Day 6 – Mobile Game Progress

Its been a few more days now and quite a bit of progress has been made so I thought I would update. First, have a go at the game in its current form above (use the keyboard to control).

Since last time I have concentrated on the parts I was less technically confident about such as the correct rendering method of animated sprites and how I was going to handle the rotation of the world.

First I tackled rendering. In my previous update the tiles in the world rendered simply by drawing vector squares to Sprite.graphics, then shifting their container to move them. When I tried to run this on an actual mobile device however the performance was extremely poor, less than 1FPS. The reason for this was because drawing vectors on mobile devices doesn’t really fit well into their GPU-orientated graphics pipeline. To get decent frame rates on mobile I needed to find a way to take advantage of the mobile GPU.

Fortunately for me those clever guys who wrote NME were way ahead of me and developed the “Tilesheet.drawTiles()” API:

[codesyntax lang=”actionscript3″ lines=”normal”]

extern class Tilesheet
{
	static var TILE_SCALE:Int;
	static var TILE_ROTATION:Int;
	static var TILE_RGB:Int;
	static var TILE_ALPHA:Int;
	static var TILE_TRANS_2x2:Int;

	static var TILE_BLEND_NORMAL:Int;
	static var TILE_BLEND_ADD:Int;

	function new(inImage:BitmapData):Void;
	function addTileRect(rectangle:Rectangle, centerPoint:Point = null):Void;

	/**
	 * Fast method to draw a batch of tiles using a Tilesheet
	 * 
	 * The input array accepts the x, y and tile ID for each tile you wish to draw.
	 * For example, an array of [ 0, 0, 0, 10, 10, 1 ] would draw tile 0 to (0, 0) and
	 * tile 1 to (10, 10)
	 * 
	 * You can also set flags for TILE_SCALE, TILE_ROTATION, TILE_RGB and
	 * TILE_ALPHA.
	 * 
	 * Depending on which flags are active, this is the full order of the array:
	 * 
	 * [ x, y, tile ID, scale, rotation, red, green, blue, alpha, x, y ... ]
	 * 
	 * @param	graphics		The nme.display.Graphics object to use for drawing
	 * @param	tileData		An array of all position, ID and optional values for use in drawing
	 * @param	smooth		(Optional) Whether drawn tiles should be smoothed (Default: false)
	 * @param	flags		(Optional) Flags to enable scale, rotation, RGB and/or alpha when drawing (Default: 0)
	 */
	function drawTiles (graphics:Graphics, tileData:Array<Float>, smooth:Bool = false, flags:Int = 0):Void;
}

[/codesyntax]

With it you pass an array of tile data with a number of optional properties such as Rotation, Alpha and Scale. It then does the heavy lifting behind the scenes of building a vertex buffer and sending it to the GPU for rendering with your Tilesheet texture. This results in two big wins for performance; 1) you can render a great many sprites on the GPU in the same render, 2) you dont perform any expensive texture switches if all your sprites are on the same texture.

As for the tilesheet itself. At first I was planning on using a manual method of putting the sprites onto the tilesheet such that each sprite was the same size and we arranged in a very simplistic “one row per animation” fashion:

The problem with this however was that it was going to be a fairly inefficient way of organising the assets and it was going to be tricky for Moh (the artist) to build these animations frame by frame and put them on there while keeping them looking good.

Fortunately for me this problem has already been encountered by those before me. Philippe Elsass (of Flash Develop fame) has written a library called TileLayer that solves two problems. Firstly it abstracts out the some of the hard work of using the low-level drawTiles() API into a more friendly parent-child-like syntax which should be very familiar to any flash developer. Secondly it provides a parser for “Sparrow” spritesheets.

Sparrow is a library for game development in pure Objective-C. Part of that library is a method for loading tightly packed, animated spritesheets. These spritesheets can be generated by a number of tools but the best one I found was Texture Packer (by Lee Brimelow Andreas L├Âw):

Using the tool you can take a number of input’s such as SWFs or other images and output a compact spritesheet and an XML document containing the data NME will use to render the sprites on the screen.

One problem I encountered with using Texture Packer with SWFs however was that for it to work you must do all your animating on the timeline, it wasn’t smart enough to populate named sprites from the library. Another thing to remember is that you must arrange your animations from the top left of the stage. If you put the animation in the middle of the stage it will be offset when it is rendered in the game. Not big problems but it did cause some head-scratching for a while before I realised what was going on.

With my sprites now loading and animating correctly in flash I decided to have a go at getting the game to run on mobile devices. Thanks to the way NME works it was a pretty simple process to generate a template project “nme update application.nmml ios” then run the generated xcode project file. And it runs! The only problems was that the frame rate was around 15FPS on my iPhone 4, which was far lower than I was hoping for.

