matf-rg-project

matf-rg-project is the base project for the Computer Graphics course at the Faculty of Mathematics, University of Belgrade for the school year of 2024/2025.

Setup

Linux

Debian based (Ubuntu, Debian...)

To setup the necessary libraries, run:
./setup.sh

To generate docs, run:
doxygen Doxyfile
Open the documentation file in your browser: docs/html/index.html

Engine

Main loop

class App {
public:
int run(int argc, char** argv) {
  try {
    engine_setup(argc, argv);
    app_setup();
    initialize();
    while (loop()) {
        poll_events();
        update();
        draw();
    }
    terminate();
  } catch(...) { ... }
  return on_exit();
}
};

• engine_setup - here, the engine controllers are setup
• app_setup - the function that the user of the App overrides and implements a custom setup for the App
• initialize - App should gather whatever Resources it needs and initialize its state.
• loop - App can check whether it should continue running. If the loop method returns false, the Main loop stops, and the App terminates.
• poll_events - App collects information about the events that happened at the Platform and collects user input for the upcoming frame.
• update - App updates the world state, processes physics, events, and world logic, and reacts to the user inputs.
• draw - App uses OpenGL and draws the current state of the world.
• terminate - App terminates its state
• on_exit - do a final cleanup, and return an exit code

The App class

Here is the interface of the engine/core/App.hpp class:

class App {
    public:
        int run(int argc, char **argv);
        virtual ~App() = default;
    private:
        void engine_setup(int argc, char **argv);
        void initialize();
        void poll_events();
        bool loop();
        void update();
        void draw();
        void terminate();
        virtual void app_setup() { // the user extends and implements setup }
        virtual int on_exit() { return 0; }
        virtual void handle_error(const Error &);
    };

How ...

How to set up a basic app?

For a basic app setup, you need to:

1. Create a class for your App, let's call it MyApp, in the app/src/.
2. Inherit from the engine::core::App and implement app_setup().
3. Instantiate MyApp object in the main function and call run on it.
4. Compile and run the program.

#include <engine/Engne.hpp>
class MyApp : public engine::core::App {
 
public:
  void setup() override;
};
 
void MyApp::user_setup() {
    spdlog::info("Hello, setup!");
}
 
int main(int argc, char** argv) {
    return std::make_unique<MyApp>()->run(argc, argv);
}

How to add a custom controller?

Controllers are a way to hook into the engine execution. To create a custom controller:

1. Create a custom controller class that extends the engine::core::Controller
2. Implement for the phase (initialize, loop, poll_events, update, begin_draw, draw, end_draw, terminate) for which you want to execute custom code.
3. Register the controller in the MainApp::user_setup.

Here is the example of creating the MainController that enables depth testing.

class MainController : public engine::core::Controller {
protected:
    void initialize() override {
        engine::graphics::OpenGL::enable_depth_testing();
    }
};
class MainApp final : public engine::core::App {
protected:
    void app_setup() override {
        auto main_controller = engine::controller::register_controller<MainController>();
        /* Make sure that the main_controller executes after all the engine controllers. */
        main_controller->after(engine::core::Controller::get<engine::core::EngineControllersEnd>());
    }
};

How does the engine manage resources?

Resources currently include: textures, shaders, models, skyboxes. The ResourcesController manages the loading, storing, and accessing the resource objects. During the App::initialize, the ResourcesController will load all the resources in the resources directory.

For every type of resource, the ResourcesController has a corresponding function that retrieves it:

• Model* ResourcesController::model("backpack")
• Shader* ResourcesController::shader("basic")
• Texture* ResourcesController::texture("awesomeface")
• Skybox* ResourcesController::skybox("skybox")

The argument is always the resource name without the file extension. For textures and shaders, the name is just the name of the file without the extension. For models and skyboxes, it's the name of the directory, because they have multiple files associated with them.

The pointer to the resource that the ResourcesController returns is a non-owning pointer, meaning you should never call delete on it. All the memory is managed internally by the ResourcesController.

How to add a model?

