHIP A
cannons={  
+
cannons={
{ x=49,y=0, type="laser", rate=30 },
+
{ x=0, y=-122/2, type="laser", rate=10*8 }, -- single cannon
 
}},
 
}},
B={ file="ship_2.png", armour=15, size=2,
+
B={ file="ship_2.png", armour=15, size=2, -- SHIP B
 
cannons={
 
cannons={
{x=20,y=30, type="laser", rate=25 },
+
{x=-115/5, y=-161/2, type="laser", rate=8*8 }, -- double cannon
{x=96,y=30, type="laser", rate=25 },
+
{x=115/5, y=-161/2, type="laser", rate=8*8 }, -- double cannon
 
}},
 
}},
C={ file="ship_3.png", armour=50, size=3,
+
C={ file="ship_3.png", armour=50, size=3, -- BOSS SHIP
 
cannons={
 
cannons={
{x=108,y=0, type="missile", rate=80 },
+
{x=-283/3, y=0, type="missile", rate=6*8 }, -- quadruple cannon!
{x=172,y=0, type="missile", rate=80 },
+
{x=283/3, y=0, type="missile", rate=6*8 }, -- quadruple cannon!
{x=40,y=105, type="laser", rate=20, angle=-15 },
+
{x=-40, y=-357/2, type="laser", rate=4*8, angle=-40 }, -- quadruple cannon!
{x=240,y=105, type="laser", rate=20, angle=15 },
+
{x=40, y=-357/2, type="laser", rate=4*8, angle=40 }, -- quadruple cannon!
 
}},
 
}},
Z={ file="ship_0.png", armour=100, size=2,
+
Z={ file="ship_0.png", armour=100, size=1, -- PLAYER SHIP
 
cannons={
 
cannons={
{x=22,y=45, type="laser", rate=15 },
+
{x=-68, y=0, type="laser", rate=2*8 }, -- double cannon
{x=155,y=45, type="laser", rate=15 },
+
{x=68, y=0, type="laser", rate=2*8 }, -- double cannon
 
}},
 
}},
 
}
 
}
  
SHIP_SIZE=512/6 --Logical size of a 1 unit ship
+
SHIP_SIZE = 512/6 -- logical size of a 1 unit ship
 
</syntaxhighlight>
 
</syntaxhighlight>
Each key in our table is a ship type. I choose single letter type names to make it easier to create our levels later. Each entry describe the ship characteristics:
+
 
 +
Each key in our table is a ship ''type''. I choose single letter type names to make it easier to create our levels later. Each entry describe the ship characteristics:
 
* the texture file to use
 
* the texture file to use
 
* the armour strength
 
* the armour strength
Line 45: Line 47:
 
* an array of cannons. Cannons have a position and angle in the source image, a bullet type and a fire rate
 
* an array of cannons. Cannons have a position and angle in the source image, a bullet type and a fire rate
  
We'll want to arrange our ships on a grid when they arrive on screen, as if they were part of a wing. To do so, we need our ships to be scaled appropriately to fit cells. That is what the relative size field is for, it will tell how many cells this ship is supposed to span. The final line of the above code block specifies the size of a grid cell. We computed it so that five small ships can span the screen width, but divided by 6 to leave a bit of margin.
+
We want to arrange our ships in a grid when they arrive on screen, as if they were part of a wave. To do so, we need our ships to be scaled appropriately to fit the cells. That is what the relative size field is for, it will tell how many cells this ship is supposed to span. The final line of the above code block specifies the size of a grid cell. We computed it so that five small ships can span the screen width, but divided by 6 to leave a bit of margin.
  
 
== The Ship class ==
 
== The Ship class ==
 +
We will use the same technique as the Background, and have our Ship class extend Gideros Pixel.
  
