OGRE  1.10.7
Object-Oriented Graphics Rendering Engine
Introduction

This chapter is intended to give you an overview of the main components of OGRE and why they have been put together that way.

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

Multi-everything

I wanted to do more than create a 3D engine that ran on one 3D API, on one platform, with one type of scene (indoor levels are most popular). I wanted OGRE to be able to extend to any kind of scene (but yet still implement scene-specific optimisations under the surface), any platform and any 3D API.

Therefore all the ’visible’ parts of OGRE are completely independent of platform, 3D API and scene type. There are no dependencies on Windows types, no assumptions about the type of scene you are creating, and the principles of the 3D aspects are based on core maths texts rather than one particular API implementation.

Now of course somewhere OGRE has to get down to the nitty-gritty of the specifics of the platform, API and scene, but it does this in subclasses specially designed for the environment in question, but which still expose the same interface as the abstract versions.

For example, there is a ’Win32Window’ class which handles all the details about rendering windows on a Win32 platform - however the application designer only has to manipulate it via the superclass interface ’RenderWindow’, which will be the same across all platforms.

Similarly the ’SceneManager’ class looks after the arrangement of objects in the scene and their rendering sequence. Applications only have to use this interface, but there is a ’BspSceneManager’ class which optimises the scene management for indoor levels, meaning you get both performance and an easy to learn interface. All applications have to do is hint about the kind of scene they will be creating and let OGRE choose the most appropriate implementation - this is covered in a later tutorial.

OGRE’s object-oriented nature makes all this possible. Currently OGRE runs on Windows, Linux and Mac OSX using plugins to drive the underlying rendering API (currently Direct3D or OpenGL). Applications use OGRE at the abstract level, thus ensuring that they automatically operate on all platforms and rendering subsystems that OGRE provides without any need for platform or API specific code.