The resources/models/ directory stores all the models. Let's add a backpack model from the course.

1. Create a directory, resources/models/backpack.
2. Copy the backpack model files into the resources/models/backpack.
3. Add a configuration for your model in the config.json

"resources": {
   "models": {
     "backpack": { # <--- This will be the name of the model you use in the app
       "path": "backpack/backpack.obj", # <---- Relative path to the .obj file
       "flip_uvs": false # <---- whether the loader should flip the texture coordinates
     }
   }
 }

1. The ResourcesController will automatically load this model during ResourcesController::initialize(); you should see a log:

...
[2024-12-08 11:19:12.347] [info] load_model(backpack, resources/models/backpack/backpack.obj)
[2024-12-08 11:19:13.272] [info] Loading texture: resources/models/backpack/diffuse.jpg
[2024-12-08 11:19:14.114] [info] Loading texture: resources/models/backpack/specular.jpg
[2024-12-08 11:19:14.479] [info] Loading texture: resources/models/backpack/normal.png
...

1. Use the model in your App:

Model* backpack = engine::core::Controller::get<engine::resources::ResourcesController>()->model("backpack");
Shader* shader   = ... 
backpack->draw(shader);

How to add a texture?

1. Add a texture file awesomeface.png to the resources/textures directory
2. Use it in the App (ResourcesController will automatically load it)

Texture* texture = engine::core::Controller::get<ResourcesController>()->texture("awesomeface");

How to add a shader?

1. Create a your_shader.glsl in the resources/shaders/your_shader.glsl.
2. Implement vertex, fragment, and geometry (optional), shaders in the same file.
3. Use it in the App:

Shader* shader = engine::core::Controller::get<ResourcesController>()->shader("your_shader");
Model* backpack = ...;
backpack->draw(shader);

Vertex, fragment, and geometry shaders are written in the same file. Use the // #shader vertex|fragment|geometry to declare the start of the shader. Here is an example:

//#shader vertex
#version 330 core
layout (location = 0) in vec3 aPos;
 
void main() {
    gl_Position = vec4(aPos, 1.0);
}
 
//#shader fragment
#version 330 core
 
out vec4 FragColor;
 
void main() {
    FragColor = vec4(0.0, 0.0, 0.0, 1.0);
}

ResourcesController will load and compile all the shaders in the resources/shaders directory.

How to draw a GUI?

Engine uses the imgui library to draw a GUI. See the library page for more examples. For GUI to be visible it should be drawn last, after all the world objects. Here is an example of displaying camera info in a GUI.

auto graphics = engine::core::Controller::get<engine::graphics::GraphicsController>();
graphics->begin_gui();
// Draw Camera Info window
{
    ImGui::Begin("Camera info");
    const auto &c = *camera;
    ImGui::Text("Camera position: (%f, %f, %f)", c.Position.x, c.Position.y, c.Position.z);
    ImGui::Text("(Yaw, Pitch): (%f, %f)", c.Yaw, c.Pitch);
    ImGui::Text("Camera front: (%f, %f, %f)", c.Front.x, c.Front.y, c.Front.z);
    ImGui::End();
}
graphics->end_gui();

How to throw and handle errors?

The Engine defines a base Error type with two subclasses, EngineError and UserError. They serve as a graceful way to terminate the application and provide the user with some helpful information on how to fix the error.

For example, the ResourcesController will throw AssetLoadingError if it can't read the asset file.

if (!scene || scene->mFlags & AI_SCENE_FLAGS_INCOMPLETE || !scene->mRootNode) {
    throw util::EngineError(util::EngineError::Type::AssetLoadingError,
                            std::format("Assimp error while reading model: {} from path {}.",
                                        model_path.string(), name));
}

Exceptions shouldn't be used as a control-flow mechanism, instead they should be used to inform the user of an exceptional event that the program can't do anything about, like the missing asset file.

How to get the key/mouse press event?

