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Design II : Attack of the GraphicEngine

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Emmanuel Deloget


Abstract of the previous shows

In the previous installment of my ongoing journal activity, I decided to speak about UML design. I now see that my wording is obscure and contains a lot of (sometimes kebod related) english errors. My bad. Nevertheless I decided to continue to speak about this subject - I find it interesting. Since noone really read this journal (thanks Mushu for the info), I bet it will not hurt your brains.

Good design, bad design

The preliminary design I got yesterday is a bit small. Some pseudo-C++ gives us:1

class GraphicEngine
draw(World world);
draw(NonStaticObject object);

It raises an interesting question: who performs the drawing? Should we say that the GraphicEngine draws the World object or should we say that the World should draw itself using the GraphicEngine? The answer is not very simple.

First, let's consider a simple design issue: what if I decide to modify the graphic engine? I may decide to finally go the 3D way. I probably won't want to rebuild all the game code because of this modification- even it if is a large modification. Or I may decide to adapt the drawing to the device capabilities (a mobile phone is more restricted than a PC). Using this wording it seems that I should let my GraphicEngine draw the world. It means that my GraphicEngine will have to know how a World object is built. Here comes the problem...

OTOH the GraphicEngine is a base component that should not be aware of what the game really is. If I decide to modify my underlying World representation later - because of some important design issue - I will need to modify the GraphicEngine as well - even if the rendering itself do not change. Moreover, if I decide to add a new object type that have to be drawn then I'll have to modify the GraphicEngine as well.

Conclusion: there is something missing. We have to forget our preliminrary design and we need to find better answers to the questions I asked yesterday: who will use the graphic engine, and what will the actor do with it?

To be continued... :)
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I never was a big fan of function names like "Initialize" and "Uninitialize". I always prefer semantics like "Open" and "Close" or "Start" and "Stop" or "Create" and "Destroy".

Also, in most implementations, a graphics engine is a singleton, and because of this, it can have a single, global, point of access. The natural consequence of this is that everything that needs to be drawn should have access to it.

class SGraphicsEngine
static bool Open();//or Start, or Create
static void Close();//or Stop, or Destroy

Furthermore, in a rendering list, it should not matter what you are rendering... each object should inherit from a single base class, perhaps "IRenderable". The graphics engine would contain a list of renderables, and when it comes time to render a scene, it goes through and tells each object to draw itself.

class SGraphicsEngine
static bool Open();
static void Close();
static bool AddObject(IRenderable* pObject);
static bool Render();

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The above is a good example, however the render function of the renderable object may depend on resources stored within the graphics object. If this is the case the renderable object would need a reference to the core engine as well as an interface to retrieve the resources. If you didn't want an instance of the graphics engine stored in the renderable object you could pass it as a parameter of the render function.

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