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Brian Kirkpatrick
Brian Kirkpatrick

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Old School Is New Again: SDL2, Grad School Engines, and PacMan Clones

It made me feel really old, but I came across a CD the other day that had a backup of some projects from college and grad school. This included a lot of stuff, from CFD rocket engine simulations to a random game I wrote for job application portfolios.

But the real interesting, and timely, part of these projects were the mini-tour-de-force they represented through end-to-end game programming at the C++ level. There's a lot that's changed over the years (this was a little over a decade ago), but also a lot of valuable lessons learned from something that, even today, still builds and runs. So, pull up a chair, and join me in our high-level tour through self-contained C++ game development concepts!

With that in mind, this isn't a tutorial. You can follow along, crack your own, or even follow up on a potential series I might write as I go about refactoring the old code. But this article here, it's reminiscent. Like old leather and a fine wine. Even if the leather has a boatload of tears and the wine has long since corked.

The Bones

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You've probably heard that "real programmers" working on game development have to learn something in the C family--whether it's old school ANSI C, C++, Objective-C, C#, or others. Frankly, this is a load of male cow fecal matter. With today's software tools and hardware technology, there are AMAZING things you can do in high-performance, photo-realistic web pages. 3d in JavaScript! Even down to the GLSL shader level on the GPU! It's amazing.

But that doesn't mean thinking about (say) the C++ approach is a bad idea. Let's say you've learned a modest amount of C++ and are now thinking you want to write a game. Where to begin? There are a lot of engines (Gadot, Unity, and Unreal, to name a very small few) to choose from. But let's say we want to ignore the abstractions and role something ourselves. Where would we begin?

Cross platform is good. Write something for Windows 10, and maybe with the right approach it can also run on Windows 7, Linux, and even Mac OS. The most platform dependent part of this problem is solved by a robust window manager, which gives you the tools to get up and running with a bare-bones graphical context around which you can develop the rest of the rest of your code through the platform-agnostic OpenGL API.

Sure, there are a few other platform abstraction benefits. Handling input devices, managing sound mixers, loading fonts and images, defining reusable UI elements... But at the end of the day, the primary purpose of the cross-platform library you write on is to abstract window management so you can get to the starting line.

ArtemisLib

I'm a big fan of SDL, the Simple DirectMedia Layer. It's straightforward, modular (with a nice ecosystem of plugin-able libraries that remain well-maintained), and gives you a fast way to get to doing OpenGL. There are other options out there--explore them!--but since this is my article, we'll be building our bones on top of SDL.

Speaking of which, we should talk about LazyFoo. He's put together a fantastic set of tutorials on a variety of topics, including SDL and OpenGL. If there's one place you should start, I'd recommend part 51 of his SDL series. Get this up and running, and you're well on your way to first base.

Some Flesh

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It's time to look at our library. Using SDL and associated libraries as a starting point, take a gander at the different elements in the following GitHub project:

Tythos/ArtemisLib

It was fun to try and reverse-engineer some of my rudimentary design decisions. You can tell what pieces of this came from simply reusing basic "scaffold" code, what came from things I found myself rewriting over and over again, and what things came from some weird abstracted vision for what and how an engine should come together for future applications.

Probably the easiest way to sort through this mess is, if you open the MSVC solution in the "msvc/" folder, to start with the "TestDriver" project that uses a single "main.cpp" to build a basic application to test the library. That means looking at "int main()" to see how an application is set up, populated, and executed.

// Set up app
hApp = new aApp();
hApp->hGraphics->setScreen(600, 900, 32);
hApp->debugging = true;
hApp->externalRender = testDraw;
hApp->hGraphics->hTypewriter->setFont("arial.ttf");
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So far, pretty straightforward. There are a few things I'd change here, but it makes sense. There's a core engine object, that includes a graphics subsystem and debugging flags. In this case, we point the application to an "external renderer", which is just a function pointer for doing rendering outside the engine code. This is probably the most glaring point to start generalizing if I rewrite this; I'm a big fan of the THREE.js rendering pipeline, which looks something like this:

THREE.js Rendering Pipeline

Finally, we tell the type-rendering subsystem (basically a wrapper around SDL_ttf, which is basically a wrapper around freetype) we want to default to a specific font, for things like console rendering.