So I started thinking about what could be causing a slow down. In flash when you try to render objects that are off-screen, flash automatically works out if it needs to be rendered and if not then it skips the render and hence improves the performance. I had a suspicion that Tilelayer / NME couldnt perform this operation so all the tiles that weren’t on the screen were being unnecessarily drawn to an off-screen buffer and hence lowering the performance.

Because in this game the player is always in the centre of the screen I knew that a crude way to calculate whether a tile was on screen was to simply check the distance of the tile from the player, if it was greater than the width + height of the screen (because the world rotates) then I knew it would be off-screen and thus didn’t need to be rendered:

As suspected this had a marked improvement in FPS, the game was now running on my iPhone at about 30FPS. An improvement but still short of what I was hoping for.

After some more experimentation I discovered another problem. In the code for checking whether to render a tile or not I was doing the following:

[codesyntax lang=”actionscript3″ lines=”normal”]

public function update(delta:Int) : Void
{				
	var dx = x - game.player.x;
	var dy = y - game.player.y;
	visible = animated = (dx * dx) + (dy * dy) < game.root.stage.stageWidth * game.root.stage.stageHeight;
}

[/codesyntax]

The problem is that apparently the call to “stage.stageWidth” and “stage.stageHeight” is very expensive. I presume behind the scenes NME is making an expensive call to the device for width and height information.

Once I took out those calls (cacheing them in the game object instead) and tested it again on the device the FPS was now up to more what I was hoping for at 60FPS ­čÖé

The final technical hurdle was to rotate the world. I was worried about this one as I was contemplating all the mathematical calculations that would be needed to work out the rotation for each tile in the game. Fortunately however there was a better solution. Because NME is based on the flash API I was able to put the world within a container Sprite, then offset the container by the player’s position – screenH/2 and screenW/2 then rotate the container thus giving the illusion of the world rotating.

To my relief this idea actually worked and even more incredibly it didn’t seem to affect the frame rate! See the video below of it running one my iPhone 4, iPad 3 and my old iPhone 3G.

The 3G will need some more work to make it run at acceptable framerates, but im impressed it even runs on that thing, you cant do this with Adobe’s Stage3D!

Well that enough of me talking about this for now, I need to get on with making the damn thing! Next up is some more game-play elements and perhaps some menu structure.

3 Days into the 3-Week Challenge

.. and we have progress! See above. Use the keyboard to control the ‘player’.

The first thing you will noticed is that there is a world populated with solid blocks and some orange circle things that may look a little like collectables ­čśŤ To generate this level I decided to take a page out of Notch’s book and build the levels at the pixel level in Paint.NET. That way the image editing software becomes the level editor. So as an example, the data that builds the level for above looks like:

That’s a 50×50 pixel image which represents a world 50×50 tiles wide. Each pixel in the image has a colour which corresponds to a tile type in the game. Black is a solid wall, red is the player spawn point and orange is a ‘collectable’. As the game develops we will be adding more tile types and hence colours.

At runtime all I do is load the level data image and loop through the pixels, grab the colour value and populate the world with the appropriate object. A problem I soon encountered however is that for some reason on different platforms Haxe reads the colour value different. This made things problematic so instead what I have done is make another 8×8 image as a “key”:

Loading this key first I can then determine what colour the platform will be recognising a particular tile type as. So to generate the level the code looks like:

[codesyntax lang=”actionscript3″]

class LevelManager 
{
	public static var TYPES : Hash<Class<Dynamic>>;	

	public var gridW : Int;
	public var gridH : Int;
	public var tiles : Array<BaseObject>;

	public function new() 
	{
		if (TYPES == null)
		{
			var objectTypes = [null, SolidBlock, SpawnPoint, Ring];			
			TYPES = new Hash<Class<Dynamic>>();
			var bmd =  Assets.getBitmapData("assets/levels/key.png");
			if (bmd == null) throw new Error("key png is null for some reason!");
			var i = 0;
			for (y in 0...bmd.height) for (x in 0...bmd.width) if(i<objectTypes.length) TYPES.set("" + StringTools.hex(bmd.getPixel(x, y), 6), objectTypes[i++]);
		}
	}

	public function loadLevel(stageIndex:Int, levelIndex:Int) 
	{
		var bmd =  Assets.getBitmapData("assets/levels/s" + stageIndex + "_l" + levelIndex+"/world.png");
		gridW = bmd.width;
		gridH = bmd.height;
		tiles = [];

		for (y in 0...gridH)
		{
			for (x in 0...gridW)
			{
				var c = StringTools.hex(bmd.getPixel(x, y), 6);
				var t = TYPES.get(c + "");
				if (t == null) { continue; }			
				var o : BaseObject = Type.createInstance(t, []);
				o.x = x * Game.GRID_SIZE;
				o.y = y * Game.GRID_SIZE;
				Game.I.addObject(o);
				tiles[y * gridW + x] = o;
			}			
		}
	}
...

