A Late Postmortem of "Monster Defense"

May 18, 2019 code rambling

A few months ago, I uploaded the full source code and assets I used in an unfinished project I worked on back in 2011/2012 to Github. However, as is typical of me lately, I forgot to take the opportunity to write about it here. This is going to be a very informal postmortem that is probably better described as me writing random things in an unorganized manner about an old project of mine.

"Monster Defense" was to be an arena shoot-em-up / bullet-hell type of arcade game where you are supposed to survive an onslaught of monsters that come at you in waves. Monsters you kill can drop power-ups and other random power-ups will spawn periodically throughout the level as you go. There were two different planned modes of play, "timed" and "endless" which are probably fairly self-explanatory.

The artwork is all placeholder. Because I opted to use Quake 2 MD2 models for reasons discussed later in this post, I found that using the wealth of character models from Egoboo (whose assets are freely available under a GPL3 license) was helpful in getting reasonable placeholder artwork going. The problem is I of course never got "real" artwork in the end to replace the placeholders. D'oh.

Not exactly a project worthy of any awards in unique design, heh. Good thing I don't fancy myself a game designer.

Anyway, I suppose I might as well start with a little background on how this project came to be.

Background and Timeline

For most of my side-projects, especially game projects, I find I tend to have a very strong Not Invented Here syndrome approach where I rather enjoy building up the groundwork myself rather then using some preexisting framework or whatnot that may be available. For game projects, this means that you will probably never find me using something like Unity or Unreal Engine or something like that. Lower-level libraries like SDL are a different story, especially if you want to target multiple platforms. In fact, I think something like SDL is really a sweet spot for people like me in that it provides just enough of an abstraction that I can easily target multiple platforms but build up my own engine from scratch if I so choose. In recent years, I've found myself becoming more accepting of using something for game projects that provides a little bit more abstraction, such as libGDX, while still not going to all-out Unity-levels of abstraction.

But back in 2011, I had been working on my own C/C++ framework for my game projects and it was just based on SDL for desktop PC builds. It was coming along well, and in early 2012 I actually left my job to start working on the project that would become Monster Defense full-time. I had grandiose ideas that I was going to somehow make it as an indie game developer. Heh. Well, that was part of the reason I left my job. The other big part was that I was getting a little bit fed up and noticed that I was really just coasting along at work so I guess this was my own way of shaking things up for myself. I think I knew in the back of my head at the time that there was absolutely no chance I was going to make it as any kind of remotely successful indie game developer. I have barely any artistic talent (at least, not enough to be able to draw my own complete set of game art assets) and as you can tell from the fairly uninspired "design" of Monster Defense described previously, I'm not exactly a game design genius.

Actually, I should point out that I was originally going to work on a completely different game. I even mocked up a promo image for it.

I don't now remember why I switched the project to Monster Defense instead. I think the reason might have been that I thought it would be an easier project to finish or something like that. Of course that wasn't true. Both project ideas were quite involved. But hindsight is (usually) 20/20...

I remember that within two or three months of quitting my job, I had fully accepted the fact that Monster Defense was going to just be a portfolio piece I would use to get a job in the games industry and that there was no way it was going to become a fully polished and published game that would make me money. Didn't really take long for reality to fully set in, right?

I continued working on it and around late fall or so of 2012, I think I had come to realize that I actually didn't want to work in the games industry. Instead, I was fine with game development just being a hobby. This was actually the second time in my life I had come to that same conclusion. The first was in 2005 after I had finished a two year college diploma and was about to commit to moving out west to attend the game development course at the Art Institute of Vancouver (apparently this has all changed significantly in the past decade or so, I don't think it exists in Vancouver anymore?). I re-thought the decision and ended up getting a job as a "normal" software developer instead.

Anyway, during summer and fall of 2012 I had started to notice that I was becoming less enthusiastic about working on Monster Defense as time went on. A big part of it was the realization that I was not ever going to be able to release it as a finished product using placeholder art assets as I was currently using. And I was still unsure exactly what was the best way to solve this problem. You could say that, obviously, recruiting an artist was the solution here. I was not so enthusiastic about that idea because the last time I worked on a game project in a team with an actual artist, the experience left a really bad taste in my mouth.