We'll use the same technique as the Background, and have our Ship class extend Gideros Pixel.
+
Below the ships definition we create our Ship class:
It will look like this:
 
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
Ship=Core.class(Pixel,function (type) end)
+
Ship = Core.class(Pixel, function(type) end)
  
 
function Ship:init(type)
 
function Ship:init(type)
type=type or "Z"
+
type = type or "Z" -- "A", "B", "C" = enemies, "Z" = player
local ship_def=SHIPS[type]
+
local ship_def = SHIPS[type]
assert(ship_def,"No such ship type: "..type)
+
assert(ship_def, "No such ship type: "..type)
-- Set up our ship based on its definition
+
-- set up our ship based on its definition
 
--- ...
 
--- ...
-- Add to actors list
+
-- add to actors list
ACTORS[self]=true
+
ACTORS[self] = true
 
end
 
end
  
 
function Ship:destroy()
 
function Ship:destroy()
-- Clean up
+
ACTORS[self] = nil -- remove from ACTORS list
ACTORS[self]=nil
+
self:removeFromParent() -- remove from screen
-- Remove from screen
 
self:removeFromParent()
 
 
end
 
end
  
 
function Ship:tick(delay)
 
function Ship:tick(delay)
-- Do whatever needs to be done on the game loop (move, fire cannons, etc)
+
-- do whatever needs to be done on the game loop (move, fire cannons, etc)
 
end
 
end
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
 
The 'Ship:init' method will look up the given ship type from the ships definition list, defaulting to the "Z" type if no type was supplied, then initialize itself from that ship description.
 
The 'Ship:init' method will look up the given ship type from the ships definition list, defaulting to the "Z" type if no type was supplied, then initialize itself from that ship description.
As a last step of its initialization, it will register itself to a global list of actors in the game. The idea here is to have each ship take care of itself without cluttering the main game loop. For now, append the above code block to your ships.lua file. We will complete this skeleton shortly.
 
  
For now switch back to main.lua file, some changes must be made already. Our game loop must call the 'tick' method of each object in the ACTORS list. While we are at it, let's create the collision world with cbump plugin, we will need it shortly. Don't forget to add the 'bump' plugin to your project settings!
+
As a last step of its initialization, it will register itself to a global list of actors in the game. The idea here is to have each ship take care of itself without cluttering the main game loop.
 +
 
 +
 
 +
Now switch back to main.lua file, some changes will be made. Our game loop must call the 'tick' method of each object in the ACTORS list.
  
 
Add this early in your main.lua code:
 
Add this early in your main.lua code:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
-- Create the collision world
+
-- a list of all ship objects that should receive frame ticks
BUMP=require "cbump"
+
ACTORS = {}
BUMP_WORLD=BUMP.newWorld()
 
-- A list of all objects that should receive frame ticks
 
ACTORS={}
 
 
</syntaxhighlight>
 
</syntaxhighlight>
  
 
And the following code inside your game loop:
 
And the following code inside your game loop:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
for k,_ in pairs(ACTORS) do
+
for k, _ in pairs(ACTORS) do
 
k:tick(1)
 
k:tick(1)
 
end
 
end
Line 98: Line 98:
 
The game loop code should look like this now:
 
The game loop code should look like this now:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
-- This is our game loop
+
-- this is our game loop
stage:addEventListener(Event.ENTER_FRAME,function ()
+
stage:addEventListener(Event.ENTER_FRAME, function()
 
background:advance(1)
 
background:advance(1)
for k,_ in pairs(ACTORS) do
+
for k, _ in pairs(ACTORS) do
 
k:tick(1)
 
k:tick(1)
 
end
 
end
Line 108: Line 108:
  
 
== Ship code ==
 
== Ship code ==
Our code should still run so far, but we won't see anything new. Lets complete our ship code and show it on screen.
+
Our code should still run so far, but we won't see anything new. Let's complete our ship code and show it on screen.
  
