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Maverick Programmer

OpenGL What would you look for in a new game engine?

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I've been working on a game engine for OpenGL (in C++) for sometime now and I'm almost at a point to where I can release it for testing ( to see how people like it ) soon. It was designed for simplicity and able to make not only 3D, but 2D games and not a load of syntax hell like this: myGameEngine* pGame; pGame->GetWindowParameters() = myGameEngine::WindowProto()->GetWindow(&data); pGame->SetFrameBuff(data); You know. Stuff like that. Things like those I have seen in many engines and they make you do this for every component such as sound, graphics, 3d objects. I'm sure it scares some of you. I've designed this engine to do this: mySprite* pSprite; loadSprite( pSprite, "happy_face_sprite", BITMAP); setSpritei( pSprite, TOTAL_NUMBER_OF_FRAMES, 40 ); setSpritei( pSprite, ANIMATE, true ); Somewhat to that. This engine has components/objects for 2D or 3D sound ( WAVs and OGGs so far ), 2D or 3D fonts, 2D or 2.5D sprites, model loading for a variety of sprites, easy camera usage and so forth. It also has built in things that people get tired of doing in OpenGL: frustum culling, input, framework. Well, this does it all for you. SO, before I shamelessly this off, I'll ask you this question: What else would you like to see in an OpenGL game engine? Thank you for your time! ~Maverick

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Quote:
Original post by PCN
I've designed this engine to do this:

mySprite* pSprite;
loadSprite( pSprite, "happy_face_sprite", BITMAP);
setSpritei( pSprite, TOTAL_NUMBER_OF_FRAMES, 40 );
setSpritei( pSprite, ANIMATE, true );


// Drop the 'my' prefix and use a namespace instead
// Also, what's the BITMAP for?
myName::Sprite sprite("happy_face_sprite");

// Don't you think this is much more natural?
sprite.setNumFrames(40);
sprite.animate(true);


Also, it might be better not to expose functions like setNumFrames() and instead have the sprite read data from a file and setup it's internal state accordingly.

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Quote:
Original post by Gage64
Quote:
Original post by PCN
loadSprite( pSprite, "happy_face_sprite", BITMAP);

// Also, what's the BITMAP for?
myName::Sprite sprite("happy_face_sprite");


I think he planned to used a BITMAP constant as opposed to appending ".bmp" to the filename string.

Having the sprite class both load and animate though? Hmmm...
Edit: Actually the OP hasn't really used a class - it looks rather C-like for a C++ library.

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Quote:
Original post by dmatter
Having the sprite class both load and animate though? Hmmm...


I would certainly move the animation functionality to another class, but I'm not sure about the loading part. How would you handle it?

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Quote:
Original post by Gage64
I'm not sure about the loading part. How would you handle it?

I would say that a sprite is a container for images; an image is, in turn, a container for pixel data.

Thus:
A sprite is constructible from a set of one or many images.
An image is constructible from a range of pixel data.
This data comes from a bmp/jpeg/etc loading function, reading from a stream.

The pixel data would probably be represented as a std::vector and so, when passed to the image constructor, I suppose it might use std::vector::swap to efficiently gain ownership of that data, rather than copy it all across - of course the loader needs to be aware that their vector is then empty.

Alternatively, you can construct an empty image and use its accessors/mutators to load in the pixel data directly; this is often the case when the loading function is the overloaded << (stream) operator.

Either way, the loading functionality is kept separate from the image class.


You could wrap it all up in a convenience function to make common use cases more friendly for the client:

Sprite loadSpriteFromBitmap(const std::string & filename)
{
return Sprite(loadBitmapImage(std::ifstream(filename)));
}

Sprite mySprite = loadSpriteFromBitmap("picture.bmp");

Return value optimisation makes this far cheaper than it might first appear.

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When the sprite class is loaded, it also constructs the sprite and retrieves the filename from the load function and then send that to a function the game engine has built-in and returns the texture ID to that sprite.

Anyway this wasn't about how I load sprites. Also, those aren't how the real functions look ( or class names ), those were examples of how they look. I designed them to look like setSpritei( object, parmeter, value ) so it would be familiar with OpenGL users. It was based off of the OpenGL lighting design:

glLightf( light object 0-7, param, value ) //OpenGL lighting function
glennSpritef( sprite object, param, value ) //Engine spriting function

So, back on topic please?

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Quote:
Original post by PCN
Also, those aren't how the real functions look ( or class names ), those were examples of how they look. I designed them to look like setSpritei( object, parmeter, value ) so it would be familiar with OpenGL users.