Around 2006/2007 or so, I was working on a Zombie survival / puzzle game for Nintendo DS (using homebrew development tools, like devkitARM) in a team of friends, one of whom was an artist and two others who also had some varying artistic talents as well. That project ended abruptly with our artist seemingly just disappearing (I actually never met him in person, but he was a friend of a friend and lived in Vancouver). In the end no artwork had actually been produced. At least, none that I ever saw. In fact, the only thing that had ever been produced during this failed project was a fairly basic 3D engine for the Nintendo DS (coded from the ground-up by yours truly) and a very, very overly ambitious game design document that detailed a plan we were honestly never going to come close to completing. The overly ambitious design is a different problem altogether and I think is a very common problem, but the artist randomly disappearing was hugely problematic and ruined whatever trust I may have had in a team composed mostly of friends being reliable. Actually, now that I think about it, this was the second time that I've had an artist disappear on me (the first was for a QBasic game project around 2001/2002 or so). Maybe the problem is me? Heh. *shrugs*

Very early test build of the Nintendo DS project. Bits of this code ended up being used in Monster Defense too!


But regardless, even if I had wanted to try finding an artist again to see how it went, I did not have much money left to pay for one and I was skeptical of being able to find one who would volunteer to help me out on the promise of splitting future profits once the game was ready to be sold. Especially since I no longer really had that end goal in mind anyway. I didn't want to be dishonest, recruiting someone on that premise when I no longer believed it would happen at all.

As a result the project basically ended right there during fall 2012 and in early 2013 I found a new full-time job. I continued working on Monster Defense in bits here and there during early 2013 but more just as "something to do."

The Project Itself and How I Built It

As mentioned earlier, Monster Defense was built from the ground up using SDL for desktop PC builds. I developed it using C/C++, in a "C with classes" coding style, targeting OpenGL ES 2.0 (or the equivalent desktop OpenGL feature-set). I built my own framework and game engine for this project. I also was targeting mobile devices through the use of the now defunct Marmalade SDK (formerly Airplay SDK).

Before SDL 2.0 really stabilized and ironed out their mobile device support, Marmalade SDK was really quite awesome I think. It provided an environment where you could code your game using a quite familiar feeling game loop, C-style, in your main() function. It had some higher-level graphics/audio/input abstraction support but you could also opt to just use OpenGL directly and ignore their own graphics library which is what I did.

I updated my game framework to support different platforms mainly via an abstraction provided in two interfaces, OperatingSystem and GameWindow. I had two implementations, one for SDL for desktop builds, SDLSystem and SDLGameWindow, and the other for Marmalade SDK for mobile builds, MarmaladeSystem and MarmaladeGameWindow. This platform abstraction carried on a little bit further with interfaces like Keyboard, Mouse, Touchscreen, File, and FileSystem.

I was pretty happy with this architecture. Maybe it was not perfect, but it seemed to work well enough for me. The game loop was handled in a base class called BaseGameApp which the game project itself would implement (what I always just called GameApp). The main() function just served to instantiate the OperatingSystem, GameWindow and GameApp classes and then called GameApp:Start() which would get the game loop running.

BaseGameApp provided access to the system objects mentioned above as you can probably imagine.

OperatingSystem* GetOperatingSystem();
GraphicsDevice* GetGraphicsDevice();
Keyboard* GetKeyboard();
Mouse* GetMouse();
Touchscreen* GetTouchscreen();
ContentManager* GetContentManager();

The ContentManager was an idea I took from XNA although I'm sure many other game frameworks use a similar approach. The content manager object took care of caching of loaded assets and loading them if they were not already when assets were requested.

Texture* texture = GetContentManager()->Get<Texture>("assets://textures/mytexture.png");

The above snippet would return a Texture* for the texture in the specified file. If it was not yet loaded it would be right then and there. Thusly, asset file paths were used as a unique identifier. In hindsight perhaps using numeric IDs to identify assets via the ContentManager might have been better. At any rate, what I ended up doing in an attempt to simplify re-use of common assets was to have a singleton class ContentCache that had methods like this on it:

Texture* GetUISkin();
Texture* GetGamePadButtons();
SpriteFont* GetTitleFont();
SpriteFont* GetFont();
SpriteFont* GetSmallFont();
SpriteFont* GetUIFont();
TextureAtlas* GetEnvironment();
TextureAtlas* GetParticles();
TextureAtlas* GetItems();
Texture* GetShadow();

Looking at it now, I feel like the addition of ContentCache highlighted perfectly how ContentManager was not providing the right level of abstraction and I probably should have merged the two. ContentManager just ended up being a fancy general-purpose thing which I did not really need. At least, not in that fully general-purpose way (I definitely did need the caching and re-use of loaded assets obviously).

