ADD: Texture cache system improvement 2

docs/GameAPI.md

@@ -26,8 +26,12 @@ Implementation: `src/core/game_api.cpp`
 - Instances and animation.
 - Post‑processing (tonemap, bloom, FXAA).
 - Camera control.
+- Lighting (directional, point, spot).
+- Volumetrics (clouds, smoke, flame).
+- Particle systems.
+- Debug drawing.
 - Picking and render‑graph pass toggles.
-- Input handling (keyboard, mouse, cursor modes).
+- Time and statistics.
 
 Typical creation:
 
@@ -49,10 +53,15 @@ Relevant methods:
 
 - `size_t get_texture_budget() const;`
 - `void set_texture_loads_per_frame(int count);`
+- `int get_texture_loads_per_frame() const;`
 - `void set_texture_upload_budget(size_t bytes);`
+- `size_t get_texture_upload_budget() const;`
 - `void set_cpu_source_budget(size_t bytes);`
+- `size_t get_cpu_source_budget() const;`
 - `void set_max_upload_dimension(uint32_t dim);`
+- `uint32_t get_max_upload_dimension() const;`
 - `void set_keep_source_bytes(bool keep);`
+- `bool get_keep_source_bytes() const;`
 - `void evict_textures_to_budget();`
 
 At a lower level, `VulkanEngine::query_texture_budget_bytes()` computes a conservative per‑frame texture budget using VMA heap info and constants in `src/core/config.h`:
@@ -63,6 +72,65 @@ At a lower level, `VulkanEngine::query_texture_budget_bytes()` computes a conser
 
 To globally change how aggressive streaming can be, edit these constants in `config.h` and rebuild. Use the `GameAPI::Engine` setters for per‑scene tuning (e.g. reducing upload bandwidth on low‑end machines).
 
+#### Texture Loading
+
+```cpp
+// Load from file (relative to assets/textures/ or absolute path)
+TextureHandle load_texture(const std::string& path, const TextureLoadParams& params = {});
+
+// Load from memory (compressed image data: PNG, JPG, KTX2, etc.)
+TextureHandle load_texture_from_memory(const std::vector<uint8_t>& data, const TextureLoadParams& params = {});
+
+// Check if texture is loaded and resident in VRAM
+bool is_texture_loaded(TextureHandle handle) const;
+
+// Get internal Vulkan image view (VkImageView) for advanced use
+void* get_texture_image_view(TextureHandle handle) const;
+
+// Pin texture to prevent automatic eviction (for UI, critical assets)
+void pin_texture(TextureHandle handle);
+void unpin_texture(TextureHandle handle);
+bool is_texture_pinned(TextureHandle handle) const;
+
+// Unload texture and free VRAM (optional - cache auto-manages)
+void unload_texture(TextureHandle handle);
+
+// Create ImGui descriptor set for use with ImGui::Image()
+void* create_imgui_texture(TextureHandle handle, void* sampler = nullptr);
+void free_imgui_texture(void* imgui_texture_id);
+```
+
+**TextureLoadParams:**
+```cpp
+struct TextureLoadParams
+{
+    bool srgb{false};                      // Use sRGB color space (true for albedo/emissive)
+    bool mipmapped{true};                  // Generate mipmap chain
+    TextureChannels channels{Auto};        // Channel hint (Auto, R, RG, RGBA)
+    uint32_t mipLevels{0};                 // 0 = full chain, otherwise limit to N levels
+};
+```
+
+**Usage Example:**
+```cpp
+GameAPI::Engine api(&engine);
+
+// Load UI texture and pin it to prevent eviction
+GameAPI::TextureLoadParams params;
+params.srgb = true;
+params.mipmapped = false;  // UI textures don't need mipmaps
+TextureHandle uiTex = api.load_texture("ui/button.png", params);
+api.pin_texture(uiTex);
+
+// Create ImGui descriptor for rendering
+void* imguiId = api.create_imgui_texture(uiTex);
+ImGui::Image(imguiId, ImVec2(128, 64));
+
+// Later: cleanup
+api.free_imgui_texture(imguiId);
+api.unpin_texture(uiTex);
+```
+
 ### Shadows: Resolution, Quality, and RT Modes
 
 Shadows are controlled by a combination of:
@@ -121,6 +189,95 @@ The following quality‑related shadow constants also live in `config.h`:
 
 These affect how cascades are distributed and how soft/filtered the resulting shadows are. Changing them is safe but should be tested against your content and FOV ranges.
 
