AnimationSystem

The AnimationSystem handles the visual progression of sprite animations. It updates the texture coordinates (sf::IntRect) of entities based on elapsed time, creating the illusion of movement.

Required Components

Entities processed by this system must have the following components: 1. Animation: Contains timing, frame counts, and loop settings. 2. Sprite: Contains the visual rectangle (rect) that determines which part of the texture is displayed.

Logic & Algorithm

The system performs the following logic per entity per frame:

1. Time Accumulation:

   • Adds dt (Delta Time) to the elapsedTime accumulator of the Animation component.

2. Frame Advance:

   • If elapsedTime > frameDuration, the frame index (currentFrame) is incremented.
   • The timer is reset to 0.

3. Boundary Check (Looping):

   • Checks if currentFrame exceeds endFrame.
   • If loop is true: Resets currentFrame to startFrame.
   • If loop is false: Clamps currentFrame to endFrame (stops animating).

4. Rect Update (Sprite Sheet):

   • Calculates the new left coordinate for the texture rectangle: newLeft = currentFrame * sprite.width
   • Updates the Sprite component so the RenderSystem draws the correct image.

Code reference

src/game/src/systems/AnimationSystem.cpp

void AnimationSystem::onUpdate(float dt) {
    auto& animations = _engine.getComponents<Animation>();
    auto& sprites = _engine.getComponents<Sprite>();
    for (size_t e : _entities) {
        if (!animations[e] || !sprites[e]) continue;
        auto& anim = animations[e].value();
        auto& sprite = sprites[e].value();
        anim.elapsedTime += dt;
        if (anim.elapsedTime > anim.frameDuration) {
            anim.currentFrame++; anim.elapsedTime = 0;
        }
        if (anim.currentFrame > anim.endFrame)
            anim.currentFrame = anim.loop ? anim.startFrame : anim.endFrame;
        sprite.rect.left = anim.currentFrame * sprite.rect.width;
    }
}

Visual Representation

Assuming a sprite sheet where frames are aligned horizontally:

[Frame 0] [Frame 1] [Frame 2] ...
    ^
    |
  rect.left (shifts every X milliseconds)

Usage

Entity entity = gameEngine.createEntity("Entity");

gameEngine.addComponent<Sprite(entity, Sprite(ASSET_ID, Z_INDEX, sf::IntRect(0, 0, RECT_WIDTH, RECT_HEIGHT)));
gameEngine.addComponent<Animation>(entity, Animation(
    RECT_WIDTH,
    RECT_HEIGHT,
    CURRENT,
    ELAPSED_TIME,
    DURATION,
    START_FRAME,
    END_FRAME,
    IS_LOOP,
));