Anyway, back to the game loop, which was implemented via callbacks in GameApp that were called every frame. Additionally, other important system event callbacks could be invoked for certain things, such as the window being minimized or the app running on the mobile device being paused.

void OnAppGainFocus();
void OnAppLostFocus();
void OnAppPause();
void OnAppResume();
bool OnInit();
void OnLoadGame();
void OnLostContext();
void OnNewContext();
void OnRender();
void OnResize();
void OnUpdate(float delta);

At the time I remember that this style of game loop, implemented via callbacks (in this case, the main callbacks being OnUpdate() and OnRender() of course) seeming to be "obvious" to me in a "well, duh, why wouldn't you do it like this" kind of way. I had been doing game projects (such as the aforementioned Nintendo DS project) in this style for a while already. Nowadays, I'm not so certain. It might be because of my recent return to "old-school coding" with MS-DOS, but I feel like today I would prefer (if at all possible) to just put everything into a simple while(true) { ... } type of loop. No fully separated update and render operations. I would still separate update and render calls so that updates generally all happen before rendering each frame as that only makes sense. But I feel like sometimes there are some grey areas and having the full, forced, separation makes you sometimes do silly things to work around the 1-5% of the time you need to do something weird. Plus I guess that I just like the simplicity of a simple loop. Meh.

However, one plus of using this game app object callbacks architecture was that it made it pretty simple to plug in a game state/process system which still allowed each state and process to react to the very same types of callbacks/events.

The idea with the game state/process system was that in a typical game there are multiple different states where the player needs to interact with the game in a very different way. For example, they launch the game, and a main menu appears. They make some choices and then the game starts. Now they are playing the game. They can pause the game which maybe brings up a different menu. Or they could open some status/inventory display during game play which requires them to interact with a UI of some sort.

Each of these things can be represented by a different state, and all of the states together are represented by a stack. And that is exactly what StateManager and GameState are for. StateManager has a bunch of useful methods for manipulating the stack of game states.

template<class T> T* Push();
template<class T> T* Push(const stl::string &name);
template<class T> T* Overlay();
template<class T> T* Overlay(const stl::string &name);
template<class T> T* SwapTopWith();
template<class T> T* SwapTopWith(const stl::string &name);
template<class T> T* SwapTopNonOverlayWith();
template<class T> T* SwapTopNonOverlayWith(const stl::string &name);
void Pop();
void PopTopNonOverlay();

The reason for the use of templates in most of these methods is so that the class type of the game state is passed but not the actual object instance itself since I guess I didn't want the caller to have to instantiate the object itself (I honestly don't recall my reasoning for that decision now).

GetStateManager()->Push<MainMenuState>();

The code in StateManager always felt overly complex to me but I do feel like the end result worked quite well. In addition to normal game states that could be pushed/popped or swapped, you could also use "overlay states." This essentially let you have the current "main" state running in the background while overlaying another state over it to allow some limited processing to continue happening in the "main" state, but where the overlay state was the one that was really the current state. The best example of this is where the player is in game and they open their inventory. You might want to overlay a menu UI over-top of the game screen which still continues to be rendered and even maybe with animations still continuing in the background, but you want to pause game logic (so enemies cannot attack the player while they are in their inventory, for example). This type of thing could be implemented by having the inventory game state be an "overlay state."

GameState objects implemented all the same basic callbacks/events that GameApp had in addition to having a few extras to allow them to react to manipulation of the game state stack.

void OnPush();
void OnPop();
void OnPause(BOOL dueToOverlay);
void OnResume(BOOL fromOverlay);
BOOL OnTransition(float delta, BOOL isTransitioningOut, BOOL started);

OnTransition() in particular was neat because it allowed game states to very simply do things like implement their own fading in/out or similar type of effects.

Now, game states were not by themselves enough. At least I didn't think so at the time. Very often you may want to do multiple things each frame while a game state is running and they must run together. The best example of this I think is during a GamePlayState where the player has full control over the gameplay, you probably also want to show some kind of game status (e.g. score, lives, health, etc). You could obviously just call whatever code to show this in your GamePlayState or you could do what I did and have each game state have it's own ProcessManager which containers a list of GameProcesses, and then have a StatusUI process do the game status display.

ProcessManager worked quite similarly to StateManager except that instead of a stack of processes, it was just a list. Each GameProcess added was always processed each frame in sequence.