+### IBL (Image-Based Lighting)
+
+**API:**
+```cpp
+// Load global IBL asynchronously
+bool load_global_ibl(const IBLPaths& paths);
+
+// Get/set global IBL paths (does not trigger reload)
+IBLPaths get_global_ibl_paths() const;
+void set_global_ibl_paths(const IBLPaths& paths);
+
+// Add/remove local IBL volumes
+size_t add_ibl_volume(const IBLVolume& volume);
+size_t add_ibl_volume(const IBLVolumeD& volume);  // double-precision
+bool remove_ibl_volume(size_t index);
+
+// Get/set IBL volume properties
+bool get_ibl_volume(size_t index, IBLVolume& out) const;
+bool set_ibl_volume(size_t index, const IBLVolume& volume);
+bool get_ibl_volume(size_t index, IBLVolumeD& out) const;  // double-precision
+bool set_ibl_volume(size_t index, const IBLVolumeD& volume);
+
+// Query active volume
+int get_active_ibl_volume() const;  // -1 = global
+size_t get_ibl_volume_count() const;
+void clear_ibl_volumes();
+```
+
+**Structures:**
+```cpp
+struct IBLPaths
+{
+    std::string specularCube;  // .ktx2 specular cubemap
+    std::string diffuseCube;   // .ktx2 diffuse cubemap
+    std::string brdfLut;       // .ktx2 BRDF lookup table
+    std::string background;    // .ktx2 background (optional, falls back to specular)
+};
+
+struct IBLVolume
+{
+    glm::vec3 center{0.0f};
+    glm::vec3 halfExtents{10.0f};
+    IBLPaths paths;
+    bool enabled{true};
+};
+
+struct IBLVolumeD  // double-precision variant
+{
+    glm::dvec3 center{0.0};
+    glm::vec3 halfExtents{10.0f};
+    IBLPaths paths;
+    bool enabled{true};
+};
+```
+
+**Usage Example:**
+```cpp
+GameAPI::Engine api(&engine);
+
+// Load global IBL (outdoor environment)
+GameAPI::IBLPaths globalIBL;
+globalIBL.specularCube = "ibl/outdoor_spec.ktx2";
+globalIBL.diffuseCube = "ibl/outdoor_diff.ktx2";
+globalIBL.brdfLut = "ibl/brdf_lut.ktx2";
+api.load_global_ibl(globalIBL);
+
+// Create local IBL volume for interior (overrides global when camera inside)
+GameAPI::IBLVolume interior;
+interior.center = glm::vec3(10.0f, 2.0f, -5.0f);
+interior.halfExtents = glm::vec3(5.0f, 3.0f, 5.0f);
+interior.paths.specularCube = "ibl/indoor_spec.ktx2";
+interior.paths.diffuseCube = "ibl/indoor_diff.ktx2";
+interior.paths.brdfLut = "ibl/brdf_lut.ktx2";
+interior.enabled = true;
+
+size_t idx = api.add_ibl_volume(interior);
+
+// Query which IBL is active
+int activeVol = api.get_active_ibl_volume();
+if (activeVol == -1)
+{
+    // Using global IBL
+}
+else
+{
+    // Using local volume at index activeVol
+}
+```
+
 ### Reflections and Post‑Processing
 
 Game‑side reflection controls:
@@ -150,14 +307,17 @@ FXAA:
 Camera:
 
