OGRE Manual v1.7 ('Cthugha'): Object Orientation - more than just a buzzword

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## 1.1 Object Orientation - more than just a buzzword

The name is a dead giveaway. It says Object-Oriented Graphics Rendering Engine, and that's exactly what it is. Ok, but why? Why did I choose to make such a big deal about this?
Well, nowadays graphics engines are like any other large software system. They start small, but soon they balloon into monstrously complex beasts which just can't be all understood at once. It's pretty hard to manage systems of this size, and even harder to make changes to them reliably, and that's pretty important in a field where new techniques and approaches seem to appear every other week. Designing systems around huge files full of C function calls just doesn't cut it anymore - even if the whole thing is written by one person (not likely) they will find it hard to locate that elusive bit of code after a few months and even harder to work out how it all fits together.
Object orientation is a very popular approach to addressing the complexity problem. It's a step up from decomposing your code into separate functions, it groups function and state data together in classes which are designed to represent real concepts. It allows you to hide complexity inside easily recognised packages with a conceptually simple interface so they are easy to recognise and have a feel of 'building blocks' which you can plug together again later. You can also organise these blocks so that some of them look the same on the outside, but have very different ways of achieving their objectives on the inside, again reducing the complexity for the developers because they only have to learn one interface.
I'm not going to teach you OO here, that's a subject for many other books, but suffice to say I'd seen enough benefits of OO in business systems that I was surprised most graphics code seemed to be written in C function style. I was interested to see whether I could apply my design experience in other types of software to an area which has long held a place in my heart - 3D graphics engines. Some people I spoke to were of the opinion that using full C++ wouldn't be fast enough for a real-time graphics engine, but others (including me) were of the opinion that, with care, and object-oriented framework can be performant. We were right. In summary, here's the benefits an object-oriented approach brings to OGRE:

Abstraction
  Common interfaces hide the nuances between different implementations of 3D API and operating systems
Encapsulation
  There is a lot of state management and context-specific actions to be done in a graphics engine - encapsulation allows me to put the code and data nearest to where it is used which makes the code cleaner and easier to understand, and more reliable because duplication is avoided
Polymorphism
  The behaviour of methods changes depending on the type of object you are using, even if you only learn one interface, e.g. a class specialised for managing indoor levels behaves completely differently from the standard scene manager, but looks identical to other classes in the system and has the same methods called on it

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This document was generated by Steve Streeting on December, 31 2009 using texi2html