Circling back to what I was talking about earlier with regards to the game loop and the pros/cons of implemented via some really simple while(true) { ... } with all the code right there in the body of the loop, or by doing it how I described here using callbacks in GameApp... I think that (at least to me) my StateManager and ProcessManager setup made a pretty strong case for sticking with this approach. In fact I liked this architecture so much that I kept it for the next game project I worked on in 2014, even porting it all to Java/libGDX in the process.

Another really important thing for a game to have is some sort of events system. I'm not actually talking about events from the operating system or input devices, etc, but game logic events. Such as when the timer runs out, the player gets hurt or even dies, the player picks up an item, etc. My framework had support for this too, and this was another aspect that I think worked well in practice but had some little things to cleanup in the future maybe.

In my event system, classes can subscribe to events by implementing EventListener and then registering themselves with the EventManager (which was managed by BaseGameApp). I can remember it was very important to me that game states and processes (and, well, really anything else) should be able to subscribe to events via code like the following:

ListenFor<IntroFinishedEvent>();
ListenFor<OptionsUpdatedEvent>();
ListenFor<QuitGameEvent>();
ListenFor<DespawnedEvent>();
ListenFor<TimerRanOutEvent>();

And then unsubscribe from events via StopListeningFor() called in a very similar way. And this is exactly the way it worked. Event handling happened in a Handle() method that any class implementing EventListener would need to provide.

BOOL GamePlayState::Handle(const Event *event)
{
    if (event->Is<QuitGameEvent>())
    {
        // do stuff
    }
    else if (event->Is<IntroFinishedEvent>())
    {
        // do other stuff
    }
    
    // ...
}

It was a very minor bit of syntax sugar, but I really remember the use of templates here to provide the information about what type of event to subscribe to, or here with the Is() method so it knows what to compare against, was a really important feature I wanted the API to have. *shrug* Looking back at it now, I do have to admit that I quite like the way it reads so I guess it was worth the effort.

Events were used quite a bit in Monster Defense. I had defined 35 different event types at the time I stopped working on the project. Definitely there would have been more had it continued. There were events for things like animation updates, player buffs, entity state changes, healing, damage, movement, score, weapon switching, timers, and other things.

Which brings us to entity management. I remember at this time there had been a number of articles written on the subject of "entity/component systems." I was initially intrigued by the idea as I was not liking the idea of going with some object hierarchy approach. I remember reading this article (and a number of others by the same author) and really liking the approach but that it clearly seemed to be missing some necessary extras and what I deemed "basic functionality." As well, I didn't like the idea of using ID's to represent entities and figured I'd instead just use an Entity object (but purely as a code convenience... it still would not be meant to contain any data directly).

class Entity
{
public:
    Entity(EntityManager *entityManager);
    virtual ~Entity();
    
    template<class T> T* Get() const;
    template<class T> T* Add();
    template<class T> void Remove();
    template<class T> BOOL Has() const;
    
    template<class T> BOOL WasCreatedUsingPreset() const;
    BOOL WasCreatedUsingPreset(ENTITYPRESET_TYPE type) const;
};

The Get(), Add(), Remove() and Has() methods all operated on Component objects which you attached to entities and these were the things that contained the actual data that belonged to the entity itself.

class Component
{
public:
    virtual ~Component();
    virtual void Reset();
};

As you can see from this, a base Component is incredibly simple and itself contains no data. The specific component types would actually have the data defined. For example, a PositionComponent to hold an entity's position in the game:

class PositionComponent : public Component
{
public:
    PositionComponent();
    void Reset();
    
    // the actual data this component holds
    Vector3 position;
};

ComponentSystem objects are registered with the EntityManager. Then each time during the game's OnUpdate() and OnRender() callbacks, each component system is called in turn.

When a component system is called to do it's processing (either during OnUpdate() or OnRender()), it queries the EntityManager for a list of all entities that contain a component it cares about. For example, the PhysicsSystem would fetch a list of entities that currently have a PhysicsComponent.

void PhysicsSystem::OnUpdate(float delta)
{
    EntityList list;

    GetEntityManager()->GetAllWith<PhysicsComponent>(list);
    for (EntityList::iterator i = list.begin(); i != list.end(); ++i)
    {
        Entity *entity = *i;
        PhysicsComponent *physics = entity->Get<PhysicsComponent>();

        // do stuff
    }
}

Generally, component systems would do their main processing loop based on entities having one "main" component type that that component system cared about, but component systems could very well retrieve other components from a given entity if needed (for example, the PhysicsComponent will need access to the entity's position via the PositionComponent in addition to whatever current physics state is present in its PhysicsComponent).