 - `void set_camera_position(const glm::vec3 &position);`
+- `void set_camera_position(const glm::dvec3 &position);`  // double-precision
 - `glm::vec3 get_camera_position() const;`
+- `glm::dvec3 get_camera_position_d() const;`  // double-precision
 - `void set_camera_rotation(float pitchDeg, float yawDeg);`
 - `void get_camera_rotation(float &pitchDeg, float &yawDeg) const;`
 - `void set_camera_fov(float fovDegrees);`
 - `float get_camera_fov() const;`
 - `void camera_look_at(const glm::vec3 &target);`
+- `void camera_look_at(const glm::dvec3 &target);`  // double-precision
 
-These functions internally manipulate the quaternion‑based `Camera::orientation` and `position` in `SceneManager`. They respect the engine’s `-Z` forward convention.
+These functions internally manipulate the quaternion‑based `Camera::orientation` and `position` in `SceneManager`. They respect the engine's `-Z` forward convention. Double-precision variants allow precise camera positioning in large worlds.
 
 Render resolution scaling:
 
@@ -171,9 +331,27 @@ This scales the internal draw extent relative to the swapchain and main HDR imag
 Picking:
 
 - `Engine::PickResult get_last_pick() const;`
+- `Engine::PickResultD get_last_pick_d() const;`  // double-precision
 - `void set_use_id_buffer_picking(bool use);`
 - `bool get_use_id_buffer_picking() const;`
 
+**PickResult structure:**
+```cpp
+struct PickResult
+{
+    bool valid{false};
+    std::string ownerName;
+    glm::vec3 worldPosition{0.0f};
+};
+
+struct PickResultD  // double-precision variant
+{
+    bool valid{false};
+    std::string ownerName;
+    glm::dvec3 worldPosition{0.0};
+};
+```
+
 These mirror `VulkanEngine::get_last_pick()` and `_useIdBufferPicking`, letting you choose between:
 
 - CPU raycast picking (immediate, cheaper VRAM).
@@ -372,17 +550,99 @@ Header: `src/core/engine.h`
 Header: `src/scene/vk_scene.h`
 Docs: `docs/Scene.md`
 