Personally, I think you're targetting the wrong demographic, let me explain:

A library that abstracts boilerplate OpenGL code and targets OpenGL users is somewhat silly - In order for users to even qualify as your target audience they must have already written that boiletplate code themselves anyway - What reason would they have to switch to your library?

You might argue that perhaps they didn't make a very good job at it themselves and so have turned to you, in which case they're probably not all too familiar with OpenGL anyway and so won't benefit from you mimicking OpenGL's API.

For all other users, they're modern C++, non-OpenGL, programmers who want to make graphics. They would expect a C++ library to employ idiomatic C++ and not adopt an old C-style API model.

In short:
  • Good OpenGL users won't need/want your library.
  • Bad OpenGL users won't benefit from the awkward API (and it is awkward: more key strokes, less auto-complete intellisense, harder to mentally parse).
  • Non OpenGL (C++) users won't like your API model.

Of course, I shouldn't forget that you've added in extra niceties, like sounds and fonts. They do provide existing OpenGL users benefits and as such it would be ideal if they were separate modules that users can link to without having to include everything else.

On topic, my suggetion for what I'd like to see in your OpenGL game engine, is to make it more C++ friendly which, by implication, translates to being more user friendly and more useful overall. With everything nicely modularised. [smile]

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I see what you are saying, but is it all that bad? Also you have the ability to disable/turn off every component that uses other libraries you may not have. For example, the sounds use OpenAL and you need the correct headers and libraries to use them. If you don't want to use the sound objects, define a macro like so:

#define _GLENN_DISABLE_OPENAL 1

If you want to disable the built in main loop with your own routines to the frame work, then do so:

#define _GLENN_DISABLE_FRAMEWORK 1

and then define the drawing/updating routines elsewhere.

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Having #define macros misses the overall point. An engine is really just a series of libraries that gives the functionality to create a game. If you don't need the functionality the library provides then you just don't link it. Since you've done some OpenAL work which is orders of magnitude simpler than OpenGL

With OpenAL you have essentially 3 constructs.
-- Listener - The point at which the sound is being heard from
-- Source - The point at which a sound is being emitted from
-- Buffer - The data used to reproduce the sound.

*NOTE* If you start delving into EFX there are some more options such as filters and effects but to keep it simple I'll stick with the core of OpenAL

So for creating a low-level C++ style wrapping of OpenAL you would have the following classes


// Listener.hpp

class Listener
{
public:
Listener();

// Some potential methods
void setPosition(const vector3& position);
void setVelocity(const vector3& velocity);

// Add any other methods needed

protected:
vector3 position;
vector3 velocity;
} ;

// Sound.hpp

class Sound // Conceptually a buffer in OpenAL terms
{
public:
Sound();

// Methods to set the buffer
} ;

// Source.hpp

class Source
{
public:
Source();

// Methods to attach buffers
void attachSound(Sound* sound);

// Any other needed methods
}



As an example of thinking in C++ we can have a static sound, maybe a gun sound stored in a wav file, and we can have streaming sounds, such as a music file. So maybe its better to approach the sound class as follows


// Sound.hpp

class Sound
{
protected:
Sound();

public:
// Any other needed methods
} ;

// SoundStreaming.hpp

class SoundStreaming : public Sound
{
public:
SoundStreaming();
} ;

// SoundStatic.hpp

class SoundStatic : public Sound
{
public:
SoundStatic();
} ;



With that when a sound is attached to a source it won't need to know all the necessary details on how to operate on a streaming source vs a static source.

These are all off the top of my head examples of what a low-level library for OpenAL would attempt to accomplish so there are other things that should be in there. But it gets at the jist of how to approach the problem.

One of these days I gotta finish another article for this site.

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PCN what ild like to see seriously (+ i am)
is a clone of galaga. or better mr do.

bugger your engine give me a game, look at the cryengine (perhaps the top engine ATM, written by heaps of fulltime ppl),
hands up whos using it, ...silence...

+ u expect ppl to give a monkeys about your engine, give us a game, I will be happier, harry will be happier + most important U will be happier, u will have something to show your folks, now with an engine how does that work mom have a play with my engine? ... hmmmm

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Quote:
Original post by PCN
I've been working on a game engine for OpenGL (in C++) for sometime now and I'm almost at a point to where I can release it for testing

[...]

SO, before I shamelessly this off, I'll ask you this question:
What else would you like to see in an OpenGL game engine?


The most important point would be to make it well-integrating with the C++ language (which isn't the case for OpenGL itself). Key points would be compatibility with the Standard C++ Library (all objects can be stored in the containers, any sequences provide iterator-based signatures...) and usage of C++ idioms (RAII...).

Because, if the point is to create yet another C library that can be used from C++, then there's not a lot of synergy to be gained from C++ itself...

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