The entity system and event system also were integrated. Component systems were also EventListeners. For example, the PhysicsSystem would respond to JumpEvent and MoveForwardEvent amongst others. These types of events were EntityEvent subclasses which required an Entity object to be set so that the thing that listened to those events (almost always a ComponentSystem) would manipulate the source entity in response to the event.

MoveForwardEvent *moveEvent = new MoveForwardEvent(entity);
GetEventManager()->Queue(moveEvent);

Most of the entities that were created and managed in Monster Defense fit into the same categories and were re-used a lot. For example, there were a few types of zombie monsters that were constantly re-used. Same for power-up objects that the player could pick up. To handle this standardized creation of new entities, the addition of new entities via EntityManager needed to be passed off to an EntityPreset.

An EntityPreset had a very important Create() method which would be called by EntityManager when an entity was being Add()ed. It was then up to that Create() method to actually add the entity to the EntityManager and then return it. The MonsterPreset and ZombiePreset together give you an idea of how much manual component adding and manipulation was required to create an entity in Monster Defense (ZombiePreset is a subclass of MonsterPreset so when a zombie monster entity is added, both of their Create()'s get called).

Entity* MonsterPreset::Create(EntityPresetArgs *args)
{
    Entity *entity = GetEntityManager()->Add();
    
    entity->Add<AffectedByGravityComponent>();
    entity->Add<AnimationComponent>();
    entity->Add<AutoAnimationComponent>();
    entity->Add<BlobShadowComponent>();
    entity->Add<BoundingSphereComponent>();
    entity->Add<ColorComponent>()->color = COLOR_WHITE;
    entity->Add<LifeComponent>();
    entity->Add<NPCComponent>();
    entity->Add<OrientationXZComponent>();
    entity->Add<PhysicsComponent>();
    entity->Add<PositionComponent>();
    entity->Add<PushableComponent>();
    entity->Add<PusherComponent>();
    entity->Add<StateComponent>();
    entity->Add<BuffsComponent>();
    
    return entity;
}

Entity* ZombiePreset::Create(EntityPresetArgs *args)
{
    Entity *entity = MonsterPreset::Create(args);
    
    ContentManagerComponent *content = GetEntityManager()->GetGlobalComponent<ContentManagerComponent>();
    
    KeyframeMesh *mesh = content->content->Get<KeyframeMesh>("assets://characters/zombie.mesh");
    
    entity->Get<AnimationComponent>()
        ->AddSequence("idle", *mesh->GetAnimation("IDLE"), 0.05f)
        ->AddSequence("walk", *mesh->GetAnimation("WALK"), 0.02f)
        ->AddSequence("dead", *mesh->GetAnimation("KILLED"), 0.10f)
        ->AddSequence("punch", *mesh->GetAnimation("BASH_LEFT_2"), 0.1f);
        
    entity->Get<AutoAnimationComponent>()
        ->Set(ENTITYSTATE_IDLE, "idle")
        ->Set(ENTITYSTATE_WALKING, "walk")
        ->Set(ENTITYSTATE_DEAD, "dead", FALSE, TRUE)
        ->Set(ENTITYSTATE_PUNCHING, "punch", FALSE, TRUE);
        
    entity->Get<BoundingSphereComponent>()
        ->bounds.radius = 0.49f;

    PhysicsComponent *physics = entity->Get<PhysicsComponent>();
    physics->friction = FRICTION_NORMAL;
    physics->maxWalkSpeed = 6.0f;
    physics->walkingAcceleration = 4.0f;
    physics->SetBoundsRadius(0.49f);
    
    entity->Add<RenderOffsetComponent>()
        ->offset = Vector3(0.0f, -0.5f, 0.0f);

    KeyframeMeshComponent *keyframeMesh = entity->Add<KeyframeMeshComponent>();
    keyframeMesh->mesh = mesh;

    CanBeAttackedComponent *canBeAttacked = entity->Add<CanBeAttackedComponent>();
    canBeAttacked->byPlayer = TRUE;
    canBeAttacked->byNPC = TRUE;
    
    return entity;
}

Monster Defense had 65 different component types. Some of these were complex (like PhysicsComponent) and others served no purpose other then as a simple "marker" or type of glorified flag on the entity (such as AffectedByGravityComponent).