-### Dynamic Mesh/GLTF Instances
+### Transform Structures
 
-- Mesh instances
+The GameAPI provides both single and double-precision transform representations:
-  - `void addMeshInstance(const std::string &name, std::shared_ptr<MeshAsset> mesh, const glm::mat4 &transform = glm::mat4(1.f), std::optional<BoundsType> boundsType = {});`
+
-  - `bool getMeshInstanceTransform(const std::string &name, glm::mat4 &outTransform);`
+```cpp
-  - `bool setMeshInstanceTransform(const std::string &name, const glm::mat4 &transform);`
+struct Transform
-  - `bool removeMeshInstance(const std::string &name);`
+{
-  - `void clearMeshInstances();`
+    glm::vec3 position{0.0f};
-  - Typical usage:
+    glm::quat rotation{1.0f, 0.0f, 0.0f, 0.0f};
-    - Spawn primitives or dynamic meshes at runtime (e.g. projectiles, props).
+    glm::vec3 scale{1.0f};
-    - Use `setMeshInstanceTransform` every frame to move them based on game logic.
+
+    glm::mat4 to_matrix() const;
+    static Transform from_matrix(const glm::mat4& m);
+};
+
+struct TransformD  // double-precision variant for large worlds
+{
+    glm::dvec3 position{0.0};
+    glm::quat rotation{1.0f, 0.0f, 0.0f, 0.0f};
+    glm::vec3 scale{1.0f};
+
+    glm::mat4 to_matrix() const;
+    static TransformD from_matrix(const glm::mat4& m);
+};
+```
+
+Use `TransformD` for positioning objects in large worlds (e.g., space games, flight sims) where single-precision floating point loses sub-meter precision at large coordinates.
+
+### GLTF Instances
+
+**API:**
+```cpp
+// Add glTF model instance (path relative to assets/models/)
+bool add_gltf_instance(const std::string& name,
+                       const std::string& modelPath,
+                       const Transform& transform = {},
+                       bool preloadTextures = true);
+bool add_gltf_instance(const std::string& name,
+                       const std::string& modelPath,
+                       const TransformD& transform,
+                       bool preloadTextures = true);
+
+// Add glTF model asynchronously (returns job ID, 0 on failure)
+uint32_t add_gltf_instance_async(const std::string& name,
+                                  const std::string& modelPath,
+                                  const Transform& transform = {},
+                                  bool preloadTextures = true);
+uint32_t add_gltf_instance_async(const std::string& name,
+                                  const std::string& modelPath,
+                                  const TransformD& transform,
+                                  bool preloadTextures = true);
+
+// Remove glTF instance
+bool remove_gltf_instance(const std::string& name);
+
+// Get/set glTF instance transform
+bool get_gltf_instance_transform(const std::string& name, Transform& out) const;
+bool set_gltf_instance_transform(const std::string& name, const Transform& transform);
+bool get_gltf_instance_transform(const std::string& name, TransformD& out) const;
+bool set_gltf_instance_transform(const std::string& name, const TransformD& transform);
+
+// Preload textures for an instance
+void preload_instance_textures(const std::string& name);
+
+// Clear all dynamic instances
+void clear_all_instances();
+```
+
+### Primitive Mesh Instances
+
+**API:**
+```cpp
+// Add primitive mesh instance
+bool add_primitive_instance(const std::string& name,
+                            PrimitiveType type,
+                            const Transform& transform = {});
+bool add_primitive_instance(const std::string& name,
+                            PrimitiveType type,
+                            const TransformD& transform);
+
+// Remove mesh instance
+bool remove_mesh_instance(const std::string& name);
+
+// Get/set mesh instance transform
+bool get_mesh_instance_transform(const std::string& name, Transform& out) const;
+bool set_mesh_instance_transform(const std::string& name, const Transform& transform);
+bool get_mesh_instance_transform(const std::string& name, TransformD& out) const;
+bool set_mesh_instance_transform(const std::string& name, const TransformD& transform);
+```
+
+**Typical usage:**
+- Spawn primitives or dynamic meshes at runtime (e.g. projectiles, props).
+- Use `set_mesh_instance_transform` every frame to move them based on game logic.
 
 ### Textured Primitives
 
@@ -488,29 +748,50 @@ api.add_textured_primitive("chrome_sphere", GameAPI::PrimitiveType::Sphere, meta
 
 ### Animations (GLTF)
 
-- Scene‑level
+**API:**
-  - `bool setSceneAnimation(const std::string &sceneName, int animationIndex, bool resetTime = true);`
+```cpp
-  - `bool setSceneAnimation(const std::string &sceneName, const std::string &animationName, bool resetTime = true);`
+// Set animation by index for a glTF instance (-1 to disable)
-  - `bool setSceneAnimationLoop(const std::string &sceneName, bool loop);`
+bool set_instance_animation(const std::string& instanceName, int animationIndex, bool resetTime = true);
-- Instance‑level
+
-  - `bool setGLTFInstanceAnimation(const std::string &instanceName, int animationIndex, bool resetTime = true);`
+// Set animation by name for a glTF instance
-  - `bool setGLTFInstanceAnimation(const std::string &instanceName, const std::string &animationName, bool resetTime = true);`
+bool set_instance_animation(const std::string& instanceName, const std::string& animationName, bool resetTime = true);
-  - `bool setGLTFInstanceAnimationLoop(const std::string &instanceName, bool loop);`
+
-- Notes:
+// Set animation looping for a glTF instance
-  - All functions return `bool` indicating whether the named scene/instance exists.
+bool set_instance_animation_loop(const std::string& instanceName, bool loop);
-  - Animation state is **independent per scene and per instance**:
+```
-    - Each named scene has its own `AnimationState`.
+
+**Notes:**
+- All functions return `bool` indicating whether the named instance exists.
+- Animation state is **independent per instance**:
   - Each glTF instance has its own `AnimationState`, even when sharing the same `LoadedGLTF`.
-  - An index `< 0` (e.g. `-1`) disables animation for that scene/instance (pose is frozen at the last evaluated state).
+- An index `< 0` (e.g. `-1`) disables animation for that instance (pose is frozen at the last evaluated state).
-  - `SceneManager::update_scene()` advances each active animation state every frame using engine delta time.
+- `SceneManager::update_scene()` advances each active animation state every frame using engine delta time.
+
+**Usage Example:**
+```cpp
+GameAPI::Engine api(&engine);
+
+// Play walk animation by index
+api.set_instance_animation("player", 0, true);  // Reset to start
+api.set_instance_animation_loop("player", true);
+
+// Switch to run animation by name
+api.set_instance_animation("player", "run", true);
+
+// Stop animation (freeze pose)
+api.set_instance_animation("player", -1);
+```
 