 
First complete the 'Ship:init' method:
 
First complete the 'Ship:init' method:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
 
function Ship:init(type)
 
function Ship:init(type)
type=type or "Z"
+
type = type or "Z" -- "A", "B", "C" = enemies, "Z" = player
local ship_def=SHIPS[type]
+
local ship_def = SHIPS[type]
assert(ship_def,"No such ship type: "..type)
+
assert(ship_def, "No such ship type: "..type)
-- Load the texture
+
-- load the texture
local texture=Texture.new("gfx/"..ship_def.file,true)
+
local texture = Texture.new("gfx/"..ship_def.file, true)
-- Compute ship scale
+
-- compute ship scale
local tw,th=texture:getWidth(),texture:getHeight()
+
local tw, th = texture:getWidth(), texture:getHeight()
local size=(ship_def.size or 1)*SHIP_SIZE
+
local size = (ship_def.size or 1)*SHIP_SIZE
local scale=size/(tw<>th)
+
local scale = size/(tw<>th)
-- Set up texture and dimensions
+
-- set up texture and dimensions
 
self:setTexture(texture)
 
self:setTexture(texture)
self:setDimensions(tw*scale,th*scale)
+
self:setDimensions(tw*scale, th*scale)
-- Position the ship by its center
+
-- position the ship by its center
self:setAnchorPoint(0.5,0.5)
+
self:setAnchorPoint(0.5, 0.5)
-- Keep a few variables in our object
+
-- keep a few variables in our object
self.scale=scale
+
self.scale = scale
self.armour=ship_def.armour
+
self.armour = ship_def.armour
-- Set up cannons
+
-- by default the ships are enemy ships
self.cannons={}
+
self.isplayer = false
for k,cdef in ipairs(ship_def.cannons) do
+
if type == "Z" then self.isplayer = true end -- Z = player ship
--self.cannons[k]=Cannon.new(cdef,scale,self)
+
-- set up cannons
 +
self.cannons = {}
 +
for k, cdef in ipairs(ship_def.cannons) do
 +
--self.cannons[k] = Cannon.new(cdef, scale, self)
 
end
 
end
-- Add to collision world
+
-- add to actors list
BUMP_WORLD:add(self,0,0,tw*scale,th*scale)
+
ACTORS[self] = true
-- Add to actors list
 
ACTORS[self]=true
 
 
end
 
end
 
</syntaxhighlight>
 
</syntaxhighlight>
 +
 
A few remarks:  
 
A few remarks:  
* I used the <> operator, which is something specific to gideros. It stands for 'take the maximum of the two values'. Likewise, the >< operator can be used to take the lowest value. While it makes the lua code incompatible with other engines, it also makes it faster and more difficult to recover from compiled code.
+
* I used the <> operator, which is something specific to Gideros. It stands for 'take the maximum of the two values'. Likewise, the >< operator can be used to take the lowest value. While it makes the lua code incompatible with other engines, it also makes it faster and more difficult to recover from compiled code
* The cannons will be handled by their own objects. I left that line commented because we'll that that later.
+
* the cannons will be handled by their own objects. I left that line commented because we'll do that later
* The collision rectangle in bump world is initially placed at coordinate 0,0 (but width and height are set up correctly). We will update them in the 'Ship:tick' method below.
+
* by default all the ships are flagged as enemy ships ''self.isplayer = false'' but for the player ship (Z) we set it to true. This flag will be used for the bullets and collision checking!
  
Our 'tick' method will update the collision world according to our ship position, and fire the cannons if appropriate.
+
Our 'tick' method will fire the cannons if appropriate:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
 
function Ship:tick(delay)
 
function Ship:tick(delay)
local shipax,shipay=self:getAnchorPosition()
 
local shipx,shipy=self:getPosition()
 
BUMP_WORLD:update(self,shipx-shipax,shipy-shipay)
 
 
if self.fire then
 
if self.fire then
for _,cannon in ipairs(self.cannons) do
+
for _, cannon in ipairs(self.cannons) do
 
cannon:fire(delay)
 
cannon:fire(delay)
 
end
 
end
Line 160: Line 159:
 
</syntaxhighlight>
 
</syntaxhighlight>
  
We must not forget to remove our ship from the collision world when it gets destroyed. Add the following lines inside the 'Ship:destroy' method.
+
== Showing our ship ==
 +
We have done a lot of hidden work in this chapter, to see a bit of evolution from the previous chapter, let's add our ship on screen.
 +
 