Now, a decision I am in hindsight a bit bothered by was that EntityPresets also sometimes subscribed to events and did processing on EntityEvents only when the entity in question was created by that same EntityPreset (when an entity was added using an EntityPreset the EntityManager would automatically attach an EntityPresetComponent to that entity to mark the EntityPreset type that was used to create it). Anyway, EntityPresets subscribed to stuff like DespawnedEvent, HealEvent, HurtEvent and KilledEvent so that common functionality that should occur for entities when these events occurred could be customized per entity-type. For example, when a bullet entity dies (KilledEvent) different things should happen then when a zombie entity dies. In the bullet's case, some particles should be created where it was last located at and nothing else. When a zombie dies, some smoke particles are created and a random chance for a power-up entity to be created needs to happen.

Looking back at this, I like the idea of the EntityPreset::Create() but I think that the other additions to EntityPresets made it a bit too complicated in practice. It's hard to put my finger on it exactly though, but I know if I were to start a new project today using this same entity/component system, I would probably spend a good long while thinking about a better way of organizing this.

However, in general I really liked the whole entity/component system concept. It felt very powerful to me. Especially with the physics system I had, I liked how easy it was to add full physics processing to basically anything. Added some smoke particles and didn't like them flying through solid walls? Fixed by adding a PhysicsComponent to it. Want to attach a "blob" (stupid simple black circle) shadow to some entity? Add a BlobShadowComponent. Want to make it so that one entity can push other entities around? Add a PusherComponent.

That all being said, I do feel like this also made the code feel a little bit more like spaghetti in some ways. It could be a bit harder to fully figure out how an entity was going to work once all it's components were added because the code for fully processing that entity was now spread out over (at most) 19 different component systems.

I won't be able to talk about every entity subsystem (of which there were many), but I do want to talk about the physics system a bit more because I spent so much freaking time on it. I think this took up at least half of my development time over the entire project, but it is hard to say for sure.

I remember initially investigating things like Bullet Physics and Open Dynamics Engine (ODE) and coming away from each of these feeling incredibly stupid. I never did particularly well in math 'nor physics classes in high school. I was at best a 'C' student in both subjects overall. Some years I did better then others. Of course, this is why things like the aforementioned two physics libraries exist... to make it possible for math "dummies" like myself to not have to fret over the scary details as much. Note the emphasized part of that last sentence. You do still need to know some stuff. Anyway, personally I felt like the documentation and examples for both of these libraries just flew over my head and left me almost clueless. I remember having a weak moment after investigating both of these and coming away empty handed where I almost decided that I really should have gone with something like Unity 3D instead from the very beginning ("Things like this must by one of the major reasons why these frameworks are so popular! I cannot possibly be the only one so stupid!"). I remember reading discussions at the time how the Bullet documentation was not the greatest so at least I didn't feel totally alone there.

Anyway, I decided to pick up a prototype I had worked on a few years earlier for a physics system that was based on the old Peroxide "Improved Collision detection and Response" article from 2003. I had it half working back then and figured I might see if I could fix the remaining problems and integrate it into my engine here for Monster Defense. I think after about a month of futzing about with this I became a victim of the sunk cost fallacy, unable to pull myself away from this and admit defeat. I just had to get it working.

I rather "enjoyed" spending lots of time working on problems with ramps. Of course, the single final level included in Monster Defense doesn't even have ramps in it, but hey, let's just ignore that fact. In particular I remember there was some bug that would only occur sometimes, where an entity that climbed up to the top of the ramp could suddenly disappear (in reality, they would be launched either upward or downward – I forgot which – at incredible velocity). This was due to some floating point inaccuracy that could cause a calculation to result in NaN and propagate into other calculations and cause mayhem. Took me a looong time to narrow that down.

Lots and lots of time was spent tweaking the "feel" of the physics system too, from the player's perspective. And I still don't think it's tweaked quite right. I remember reading something from Twitter at the time where an indie developer was saying something like how typically when a game's physics system feels just right, it's usually not the most accurate (real-world accurate that is). Like it's kind of "cartoon physics" in a way or something. I really agree with this. I definitely never nailed this either, heh. A lot of my time was spent on jump physics. Things that you never thing of, like how it should react when you jump up and hit a flat surface. Or you jump up and hit the corner of a ceiling edge. Or when you jump up and just as you're falling you catch the edge/lip of the top of a wall (there's a lingering "bug" in Monster Defense where you can double-jump in this scenario if you time it right, but the single level in it doesn't give you any opportunity to test this).

Test level for working on ramps, steps and other physics bugs.

One thing I did learn was that debugging physics problems by overlaying wireframe geometry onto the display was incredibly helpful. Looking at tons of numbers (so many you have to use 2 letter labels as in the above screenshot) is difficult. As well, stepping through with a debugger is not always the most helpful either.