The PlatformController differentiates between four types of button/key states:

1. engine::platform::Key::State::JustPressed -> Only in the first frame when the button was registered as pressed
2. engine::platform::Key::State::Pressed -> Every subsequent frame if the button is still held
3. engine::platform::Key::State::JustReleased -> Just the first frame in which the button was registered as released
4. engine::platform::Key::State::Released -> Button is not pressed

Use the PlatformController::key method to get the state of the key in a given frame:

Here is an example of turning GUI on/off drawing using the F2 key:

void poll_events() override {
    const auto platform = engine::core::Controller::get<engine::platform::PlatformController>();
    if (platform->key(engine::platform::KeyId::KEY_F2).state() == engine::platform::Key::State::JustPressed) {
        m_draw_gui = !m_draw_gui;
    }
}

Keys have a unique identifier: via engine::platform::KeyId.

How to register a callback for platform events?

1. Implement the event observer by extending the class engine::platform::PlatformEventObserver, and override methods you'd like to have a custom operation executed once the event happens
2. Register the observer instance in the PlatformController.

class MainPlatformEventObserver final : public engine::platform::PlatformEventObserver {
public:
    void on_keyboard(engine::platform::Key key) override {
        spdlog::info("Keyboard event: key={}, state={}", key.name(), key.state_str());
    }
};
class MainController : public engine::core::Controller {
protected:
    void initialize() override {
        // ... 
        auto observer = std::make_unique<MainPlatformEventObserver>();
        engine::core::Controller::get<engine::platform::PlatformController>()->register_platform_event_observer(std::move(observer));
    }
    // ...
}

Now, for every keyboard event, the PlatformController will call MainPlatformEventObserver::on_keyboard and pass the key on which the event occurred as an argument.

How to get Window properties?

PlatformController initializes and stores the Window handle, which you can access via:

auto platform = engine::core::Controller::get<engine::platform::PlatformController>();
platform->window()->height()
platform->window()->width()
platform->window()->title()

Also, the PlatformController will update the window properties if the size of the window changes.

How to add new OpenGL calls?

Rendering actions that require more than one OpenGL call should be abstracted in the engine::graphics::OpenGL class. For example, OpenGL::compile_shaer compiles a GLSL shader and returns the shader_id.

uint32_t OpenGL::compile_shader(const std::string &shader_source,
                                resources::ShaderType shader_type) {
    uint32_t shader_id             = CHECKED_GL_CALL(glCreateShader, shader_type_to_opengl_type(shader_type));
    const char *shader_source_cstr = shader_source.c_str();
    CHECKED_GL_CALL(glShaderSource, shader_id, 1, &shader_source_cstr, nullptr);
    CHECKED_GL_CALL(glCompileShader, shader_id);
    return shader_id;
}

If the OpenGL call fails, the CHECKED_GL_CALL macro throws an OpenGLError in DEBUG mode. The engine will print the error description of and the source location in which it occurred.

Why this way? It's less error-prone and more straightforward to add debugging assertions and error checks if needed.

How do you add a configuration option?

You can configure some parts of the engine in the config.json. For example, we can configure the size and the title of the window when the application starts.

{
  "window": {
    "height": 600,
    "title": "Hello, window!",
    "width": 800
  }
}

You can also add your custom configuration options:

{
  "exposure": 2.45,
  "on_exit_message": "Bye..."
}

and use them:

auto &config = util::Configuration::config();
float exposure = config["exposure"].get<float>();
std::string gamma = config["on_exit_message"].get<std::string>();

The Configuration class uses nlohmann/json library to load and store json files, see the library page for documentation.

How to pass a command line argument?

You use the ArgParser to parse the command line arguments anywhere from the program. For example, for the invocation command: ./matf-rg-engine ... --fps 120, the parser->arg("--fps") will return the value 60. If the argument is not present, it will return the default value passed as the second argument to the arg method.

void setup() override {
    auto parser = engine::util::ArgParser()->instance();
    auto fps = parser->arg<int>("--fps", 60);
}

Tutorials

App test tutorial

Here you can find a walkthrough tutorial for recreating the engine/test/app that demonstrates how to use different engine systems.