 +
In main.lua, above the game loop, add the following code:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
-- Remove from collision world
+
-- a graphic layer for all enemies
BUMP_WORLD:remove(self)
+
ENEMIES_LAYER = Sprite.new()
 +
stage:addChild(ENEMIES_LAYER)
 +
 
 +
-- the player ship
 +
local player = Ship.new()
 +
player:setPosition((SCR_RIGHT-SCR_LEFT)/2, SCR_BOTTOM-128)
 +
stage:addChild(player)
 +
 
 +
-- a graphic layer for all bullets
 +
BULLETS_LAYER = Sprite.new()
 +
stage:addChild(BULLETS_LAYER)
 
</syntaxhighlight>
 
</syntaxhighlight>
  
== Showing our ship ==
+
With this code, we are declaring the enemies layer, the player ship and the bullets layer and we place our ship centered at the bottom of the screen.
We have done a lot of hidden work in this chapter, to see a bit of evolution from the previous chapter, lets add our ship on screen.
 
  
In main.lua add the following code at the end:
+
Your main.lua should look something like this:
 
<syntaxhighlight lang="lua">
 
<syntaxhighlight lang="lua">
-- A graphic layer for all enemies
+
-- a list of all ship objects that should receive frame ticks
ENEMIES=Sprite.new()
+
ACTORS = {}
stage:addChild(ENEMIES)
+
 
 +
-- compute screen bounds
 +
SCR_LEFT, SCR_TOP, SCR_RIGHT, SCR_BOTTOM = application:getLogicalBounds()
 +
 
 +
-- create the background object with the screen size
 +
-- width should be 512 per project settings, but we'll compute it anyway to avoid relying on constants
 +
local background = Background.new(SCR_RIGHT-SCR_LEFT, SCR_BOTTOM-SCR_TOP)
 +
-- add it to stage
 +
stage:addChild(background)
 +
background:setY(SCR_TOP)
 +
 
 +
-- a graphic layer for all enemies
 +
ENEMIES_LAYER = Sprite.new()
 +
stage:addChild(ENEMIES_LAYER)
  
local player=Ship.new()
+
-- the player ship
player:setPosition(256,SCR_BOTTOM-128)
+
local player = Ship.new()
 +
player:setPosition((SCR_RIGHT-SCR_LEFT)/2, SCR_BOTTOM-128)
 
stage:addChild(player)
 
stage:addChild(player)
  
-- A graphic layer for all bullets
+
-- a graphic layer for all bullets
BULLETS=Sprite.new()
+
BULLETS_LAYER = Sprite.new()
stage:addChild(BULLETS)
+
stage:addChild(BULLETS_LAYER)
 +
 
 +
-- this is our game loop
 +
stage:addEventListener(Event.ENTER_FRAME, function()
 +
background:advance(1)
 +
for k, _ in pairs(ACTORS) do
 +
k:tick(1)
 +
end
 +
end)
 
</syntaxhighlight>
 
</syntaxhighlight>
  
With this code, we are declaring the enemy layer, the player ship and the bullets layer. We place our ship centered at the bottom of the screen. Now we just need to add our [[Media:2D_Spaceshooter_Ships.zip|ship images]] to our project (under the gfx folder), and see the result.
+
Let's see the result.
  
 
{{#widget:GApp|app=SpaceShooter_SH1.GApp|width=320|height=480|plugins=bump}}
 
{{#widget:GApp|app=SpaceShooter_SH1.GApp|width=320|height=480|plugins=bump}}
  
[[2D Space Shooter Part 4: Player]]
+
 
 +
Prev.: [[2D Space Shooter Part 2: Background]]</br>
 +
'''Next: [[2D Space Shooter Part 4: Player]]'''
 +
 
 +
 
 +
'''[[Tutorial - Making a 2D space shooter game]]'''
 +
{{GIDEROS IMPORTANT LINKS}}

Latest revision as of 22:08, 15 November 2023

The objective in a space shooter game is to destroy enemy spaceships, so let's work on that part.