The game world was represented using 3D tiles. Kind of Minecraft-inspired, where the world is subdivided into 16x16x16 TileChunks which are all part of a larger TileMap. Each grid component in the map being represented by a Tile which had a number of different properties. Special, optimized processing was present for Tiles which were just simple cubes. But it was also possible for tiles to be represented by arbitrary 3D meshes (ramps could be done in this way for example).

TileChunks needed to be turned into 3D meshes with a ChunkRenderer of which there was also an optional subclass, LitChunkVertexGenerator which could apply lighting effects, based on tiles that were defined to be light sources and would "spread" light to adjacent tiles (if there was empty space for light to pass though, or a tile was marked as not solid).

The full TileMap could be rendered by simply looping through all of the TileChunks in the map and rendering their pre-generated 3D meshes (which would each just be a VBO).

The use of a 3D tilemap for the game world made integration with the physics engine easier. Scanning the world for nearby world geometry to test for potential collisions with was made incredibly simple due to this. Once I did solve the seemingly insurmountable task of getting the physics engine to work decently enough, it really did feel like it all came together well. I remember when I finally got it all working, I just spent a day moving around one of my test worlds, running past the same set of walls and ramps and whatever other objects again and again, marveling at the fact that it actually worked. If I had to choose one single thing in my "programming career" that I worked on to date that gave me the most satisfaction or that I was most proud of, it would be this thing. Not this project (Monster Defense), but just getting this physics engine working as well as it is now, despite whatever bugs remain. I truthfully did not even think I would end up getting it working this well when I first began working on it.

There are a bunch of other things in Monster Defense that I could talk about in detail but nothing else feels like it is super worthy of writing lots of words about. One thing I feel that is missing that probably would have been added had I continued working on it was some sort of scripting integration, probably with Lua in particular since it's fairly easy to integrate with C. I think that using it to define entity behaviours and even creation of entities (versus the previously described EntityPreset system) would probably have been the best approach. Assuming of course that I could have arrived at a decent abstraction through Lua for interacting with the entity/component system. I suspect that adding/manipulating entity components through simple maps would have been the best way and is very likely something I will explore in future projects.

The last thing I think I will talk about is the graphics and math engine used in Monster Defense. There was a fair bit of inspiration taken from XNA here.

A GraphicsDevice is used to manage an OpenGL context, and allows Texture, Shader, VertexBuffer and IndexBuffer objects to be bound.

The integration between Shaders, VertexBuffers and IndexBuffers is something I was pretty proud of at the time, but am not sure how well it might hold up today (I've not really kept up to date on graphics APIs so I cannot say honestly).

VertexBuffer and IndexBuffer objects are both subclasses of BufferObject which handles the OpenGL details of VBO's or client-side buffers. Each of these two buffer subclasses had nice, easy to use methods for programmatically manipulating the buffer's contents, such as Move(relativeOffset), MoveTo(absoluteOffset), MoveNext(), MovePrevious(), SetCurrentPosition3(Vector3), GetCurrentPosition3(), etc. If the buffer was initially set to be backed by a VBO, using these "setter" methods would set a dirty flag and the next time the buffer was to be rendered by GraphicsDevice it would know to upload the buffer's data via OpenGL. A lot of this convenience functionality was definitely not efficient, but damned if it wasn't convenient. For pure performance, you obviously could just pre-set the buffer data and upload it once and then render it many times after that.

Example usage, initializing a VertexBuffer (part of some code to set up a wireframe grid mesh).

VERTEX_ATTRIBS attribs[] = {
    VERTEX_POS_3D,
    VERTEX_COLOR,
};

points = new VertexBuffer();
points->Initialize(attribs, 2, width * 2 + 2, BUFFEROBJECT_USAGE_STATIC);

for (uint i = 0; i < height + 1; ++i)
{
    points->SetPosition3((i * 2), -(width / 2.0f), 0.0f, i - (height / 2.0f));
    points->SetColor((i * 2), 1.0f, 1.0f, 1.0f);
    points->SetPosition3((i * 2) + 1, width / 2.0f, 0.0f, i - (height / 2.0f));
    points->SetColor((i * 2) + 1, 1.0f, 1.0f, 1.0f);
}

