design

Ok ok, I am writing again! If you read my previous posts on dynamic npc actions then this is an update on my progress. If you have not, this is about how to create good file formats for your applications.

As some might have expected this project is taking more time then expected, partially because I am currently downing bosses in WoW, and partially because its very hard. So lets just cover what has been going on since my last post.

I spent a lot of time thinking of the best structure for my idea in terms of objects and file formats. I do not know if I covered this before because I am to lazy to view my past posts so heres a recap of the basic composition of what I have been calling a ‘Puppet.’ A puppet is a game object with x number of movable joints. These joints are built in a tree structure and move based on messages passed to his parent. These messages are generated from a actor class who learns actions and movements as I previously discussed. When I was designing the puppet class I hit a big fat road block. I wanted the ability for a puppet to be independent of his underlying joint structure. That way I could have a puppet control everything from a human to grass. So you might think, ‘No problem, do some tricky interface work and have derived classes for each type of puppet.’ WRONG!

See I myself have read many articles about component game object design, and I do believe I wrote an article on this in the past. However do not get the wrong idea, what I did is not EXACTLY component objects, but kind of close. Because when I thought about components I also thought how to best describe them in a file format. And after thinking about that I decided that I wanted a global file format to describe not only each type of puppet but the entire world. And that the entire game will be built out of components in a tree structure built at load time from this file format. I am getting a bit ahead of myself here so lets get some examples here.

<world>
<name type=string>Main</name>
<description type=string>
Big world for which to run tests in
</description>
<floor type=plane>
<p1 type=vector>-1000 -1000 -1000</p1>
<p2 type=vector>1000 1000 1000</p2>
<material>dirt01.mat</material>
</floor>
<joint>
<position type=vector>0 0 0</position>
<orientation type=vector>0 0 0</orientation>
<puppet>arm.puppet</puppet>
</joint>
</world>

This is my toy world I am working in. The file format is xml and I use tinyXML to load and parse these files. For those familiar with ogre, I made a resource of type XMLFile to load these. Now here comes the clincher, this XMLFile resource will load every type of game object I desire, regardless of type or purpose. Not only that, but I only added 2 methods to the class.

So let me explain a bit about these methods, they are named ‘build’ and they are called usually immediately after loading the file. Basically what they do is look at an element in the xml, find a builder for it, have the builder build it, and attach the new object to the world tree. In this example, the world has no parent and all elements inside the world tags are children of the world. I use the type descriptors so that I would not have to write many classes that do the exact same thing. Elements that load another file, like the puppet tags, will load that file in their own world and all objects created from that are children of the puppet. After the build the main program is not aware of any object other then the world. Messages are passed to the world when need be, however most thought processing is done in the world object, not by the game engine.

Lets get an example puppet up here:

<puppet>
<name type=string>arm</name>
<description type=string>
Generic arm
</description>
<joint>
<name type=string>shoulder</name>
<gib>upperarm.gib</gib>
<joint>
<name type=string>elbow</name>
<gib>forearm.gib</gib>
<joint>
<name type=string>wrist</name>
<gib>hand.gib</gib>
<joint>
<name type=string>pinky</name>
<gib>pinky.gib</gib>
</joint>
<joint>
<name type=string>ring</name>
<gib>ring.gib</gib>
</joint>
<joint>
<name type=string>middle</name>
<gib>middle.gib</gib>
</joint>
<joint>
<name type=string>index</name>
<gib>index.gib</gib>
</joint>
<joint>
<name type=string>thumb</name>
<gib>thumb.gib</gib>
</joint>
</joint>
</joint>
</joint>
</puppet>

As you can see this file format is very flexible, the same structure that built the world can build a puppet and also a gib:

<gib>
<name type=string>forearm</name>
<notes type=string>
Fore arm gib
</notes>
<mesh>forearm.mesh</mesh>
<input>
<name type=string>shoulder</name>
<position type=vector>0 0 0</position>
</input>
<output>
<name type=string>elbow</name>
<position type=vector>7 0 0</position>
</output>
</gib>

I want to clarify one thing before I go on. Elements called position or orientation, these attributes are not given any attention by the builders. Instead those objects simply exist on the tree for someone else to read them and make the correct adjustments. For example, the puppet’s name element simply sits as an element on the tree until a debugger opens and the entire tree structure is displayed. At that time the debugger will go through each element and find name tags that it will use to give more information on the node. And the description works the same way.

They are both created by the string builder, as denoted by the ‘type=string’ attribute I give them. The value is assigned and the name of the new object is set, but the type is always a string.

So what exactly is required to declare a new type of object? Just a builder and an actual object. Take this example of the string object.

class ScktString : public ISocketable
{
friend class StringBuilder;
public:
void setValue( std::string val ) { _string = val; }
std::string getString() { return _string; }
private:
ScktString( std::string propName ) : ISocketable( propName ) {}

std::string _string;
};

class StringBuilder : public IBuilder
{
public:
StringBuilder() : IBuilder() {}
Socket* build( std::string element, Socket* parent );
const std::string& getType() const;
static std::string BUILDER_TYPE_NAME;
private:
static const StringBuilder registerThis;
};

As you can see its relatively easy to create a new object. Lets start at the builder, when the xmlfile is building it will search for a builder based on its type name, which is assigned by you. A builder list is created at the start of the program, as each builder has a static ‘registerThis’ variable that adds it to the builder list. Those familar with static factories should be familiar with this. The builder’s build method gets called with the name of the element and its parent’s socket. A socket is an object that accepts links from children. The builder then builds the object passing its name. The sockable base class is for objects that can be socketed or linked with a parent.

So the string object is created and the string builder passes back the parent object. This is done in case the new object has a socket himself. In that case he would pass back his socket and the builder would build his children and attach them to him. In string’s case however, there is no socket and a little while later the builder assigns the string object a value by calling the setValue method in the parent object with the objects name and value as parameters.

After all the work we are left with one tree that contains all objects in the game. Kind of like a scene manager, but for ALL objects, not just renderable ones. And oh, don’t forget that the entire tree was created by one instance of XMLFile. As programmers we should always strive to achieve the cleanest and most powerful format we can muster. This format is not only really easy to implement, but also holds unlimited power. You want to load a texture? Create a texture socketable object, and implement file specific loading inside it. You want to script AI? Either create an AI socketable object that reads children objects that decribe a strategy, or create a python socketable object that will load your ai scripts and parse them. In this way our file format can act a proxy for another format or as the direct format. The most important thing however is that none of our in game objects need to give file formats a single thought.