There are two kind of ships in the game:

  • the player ship, which is controlled by the player and makes the game end when it is destroyed
  • the enemy ships, which are computer controlled

However both kind of ships have common characteristics: armour, cannons, they can be hit by bullets, etc. Since we want to reuse code as much as possible, we will create a Ship class to handle common features. But first let's define the actual ships we will have in our game.

Ships models

A ship in our space shooter game

Download the ship images zip file, unzip it and add each ship image to your project under the gfx folder.

Our ships will have a texture, a few cannons, and a certain armour resistance. We will define them in a lua table. Create a ships.lua file and insert the ships description:

local SHIPS = {
A = { file="ship_1.png", armour=10, size=1, -- SHIP A
	cannons={
		{ x=0, y=-122/2, type="laser", rate=10*8 }, -- single cannon
	}},
B={ file="ship_2.png", armour=15, size=2, -- SHIP B
	cannons={
		{x=-115/5, y=-161/2, type="laser", rate=8*8 }, -- double cannon
		{x=115/5, y=-161/2, type="laser", rate=8*8 }, -- double cannon
	}},
C={ file="ship_3.png", armour=50, size=3, -- BOSS SHIP
	cannons={
		{x=-283/3, y=0, type="missile", rate=6*8 }, -- quadruple cannon!
		{x=283/3, y=0, type="missile", rate=6*8 }, -- quadruple cannon!
		{x=-40, y=-357/2, type="laser", rate=4*8, angle=-40 }, -- quadruple cannon!
		{x=40, y=-357/2, type="laser", rate=4*8, angle=40 }, -- quadruple cannon!
	}},
Z={ file="ship_0.png", armour=100, size=1, -- PLAYER SHIP
	cannons={
		{x=-68, y=0, type="laser", rate=2*8 }, -- double cannon
		{x=68, y=0, type="laser", rate=2*8 }, -- double cannon
	}},
}

SHIP_SIZE = 512/6 -- logical size of a 1 unit ship

Each key in our table is a ship type. I choose single letter type names to make it easier to create our levels later. Each entry describe the ship characteristics:

  • the texture file to use
  • the armour strength
  • the relative size of the ship
  • an array of cannons. Cannons have a position and angle in the source image, a bullet type and a fire rate

We want to arrange our ships in a grid when they arrive on screen, as if they were part of a wave. To do so, we need our ships to be scaled appropriately to fit the cells. That is what the relative size field is for, it will tell how many cells this ship is supposed to span. The final line of the above code block specifies the size of a grid cell. We computed it so that five small ships can span the screen width, but divided by 6 to leave a bit of margin.

The Ship class

We will use the same technique as the Background, and have our Ship class extend Gideros Pixel.

Below the ships definition we create our Ship class:

Ship = Core.class(Pixel, function(type) end)

function Ship:init(type)
	type = type or "Z" -- "A", "B", "C" = enemies, "Z" = player
	local ship_def = SHIPS[type]
	assert(ship_def, "No such ship type: "..type)
	-- set up our ship based on its definition
	--- ...
	-- add to actors list
	ACTORS[self] = true
end

function Ship:destroy()
	ACTORS[self] = nil -- remove from ACTORS list
	self:removeFromParent() -- remove from screen
end

function Ship:tick(delay)
	-- do whatever needs to be done on the game loop (move, fire cannons, etc)
end

The 'Ship:init' method will look up the given ship type from the ships definition list, defaulting to the "Z" type if no type was supplied, then initialize itself from that ship description.

As a last step of its initialization, it will register itself to a global list of actors in the game. The idea here is to have each ship take care of itself without cluttering the main game loop.


Now switch back to main.lua file, some changes will be made. Our game loop must call the 'tick' method of each object in the ACTORS list.

Add this early in your main.lua code:

-- a list of all ship objects that should receive frame ticks
ACTORS = {}

And the following code inside your game loop:

	for k, _ in pairs(ACTORS) do
		k:tick(1)
	end

The game loop code should look like this now:

-- this is our game loop
stage:addEventListener(Event.ENTER_FRAME, function()
	background:advance(1)
	for k, _ in pairs(ACTORS) do
		k:tick(1)
	end
end)

Ship code

Our code should still run so far, but we won't see anything new. Let's complete our ship code and show it on screen.