And then rendering via GraphicsDevice

SimpleColorShader *colorShader = graphicsDevice->GetSimpleColorShader();

graphicsDevice->Clear(0.25f, 0.5f, 1.0f, 1.0f); 

graphicsDevice->BindShader(colorShader);
colorShader->SetModelViewMatrix(modelViewMatrix;
colorShader->SetProjectionMatrix(projectionMatrix;

graphicsDevice->BindVertexBuffer(points);
graphicsDevice->RenderLines(0, points->GetNumElements() / 2);
graphicsDevice->UnbindVertexBuffer();

graphicsDevice->UnbindShader();

You can see here a Shader is used (a preset Shader instance in this case, one of several that the GraphicsDevice provides, but custom shaders can be used too). Missing is some of the boilerplate that you might see in other OpenGL code to map vertex attribute to the shader (like the position and color attributes we previously set up above). This is handled automatically through the VertexBuffers use of "standard vertex attributes" which Shader objects are aware of and know how to automatically bind. This is accomplished per-shader due to the nature of the actual GLES shader source code being able to use completely arbitrary names for the attributes. But it's still easy to set up, like in the constructor of the SimpleColorShader:

SimpleColorShader::SimpleColorShader()
    : StandardShader()
{
    BOOL result = LoadCompileAndLinkInlineSources(m_vertexShaderSource, m_fragmentShaderSource);
    ASSERT(result == TRUE);

    MapAttributeToStandardAttribType("a_position", VERTEX_POS_3D);
    MapAttributeToStandardAttribType("a_color", VERTEX_COLOR);
}

Where a_position and a_color are the actual attribute names used in the GLES source code.

An additional convenience that Shader objects provided was the ability to let you pre-set uniform values (even before the Shader is bound). If a uniform value is set on a shader object, it is simply cached, and the cache flushed (uniforms set via OpenGL) when the shader is bound. This is a minor thing, but it ended up being handy in a few places where I suddenly didn't need to care about this kind of thing. Probably not the best idea for performance-reasons, but I was not (and still am not) working on anything remotely cutting-edge.

While my game framework did have some limited support for skeletal animation with 3D models, it was half-baked and only ever worked with one or two test models. I fell back to vertex interpolation for animation (via good ol' Quake 2 MD2 models) as a result of giving up on trying to extract skeletal animation data out of Autodesk FBX models via their SDK. The FBX file format feels like PDF in that it is a bit of a "kitchen sink" format and as such, it hard to grasp fully for the uninitiated.

Final Thoughts

So, Monster Defense was of course never finished and probably won't be, at least not in it's current form. It was fun working on, but I am disappointed that it was never finished.

Technology-wise, some final notes:

  • While it was fun building everything myself from scratch, it ended up taking a ton of effort and this was probably the single biggest reason it was never finished.
  • Having said that, if I could it over again, I would still build everything from scratch. I learnt a lot.
  • Inefficiencies galore! I could get away with it here because I was by no means pushing any boundaries, but it's still (in my opinion) important to recognize where you are taking inefficient shortcuts. Some of these have been noted above, but one thing that still bothers me is that I was lazy about memory allocations. In particular I was all too willing to just arbitrarily new() up memory whenever I needed it.
  • Organization of the code, in hindsight after not looking at it for six years, feels messy. I find myself jumping around a lot trying to trace through how things worked.
  • I had some terrible STACK_TRACE macro I used everywhere which is a holdover from the Nintendo DS origins for a lot of this code where the debugging facilities are much more crude. I don't know why I kept this around here in Monster Defense. Also, ASSERT()ing after every allocation was a bit silly (again, another holdover from the Nintendo DS).
  • I will probably get shunned or banished as a programmer for this next point: I would probably consider using global singletons for a lot of "system"-type objects. For example, my ContentManager, StateManager, EntityManager, EventManager ... almost anything with "Manager" in the name I guess. I just don't see the value in having to pass these things around when they are actually global state. I don't see the value in hiding from this fact.

In late 2013 and through 2014 I ported much of this game framework code to Java using libGDX and put together the beginnings of a worthy successor.

Monster Arena Defense

I cannot currently release this as-is because the 3D model assets used are paid assets so throwing it up on Github would not be a good idea (I could do it with the code, but you would not be able to run it). However, I did release a sample demo of the underlying engine in 2014.

In this project, I did not resolve all my concerns with this game framework as noted in this post. But I did confirm a lot of them, heh. In particular the spaghetti code feel of a lot of the entity system architecture. It felt a lot worse in this project (but there were a lot more entity types...).

Perhaps there will be a "take 3" on this project in the future. Or I may just bring it back to MS-DOS. Maybe!