A few things

Make child entites work.

z-sort entity list before rendering.

entites have a creation timestamp.

CMakeLists.txt

@@ -22,7 +22,7 @@ CPMAddPackage(
         URL https://git.redacted.cc/Redacted/ReWindow/archive/Prerelease-26.zip
 )
 
-set(CMAKE_CXX_FLAGS "-O3 -Wall -Wextra")
+#set(CMAKE_CXX_FLAGS "-O3 -Wall -Wextra")
 
 file(GLOB_RECURSE HEADERS "include/*.h" "include/*.hpp")
 file(GLOB_RECURSE SOURCES "src/*.c" "src/*.cpp")

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;

include/Game/Entity/Box.h

@@ -12,5 +12,5 @@ public:
     void Render() final;
     void Update() final;
 public:
-    explicit Box(const Vector2& position, float rotation = 0.0f) : Renderable(position, rotation) {}
+    explicit Box(const Vector2& position, unsigned int depth = 0, float rotation = 0.0f) : Renderable(position, depth, rotation) {}
 };

src/Engine/Entity/Entity.cpp

@@ -59,8 +59,11 @@ Vector2 Engine::Entity::GetPosition() const {
 
 bool Engine::Entity::AppendChild(Entity* entity) {
     bool success = false;
-    if (!Globals::CurrentScene->EntityListContains(entity))
-        children.push_back(entity), success = true;
+    if (!Globals::CurrentScene->EntityListContains(entity)) {
+        entity->parent = this;
+        children.push_back(entity);
+        success = true;
+    }
     return success;
 }
 
@@ -88,3 +91,19 @@ void Engine::Entity::RemoveChild(Entity* entity) {
     if (it != children.end())
         children.erase(it);
 }
+
+long Engine::Entity::GetCreationTime() const {
+    return creation_time;
+}
+
+long Engine::Entity::GetAge() const {
+    return std::chrono::duration_cast<std::chrono::microseconds>(std::chrono::high_resolution_clock::now().time_since_epoch()).count() - creation_time;
+}
+
+Engine::Entity* Engine::Entity::GetParent() const {
+    return parent;
+}
+
+std::vector<Engine::Entity*> Engine::Entity::GetChildren() const {
+    return children;
+}

src/Engine/Level/Scene.cpp

@@ -1,5 +1,6 @@
 #include <Engine/Level/Scene.h>
 #include <Engine/Entity/Camera.h>
+#include <algorithm>
 
 bool Engine::Scene::EntityListContains(const Entity* entity) const {
     for (auto* e : entity_list)
@@ -20,7 +21,7 @@ size_t Engine::Scene::FixedCount() const {
 }
 
 size_t Engine::Scene::EntityCount() const {
-    return entity_list.size();
+    return GetFlatEntityList(entity_list).size();
 }
 
 void Engine::Scene::Update() {
@@ -33,24 +34,33 @@ void Engine::Scene::Update() {
 }
 
 void Engine::Scene::Render(RenderTarget* render_target) {
-    for (auto& f : fixed_list)
-        if (f->Enabled())
-            f->Render();
+    // The Camera and the HUD are ignored here because they're special cases.
+    std::vector<Renderable*> display_list{};
+    for (auto* e : GetFlatEntityList(entity_list))
+        if (auto* r = dynamic_cast<Renderable*>(e))
+            if (auto* c = dynamic_cast<Camera*>(r); c == nullptr)
+                if (auto* h = dynamic_cast<Hud*>(c); h == nullptr)
+                    display_list.push_back(r);
 
-    // TODO Render order. In this system it's not possible for child entities to be rendered before the parent.
+    std::sort(display_list.begin(), display_list.end(), [](Renderable* a, Renderable* b) {
+        return a->GetDepth() > b->GetDepth();
+    });
 
-    J2D::Begin(render_target, true);
+    J2D::Begin( render_target, true);
 
     if (active_camera)
         active_camera->Render();
 
-    for (auto& e : entity_list)
-        if (auto* r = dynamic_cast<Renderable*>(e))
-            if (auto* c = dynamic_cast<Camera*>(r); c == nullptr)
-                r->Render();
+    for (auto& f : fixed_list)
+        if (f->Enabled())
+            f->Render();
+
+    for (auto& r : display_list)
+        r->Render();
 
     if (heads_up_display)
         heads_up_display->Render();
+
     J2D::End();
 }
 
@@ -118,3 +128,15 @@ Engine::Camera* Engine::Scene::GetActiveCamera() const {
     return active_camera;
 }
 
+std::vector<Engine::Entity*> Engine::Scene::GetFlatEntityList(const std::vector<Entity*>& ent_list) const {
+    std::vector<Entity*> result{};
+
+    for (auto* e : ent_list) {
+        auto children = GetFlatEntityList(e->GetChildren());
+        for (auto* c : children)
+            result.push_back(c);
+        result.push_back(e);
+    }
+    return result;
+}
+

src/Game/Scene/ControllableBox.cpp

@@ -4,7 +4,7 @@
 
 void ControllableBox::Init() {
     auto* hud = new Game::DemoGameHud();
-    auto* b = new Game::Box({0, 0});
-    heads_up_display = hud;
+    auto *b = new Game::Box({0, 0});
     AppendEntity(b);
+    heads_up_display = hud;
 }