RenderSystem

The RenderSystem is responsible for drawing all visual entities to the SFML window. It implements a Painter's Algorithm (Z-Ordering) to ensure correct layering of game elements (Background < Game < UI).

Required Components

Entities processed by this system must have: 1. Transform: World position, rotation, and scale. 2. Sprite: Texture ID and Z-Index.

Logic & Algorithm

The system performs a full render pass every frame:

1. Sorting (Z-Ordering):

   • Copies all valid entities into a local buffer _sortedEntities.
   • Sorts them based on Sprite.zIndex to handle depth.

2. Global Scaling:

   • Calculates a dynamic scale factor based on the window size relative to a reference resolution (1920x1080).
   • Ensures the game looks correct on any screen size.

3. Drawing:

   • Retrieves the texture from the GameEngine (Render Manager).
   • Standard Sprite: Applies Position, Rotation, and Scale, then draws.
   • Scrolling Background:
     • If ScrollingBackground.repeat is true, it sets the texture to Repeated mode.
     • Expands the texture rectangle width by 3x (width * 3) to ensure the screen is always covered during scrolling.

Code reference

src/game/src/systems/RenderSystem.cpp

void RenderSystem::onUpdate(float dt) {
    _sortedEntities.assign(_entities.begin(), _entities.end());
    auto& sprites = _engine.getComponents<Sprite>();
    std::sort(_sortedEntities.begin(), _sortedEntities.end(),
        [&sprites](size_t a, size_t b) {
            if (!sprites[a] || !sprites[b]) return a < b;
            return sprites[a]->zIndex < sprites[b]->zIndex;
        });

    sf::RenderWindow& window = _engine.getWindow();
    float scale = std::min(window.getSize().x / WINDOW_WIDTH, window.getSize().y / WINDOW_HEIGHT);
    auto& backgrounds = _engine.getComponents<ScrollingBackground>();
    auto& transforms = _engine.getComponents<Transform>();

    for (size_t entity : _sortedEntities) {
        auto& trans = transforms[entity].value();
        auto& sprite = sprites[entity].value();
        auto tex = _engine.getTexture(sprite.assetId);
        if (!tex) continue;
        bool scrolling = backgrounds[entity].has_value() && backgrounds[entity]->repeat;
        sf::Sprite s(*tex);
        s.setPosition(trans.x, trans.y);
        s.setScale(trans.scale * scale, trans.scale * scale);
        s.setRotation(trans.rotation);
        if (scrolling) {
            tex->setRepeated(true);
            s.setTextureRect(sf::IntRect(0, 0, sprite.rect.width * 3, sprite.rect.height));
        } else if (sprite.rect.width > 0) {
            s.setTextureRect(sprite.rect);
        }
        sprite.globalBounds = s.getGlobalBounds();
        window.draw(s);
    }
}

Visual Layering

Layer 0 (Background)  --> Drawn First
Layer 1 (Game Entities) --> Drawn Second (Over BG)
Layer 2 (UI/HUD)      --> Drawn Last (On Top)

Usage

Automatic. Ensure entities have Sprite and Transform.