A few things

Make child entites work.

z-sort entity list before rendering.

entites have a creation timestamp.

include/Engine/Entity/Camera.h

@@ -9,5 +9,5 @@ class Engine::Camera : public Renderable {
 public:
     void Render() override {};
 public:
-    explicit Camera(const Vector2& position) : Renderable(position) {}
+    explicit Camera(const Vector2& position) : Renderable(position, 0) {}
 };

include/Engine/Entity/Entity.h

@@ -1,6 +1,7 @@
 #pragma once
 #include <J3ML/LinearAlgebra/Vector2.hpp>
 #include <vector>
+#include <chrono>
 
 namespace Engine {
     class Entity;
@@ -8,15 +9,21 @@ namespace Engine {
 
 class Engine::Entity {
 protected:
+    // nullptr means no parent.
+    Entity* parent = nullptr;
+
+    // Epoch micro-seconds.
+    long creation_time = 0.0f;
     std::vector<Entity*> children{};
     Vector2 position = {0, 0};
     // Assuming 0 radians is up.
     float face_angle = 0.0f;
     void UpdateChildren();
 public:
-    // Movements independent of the rotation.
+    // Movements independent of the face_angle.
     void MoveX(float speed);
     void MoveY(float speed);
+    void Move(float angle_rad, float speed);
 
     // Movements dependent on face angle.
     void MoveForward(float speed);
@@ -24,20 +31,29 @@ public:
     void MoveLeft(float speed);
     void MoveRight(float speed);
 
-    void Move(float angle_rad, float speed);
-
     void Rotate(float speed);
     void SetRotation(float new_rotation);
 
+    // Parent child structure.
     [[nodiscard]] bool AppendChild(Entity* entity);
     void DestroyChild(Entity* entity);
     void RemoveChild(Entity* entity);
 public:
+    [[nodiscard]] std::vector<Entity*> GetChildren() const;
+    [[nodiscard]] Entity* GetParent() const;
+
+    // Microseconds.
+    [[nodiscard]] long GetCreationTime() const;
+    [[nodiscard]] long GetAge() const;
+
     [[nodiscard]] float GetRotation() const;
     [[nodiscard]] Vector2 GetPosition() const;
+
 public:
     virtual void Update() {}
 public:
-    explicit Entity(const Vector2& position, float rotation = 0.0f) : position(position), face_angle(rotation) {}
     virtual ~Entity();
+    explicit Entity(const Vector2& position, float rotation = 0.0f) :
+    creation_time(std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count()),
+    position(position), face_angle(rotation) {}
 };

include/Engine/Entity/Hud.h

@@ -7,5 +7,5 @@ namespace Engine {
 
 class Engine::Hud : public Engine::Renderable {
 public:
-    Hud() : Renderable({0, 0}, 0) {};
+    Hud() : Renderable({0, 0}, 0, 0) {};
 };

include/Engine/Entity/Renderable.h

@@ -8,9 +8,15 @@ namespace Engine {
 }
 
 class Engine::Renderable : public Entity {
+protected:
+    // Higher numbers are farther to the back.
+    unsigned int depth = 0;
+public:
+    [[nodiscard]] unsigned int GetDepth() const { return depth; }
+    void SetDepth(unsigned int new_depth) { depth = new_depth; }
 public:
     // *must* be overridden.
     virtual void Render() = 0;
 public:
-    explicit Renderable(const Vector2& position, float rotation = 0.0f) : Entity(position, rotation) {}
+    explicit Renderable(const Vector2& position, unsigned int depth = 0, float rotation = 0.0f) : Entity(position, rotation), depth(depth) {}
 };

include/Engine/Entity/Sprite.h

@@ -25,6 +25,6 @@ public:
     void Render() override;
 public:
     ~Sprite() override;
-    explicit Sprite(const Vector2& pos, const float face_angle = 0.0f, const Color4& base_color = Colors::White, Texture* texture = nullptr,
-                    Texture* alpha_mask = nullptr) : Renderable(pos, face_angle), texture(texture), alpha_mask(alpha_mask), base_color(base_color) {}
+    explicit Sprite(const Vector2& pos, unsigned int depth = 0, const float face_angle = 0.0f, const Color4& base_color = Colors::White, Texture* texture = nullptr,
+                    Texture* alpha_mask = nullptr) : Renderable(pos, depth,face_angle), texture(texture), alpha_mask(alpha_mask), base_color(base_color) {}
 };

include/Engine/Level/Scene.h

@@ -18,6 +18,8 @@ protected:
     Camera* active_camera = nullptr;
     std::vector<Fixed*> fixed_list{};
     std::vector<Entity*> entity_list{};
+protected:
+    [[nodiscard]] std::vector<Entity*> GetFlatEntityList(const std::vector<Entity*>& entity_list) const;
 public:
     [[nodiscard]] bool EntityListContains(const Entity* entity) const;
     [[nodiscard]] bool FixedListContains(const Fixed* fixed) const;