// Set up cameraa
hApp->camera->setRight(5.0f, 0.0f, -1.0f);
hApp->camera->setNearClip(0.1f);
hApp->camera->setFarClip(20.0f);
hApp->camera->setSkyboxTexture("playabox.png");
hApp->camera->setSkyboxResolution(1024);
hApp->debug("Finished setting up camera");
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Not much to see here, just basic camera properties. There's only one camera at this stage, and it's hard-coded into the engine model. Another place for improvement. Also worth noting we specifically set up the skybox as a property of the camera. Interesting. Let's move on to something else.

Population

The Event Manager Will See You Now

// Set up events
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*quitTrig), (*quitTarg));
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*fullscreenTrig), (*fullscreenTarg));
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*rotateRightTrig), (*rotateRightTarg));
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*rotateLeftTrig), (*rotateLeftTarg));
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*rotateUpTrig), (*rotateUpTarg));
hApp->gameEvents->createElement(ASTATE_GLOBAL, (*rotateDownTrig), (*rotateDownTarg));
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Ooooh, so here's where things get interesting. One thing I've learned over the years is, a specific application isn't really defined by its software components, the libraries it uses, or even the assets that it draws from. What makes an application unique is, how are all of those pieces put together?

And usually that comes down to events. When X occurs, Y should happen. There are much better ways to handle this (pub/sub and other message-passing systems are IDEAL here). But, for this project, I went with a combination of several "event" models:

  • First, there is a single enumeration that specifies under what application state the event should be considered. Is this only an event for glue menus? Is is something general to gameplay? Only during a specific mode of action? In this case, all of these events should always be considered (the "ASTATE_GLOBAL" enumeration value). Not sure I ever wrote anything complicated enough to do more than that.

  • Second, a function pointer to a "condition" evaluation. If this function returns false, the event isn't triggered. If it returns true, it is. There are a lot of problems with just juggling a set of global function pointers! But for this scale of application, apparently it worked well enough.

  • Finally, a function pointer to a "trigger" behavior that is only invoked when the "condition" function pointer returns true. This is obviously a more involved and expensive behavior, that usually involves some kind of change to application state. These handlers don't have a functional interface (e.g., no parameters), but since these function pointers are defined and evaluated in the global namespace they still have access to the global engine singleton hApp, from which everything else can be queried.

// Test light
aLight * testLight = new aLight();
testLight->setAmbient(0.9f, 0.1f, 0.1f);
testLight->setDiffuse(0.1f, 0.1f, 0.9f);
testLight->setPosition(2.0f, 2.0f, 2.0f);
testLight->setAttenuation(0.1f);
testLight->setVisible(true);
testLight->enable();
hApp->addLight(testLight);
hApp->setGlobalAmbient(0.5f, 0.5f, 0.5f);
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Lighting, and light management, is... kind of a pain in OpenGL? The state-based rendering assumes there are a fixed index of light sources, and unless you are writing shader code (which I hadn't learned yet on this project), you're stuck with the basic lighting model (and good luck balancing that out with material mapping). So, there's a basic "aLight" class that wraps some of that behavior, but it still has to be handled as a "special" object (much like the camera) in the scene by the engine singleton.

// Test cube
hCube = new aCube();
hCube->setSize(0.5f);
hCube->setAngularVelocity(0.8f, 0.4f, 0.2f, 0.1f);
hCube->setAmbient(0.5f, 0.5f, 0.5f);
hCube->setDiffuse(0.7f, 0.7f, 0.7f);
hApp->addObject(hCube);
hApp->debug("First cube initialized");

// Test planet
earth = new aPlanet(1.0f, 2048);
earth->setPosition(1.0f, 1.0f, -1.0f);
earth->setRotation(0.1f, 0.1f, 0.9f, 0.0f);
earth->setAngularVelocity(0.9f, 0.0f, 1.0f, 0.0f);
earth->setAmbient(0.7f, 0.7f, 0.7f);
earth->setDiffuse(0.9f, 0.9f, 0.9f);
hApp->addObject(earth);
hApp->debug("Earth initialized");