[/codesyntax]

Once I had the level populating I started getting the basics of the physics sorted. At first I thought it was going to be a nightmare as in the original version of the game it appeared as if the whole world rotated about the player (see video for reminder), I worried about how I was going to handle the complex physics of a grid at odd angles while continually rotating. After a while however I realised that what was actually going on was that the world was standing still and all that was happening was that the camera was rotating at same rate at which the gravity vectyor was changing, thus giving the illusion of a rotating world, eg:

Once I realised this fact it made my life a whole lot easier. Calculating the physics for the world should now just be a matter of solving a circle against a static grid without rotations. I decided to go with my own physics solution rather one of the existing solutions such as Box2D or Nape as I thought that it should be pretty simple to calculate and I knew from a previous project that using Box2D or Nape would have issues at the joins between tiles.

The solution it turns out took a little longer than I thought but I eventually cracked it. The key was to use the Separating Axis Theorem with Voroni Regions, there is a great tutorial on it over at magnet software, they have a handy SWF that demonstrates the concept really well:

As can be seen from above that all you need do is split the problem up into a grid, then in turn check each of the 8 neighbouring cells from the current cell. The north, east, south and west cells can be classed as one type and only need to have their relevant axis checked against the radius of the player wheres the corner cells need to be checked against the distance from the closest point. In code this looks something like:

[codesyntax lang=”actionscript3″]

// From Player.hx

override public function update(delta:Int) : Void
{
	#if !mobile
	if (Ctrl.instance.isDown("up")) vel.y -= 1;
	if (Ctrl.instance.isDown("left")) vel.x -= 1;
	if (Ctrl.instance.isDown("right")) vel.x += 1;
	//if (Ctrl.instance.isDown("down")) vel.y += 1;
	#end

	if (Ctrl.instance.mouseDown) vel.y -= 1;

	var d = delta * 0.01;
	vel.x += gravity.x * d;
	vel.y += gravity.y * d;		

	var newPos = new Vec2(x + vel.x * d, y + vel.y * d);
	var ntx : Int = Std.int(newPos.x / Game.GRID_SIZE);
	var nty : Int = Std.int(newPos.y / Game.GRID_SIZE);				

	checkTileCollide(ntx, nty, ntx - 1, nty + 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx + 1, nty + 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx - 1, nty - 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx + 1, nty - 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx, nty + 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx, nty - 1, newPos, vel);
	checkTileCollide(ntx, nty, ntx + 1, nty , newPos, vel);
	checkTileCollide(ntx, nty, ntx - 1, nty , newPos, vel);	

	x = newPos.x;
	y = newPos.y;
}

private function checkTileCollide(fromTX:Int, fromTY:Int, toTX:Int, toTY:Int, pos:Vec2, vel:Vec2) : Bool
{
	var tile = game.level.getTile(toTX, toTY);
	var dTX = fromTX - toTX;
	var dTY = fromTY - toTY;
	if (tile != null && tile.is(SolidBlock))
	{				
		if (dTX == 0)
		{
			var d =  Math.abs(pos.y-((toTY - fromTY) > 0?toTY * Game.GRID_SIZE:fromTY * Game.GRID_SIZE));
			if (d < radius)
			{
				pos.y += dTY * (radius - d);
				vel.y = 0;
				return true;
			}
		}
		if (dTY == 0)
		{
			var d =  Math.abs(pos.x-((toTX - fromTX) > 0?toTX * Game.GRID_SIZE:fromTX * Game.GRID_SIZE));
			if (d < radius)
			{
				pos.x += dTX * (radius - d);
				vel.x = 0;
				return true;
			}
		}			
		else
		{	
			var tp = new Vec2((dTX>0?fromTX:toTX)*Game.GRID_SIZE, (dTY>0?fromTY:toTY)*Game.GRID_SIZE);				
			var vToCorner = new Vec2(tp.x - pos.x, tp.y - pos.y);			
			if (vToCorner.lengthSqr() < radius * radius)
			{					
				var ang = Math.atan2(vToCorner.y, vToCorner.x);							
				pos.x = tp.x - Math.cos(ang) * radius;
				pos.y = tp.y - Math.sin(ang) * radius;				
				//vel.x = vel.y = 0;
				return true;
			}			
		}
	}
	return false;
}

[/codesyntax]

Its not 100% perfect, there is some oddness when the player hits a corner but will do for now.