First complete the 'Ship:init' method:

function Ship:init(type)
	type = type or "Z" -- "A", "B", "C" = enemies, "Z" = player
	local ship_def = SHIPS[type]
	assert(ship_def, "No such ship type: "..type)
	-- load the texture
	local texture = Texture.new("gfx/"..ship_def.file, true)
	-- compute ship scale
	local tw, th = texture:getWidth(), texture:getHeight()
	local size = (ship_def.size or 1)*SHIP_SIZE
	local scale = size/(tw<>th)
	-- set up texture and dimensions
	self:setTexture(texture)
	self:setDimensions(tw*scale, th*scale)
	-- position the ship by its center
	self:setAnchorPoint(0.5, 0.5)
	-- keep a few variables in our object
	self.scale = scale
	self.armour = ship_def.armour
	-- by default the ships are enemy ships
	self.isplayer = false
	if type == "Z" then self.isplayer = true end -- Z = player ship
	-- set up cannons
	self.cannons = {}
	for k, cdef in ipairs(ship_def.cannons) do
		--self.cannons[k] = Cannon.new(cdef, scale, self)
	end
	-- add to actors list
	ACTORS[self] = true
end

A few remarks:

  • I used the <> operator, which is something specific to Gideros. It stands for 'take the maximum of the two values'. Likewise, the >< operator can be used to take the lowest value. While it makes the lua code incompatible with other engines, it also makes it faster and more difficult to recover from compiled code
  • the cannons will be handled by their own objects. I left that line commented because we'll do that later
  • by default all the ships are flagged as enemy ships self.isplayer = false but for the player ship (Z) we set it to true. This flag will be used for the bullets and collision checking!

Our 'tick' method will fire the cannons if appropriate:

function Ship:tick(delay)
	if self.fire then
		for _, cannon in ipairs(self.cannons) do
			cannon:fire(delay)
		end
	end
end

Showing our ship

We have done a lot of hidden work in this chapter, to see a bit of evolution from the previous chapter, let's add our ship on screen.

In main.lua, above the game loop, add the following code:

-- a graphic layer for all enemies
ENEMIES_LAYER = Sprite.new()
stage:addChild(ENEMIES_LAYER)

-- the player ship
local player = Ship.new()
player:setPosition((SCR_RIGHT-SCR_LEFT)/2, SCR_BOTTOM-128)
stage:addChild(player)

-- a graphic layer for all bullets
BULLETS_LAYER = Sprite.new()
stage:addChild(BULLETS_LAYER)

With this code, we are declaring the enemies layer, the player ship and the bullets layer and we place our ship centered at the bottom of the screen.

Your main.lua should look something like this:

-- a list of all ship objects that should receive frame ticks
ACTORS = {}

-- compute screen bounds
SCR_LEFT, SCR_TOP, SCR_RIGHT, SCR_BOTTOM = application:getLogicalBounds()

-- create the background object with the screen size 
-- width should be 512 per project settings, but we'll compute it anyway to avoid relying on constants
local background = Background.new(SCR_RIGHT-SCR_LEFT, SCR_BOTTOM-SCR_TOP)
-- add it to stage
stage:addChild(background)
background:setY(SCR_TOP)

-- a graphic layer for all enemies
ENEMIES_LAYER = Sprite.new()
stage:addChild(ENEMIES_LAYER)

-- the player ship
local player = Ship.new()
player:setPosition((SCR_RIGHT-SCR_LEFT)/2, SCR_BOTTOM-128)
stage:addChild(player)

-- a graphic layer for all bullets
BULLETS_LAYER = Sprite.new()
stage:addChild(BULLETS_LAYER)

-- this is our game loop
stage:addEventListener(Event.ENTER_FRAME, function()
	background:advance(1)
	for k, _ in pairs(ACTORS) do
		k:tick(1)
	end
end)

Let's see the result.


Prev.: 2D Space Shooter Part 2: Background
Next: 2D Space Shooter Part 4: Player


Tutorial - Making a 2D space shooter game