// Load skybox
skyboxTex = new aTexture();
skyboxTex->loadFromFile("background.png");
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_S, GL_CLAMP);
glTexParameteri(GL_TEXTURE_2D, GL_TEXTURE_WRAP_T, GL_CLAMP);
skyboxMesh = new aMesh();
skyboxMesh->loadSphere(1.5f, 32);
//skyboxMesh->setColor(0.1f, 0.2f, 0.4f, 1.0f);
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This actually isn't very interesting. Just some scene graph population. A lot of this is being done manually when it should be handled by something like a geometry and material model--again, like in THREE.js, so it can be separately instantiated and resource-managed for better performance. Some sort of color model wouldn't be a bad idea so we're not throwing floats all of the place. And the kinematics... sigh. More recent stabs at this problem have led me to conclude that it's worthwhile using something like the excellent, header-only "glm" library for algebraic constructs and parameters. Just looking at the above is a pain. And why did I require texture parameters to be specified manually when creating a skybox? Who knows.

herp derp

Finally

// Run base
hApp->debug("The app is being executed...");
hApp->execute();
hApp->debug("The app has being executed.");

// Clean up and quit
hApp->debug("Finished; shutting down...");
hApp->terminate();
return 0;
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So, once everything is set up, the only thing left to do is launch the main loop (self-contained, now that we've defined all of the application logic), and then clean up when everything's finished.

Not bad, really. There's a lot of rough edges. But it's a strong enough foundation that, while it surprises me today, I was able to do a few projects here and there with the same library. Let's look at one of them.

PacMan: Vengeance

Game Design

After grad school, I was job-hunting in the Los Angeles area and put my resume in with (at the time) a small game development company (maybe 25-30 people at the time) called "Riot Games". They were a couple of years into a building a MOBA you may have heard of, and it was starting to gain some traction. I needed a basic, self-contained game to fill out my portfolio, so I started from this engine and put together "PacMan: Vengeance" from the following idea:

  • I wanted to flip the "PacMan" paradigm around, and have players control the ghosts. They'd be trying to hunt down PacMan, the evil invader, with the usual PacMan mechanics (dot-eating, etc.).

  • You would start with one ghost, in a small maze. Every few levels, you would get another ghost, and you could switch between them to try and "trap" PacMan as the mazes grew in size.

  • Each ghost would have a different special ability--sprint, teleportation, etc. You could "level" up one ability each time you finished a round. The speed bonus from "sprint" would increase, etc.

Mazes were randomly generated, and the graphics were purely 2d. But, as it turns out, this engine was enough to put it together. So, take a gander at the source code over at the following URL and let's go over some of the more interesting tidbits:

Tythos/Vengeance

New App, New Models

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Probably the first thing you'll notice is that we aren't just dealing with a single "main.cpp" file anymore. There's a few new models:

  • vSprite defines a single texture-based sprite with animation support, so you can populate multiple sprite types (different ghosts, etc.) from a single atlas

  • vActor extends vSprite with activity states and logic, including some basic AI states and pathfinding, as well as "ability" handling for the ghosts

  • vItem defines a specific powerup--small dots, large dots, fruit, etc.--randomly placed within the maze, again extending the vSprite class

  • vMaze defines the maze itself. This includes random generation logic, layout of square structures and pathfinding, etc.

I won't dive too much into these models. As mentioned before, the really interesting thing is how they come together. So, let's wander back over to "main.cpp".

The Main Thing

Game Dev

We have a number of pieces, now, largely texture-based but with good enough encapsulation to see how they can all be put together into a single game using this approach. Aside from "events.cpp"--which is just a "library" or collection of function pointers that we'll use to construct game logic--that means looking at "int main()". So, let's dive in.

// Initialize game
game = new aApp();
...
game->hConsole->setVisible(false);
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So far, pretty straight forward. We do a little more detailed setup of the in-app console here, since the debugging requirements get a lot bigger when you have multi-state game logic to verify.

// Add events
game->gameEvents->createElement(ASTATE_GLOBAL, (*quitTrig), (*quitTarg));
...
game->gameEvents->createElement(ASTATE_GLOBAL, (*levelClydeTrigger), (*levelClydeAction));
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This is pretty much what you expect. Notice that I never end up trying to organize events by application state--they're still all "ASTATE_GLOBAL"! Guess I jumped the gun a little bit there. Nonetheless, we're pretty clearly at the limit of what this global-function-pointer approach lets you do. Anything more complicated--a single additional layer of game state--and we'd need to start multiplying these handlers by a factor of 2, 3, or more. This is probably the biggest reason development down this road stopped and I moved onto other engine technologies! Robust pub/sub is really necessary, even plugging an interpreter (Lua is ideal) in for scripted game logic, to scale to something production-ready.