On the art side of the project Moh has been making good progress coming up with themes for the game. We have been playing around with the idea that the player is a Hamster lost in space, which I really like the idea of. To test this idea he made little mock-up, which looks great:

You may have noticed that currently the game is in Flash. That’s because with NME you can target Flash as one of your outputs. This makes developing and testing the game alot easyier (a lot faster to compile and run). I have however been very aware of the problems I could cause myself if I developed the whole game┬ásolely┬áin flash and only testing on mobile right at the end. Trying to track down an obscure problem in a fully written game would be a nightmare. So I have been making progress with getting the game to run on my iPhone 4.

One of the problems I faced (and I banged my against the wall for a while on this one) was that for some reason when the level was populating from the PNG, certain tiles weren’t being built. I couldn’t for the life of me work out why. To cut a long story short, apparently when building for iOS in Haxe you MUST put the super call in the constructor BEFORE any other call, else the code before the super call in the constructor wont be executed:

[codesyntax lang=”actionscript3″]

class Player extends BaseObject
{	

	public function new() 
	{		
		trace("This will not be executed when built for iOS but WILL be executed when built for flash");	
		super();
		trace("This will be execute on flash AND iOS");	
	}

...

[/codesyntax]

A small thing to remember but quite a gotcha for the NME Haxe newbie!

Another issue I have run into is the fact that my iPhone currently has iOS 5.1 on it, this means that to use it as a testing platform I had to upgrade my Macbook to OSX Lion which meant I have to leave it running over night and this morning downloading and installing. Not a biggie as I have been meaning to upgrade for a while anyways, but an inconvenience when you want to sit down to test your shiny new game out!

We have quite a way to go, but im happy with the progress we have made in 3 days thus far ­čÖé

Lets Make a Mobile Game in 3-Weeks with Haxe & NME

I have decided that I would like to learn more about mobile game development. Its an exciting area of the games industry at the moment with so many frameworks and languages to choose from to develop with. The plethora of choice however can be a hindrance, what are the best frameworks and language for this particular task? Haxe with NME?  C# MonoTouch with Monogame? or even Javascript with Titanium? One can only go so far just reading about them, eventually you have to jump in and try them out for yourself.

So on that note I have decided to team up with an artist colleague (Moh ‘Mohzart’ Mukhtar) from work to create a mobile game in three weeks of evenings and weekends. Three weeks is an ambitious target for a game on any platform let alone mobile with its added complications. The primary purpose of this project however is to gather experience on the process of making mobile games using a given langauge, framework and platform. Having a longer time-scale I feel would deter from this, the primary motivation. Having said that however, I hope that in three weeks we will have something that to be reasonably proud of.

For this particular game I have decided to explore Haxe with NME. Haxe is a programming language I have been using for my personal projects for about a year now and is ideally suited to mobile development due to its ability to target multiple platforms with the same codebase. NME is a framework written in Haxe that emulates the Flash API and thus provides an interface I am extremely familiar with having used it for many years on personal and professional games. I dabbled with NME about a year ago while at the TryHarder conference however things have moved on with the project a lot since then and im looking forward to getting stuck in and playing around with it some more.

In theory Haxe with NME should give us the ability to target Flash, HTML5, OSX, Linux, Window, iOS, Android and WebOS all with the same codebase. I hope this to be the case, however, initially we will be focusing on getting it to run on Flash and iOS, the other platforms will be of secondary concern.

So as for the actual game itself. Over the last week or so we have been racking our brains trying to come up with a game that will be suitable for this project. To be suitable it must be achievable within three week, be compatible for touch based devices, suit both our skill sets and preferably not been done before on mobile. A tough set of requirements.

We wrote down a few ideas in a GDoc, but the best one and the one we have decided upon is to do a remake /┬ávariant┬áon the “Chaos Emerald” bonus stage from Sonic1 on the Genesis. As a quick reminder of what that looks like:

It wont be a direct rip (before I have the Sega lawyers knocking down my door) but the inspiration will certainty come from that game. The reason why we finally chose this idea is because it should work quite well on mobile (single button press with accelerometer tilting) and is a fairly simple game code and art wise so hopefully we should be able to complete it within three weeks. As an added bonus I have been unable to find this already existing on the App Store. Let me know if you have seen it done already!

I intend to continue to update on the progress of the game as we go along on this blog ­čÖé

Fingers crossed we can pull it off!