// Preload music and turn looping off
mus1 = game->hSoundboard->loadSong("..\\resources\\Start.mp3");
...
snd8 = game->hSoundboard->loadSound("..\\resources\\Laser.wav");
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Hey, we have music! And sound effects! Thanks, SDL2_Mixer! Some of the dependencies are no longer maintained, so in my dust-off, I refactored to use the "mpg123" library (licensing warnings here), but your mileage may vary. Still, nice to have something for your ears. The "soundboard" property is used to load and play audio resources and can be triggered by specific event handlers, since it just lives under the global engine singleton.

// Load maze
maze = new vMaze();
...
delete game;
return 0;
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From here on out, the rest is pretty straightforward:

  • We define a new maze and initialize it for a new level (our maze instance is a global singleton, too, since there's no real scene graph implemented in 2d here and it's worth blurring the lines on context for graphical elements at this level)
  • We add a text-based sprite that shows different "tips" at different points in the game
  • We point the engine to external rendering logic
  • We set the initial state, from "level start", "play", "victory", and other possible enumeration values. This lets us control behaviors for, and trigger behaviors off of, state changes.
  • We launch the main loop, and clean up when everything's finished.

Rendering

Lastly, let's take a look at the "external" rendering logic, "bool extRender()":

float dt = 0.01f;
if (currState == VS_LEVELING) {
    renderLeveling(game->hGraphics);
} else {
    maze->renderMaze(game->hGraphics);
}
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We do rendering based off of application state, which is useful, because it lets us define a different "rendering mode" for different screens. In this case, we have a separate "leveling" screen where users choose to level-up specific ghost skills. In both cases, the basic functional interface hands off a pointer to the graphical subsystem as the only parameter; everything else is accessed through the global engine singleton.

renderInterface();
maze->update(dt);
extUpdate(dt);
return true;
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We have a separate call to another global rendering routine. This includes "typing" out different ghost states/selections, tips, and status messages. These are collectively organized under the name "interface", but this also includes some hooks into logic for ghost selection, etc. We then handle update logic, which includes keypress events and sprite movement.

Conclusion

That's actually it. I was very surprised everything built, but it still runs! Who would have thought. So, time for some "lessons-learned":

  • SDL is a solid choice. Decent way to get cross-platform support at a low level. Not having to worry about window management, in particular, is well-worth the pain of initial configuration.

  • There's no reusable 2d interface code here. We don't use Imgui, for example, or anything else that would give us buttons, UI frames, etc. This is a big weakness, even though we have the pieces to put together our own reusable classes to fill the gap. This is also one reason, since around this time WebGL starting being a thing, I started looking at other technologies where I would end up getting a lot of these things for "free" in a way that was still cross-platform.

  • We've beaten the dead horse of event management enough. Another thing I'll observe is, how much better this could work if refactored for something like an ECS-style engine where you get a real separation of concerns that can help with multi-threaded performance, update subsystem isolation, and asynchronous state management.

  • This really isn't 3d, obviously, and in fact there's no real reason to include OpenGL at all. Straight SDL surface management, for sprites and other entities, would be perfectly fine. After this engine, I would go on to do a lot more with GLSL (which was just becoming a thing around this time) but would never revisit to incorporate these technologies into this particular engine.

  • There's a lot of other non-SDL libraries that could help round out things like random number generation and similar mathematical and programming behaviors. Boost is a big one, of course, and I've already mentioned GLM. There's also a number of "new" C++ language features (and we don't even touch the STL!) that might, or might not, be useful to organize a lot of this event management logic.

That's about it. Thanks for taking a trip through the past with me. There's some great lessons-learned in here, and I suspect it would be a useful resource for those trying to understand a lower level of C++ game and application development. Don't hesitate to poke around and try it out yourself! Even hobbies are useful.

And no, I didn't get that job with Riot. ;)

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