 ### Per‑Instance Node / Joint Control (Non‑Skinned)
 
-For rigid models and simple “joints” (e.g. flaps, doors, turrets), you can apply local‑space pose offsets to individual glTF nodes per instance:
+For rigid models and simple "joints" (e.g. flaps, doors, turrets), you can apply local‑space pose offsets to individual glTF nodes per instance:
 
-- `bool setGLTFInstanceNodeOffset(const std::string &instanceName, const std::string &nodeName, const glm::mat4 &offset);`
+**API:**
-- `bool clearGLTFInstanceNodeOffset(const std::string &instanceName, const std::string &nodeName);`
+```cpp
-- `void clearGLTFInstanceNodeOffsets(const std::string &instanceName);`
+bool set_instance_node_offset(const std::string& instanceName, const std::string& nodeName, const glm::mat4& offset);
+bool clear_instance_node_offset(const std::string& instanceName, const std::string& nodeName);
+void clear_all_instance_node_offsets(const std::string& instanceName);
+```
 
 Typical usage:
 
@@ -526,20 +807,108 @@ Typical usage:
   sceneMgr->setGLTFInstanceNodeOffset("plane01", "LeftAileron", offset);
   ```
 
-### Point Lights
+### Lighting - Directional (Sunlight)
 
-- Struct:
+- `void set_sunlight_direction(const glm::vec3& dir);`
-  - `struct PointLight { glm::vec3 position; float radius; glm::vec3 color; float intensity; };`
+- `glm::vec3 get_sunlight_direction() const;`
-- API:
+- `void set_sunlight_color(const glm::vec3& color, float intensity);`
-  - `void addPointLight(const PointLight &light);`
+- `glm::vec3 get_sunlight_color() const;`
-  - `void clearPointLights();`
+- `float get_sunlight_intensity() const;`
-  - `size_t getPointLightCount() const;`
+
-  - `bool getPointLight(size_t index, PointLight &outLight) const;`
+### Lighting - Point Lights
-  - `bool setPointLight(size_t index, const PointLight &light);`
+
-  - `bool removePointLight(size_t index);`
+**Structs:**
-- Typical usage:
+```cpp
-  - On level load, add all static lights.
+struct PointLight
-  - At runtime, animate or toggle lights based on gameplay events (e.g. explosions, flickering lamps).
+{
+    glm::vec3 position{0.0f};
+    float radius{10.0f};
+    glm::vec3 color{1.0f};
+    float intensity{1.0f};
+};
+
+struct PointLightD  // double-precision variant
+{
+    glm::dvec3 position{0.0};
+    float radius{10.0f};
+    glm::vec3 color{1.0f};
+    float intensity{1.0f};
+};
+```
+
+**API:**
+- `size_t add_point_light(const PointLight &light);`
+- `size_t add_point_light(const PointLightD &light);`
+- `bool remove_point_light(size_t index);`
+- `bool get_point_light(size_t index, PointLight &out) const;`
+- `bool get_point_light(size_t index, PointLightD &out) const;`
+- `bool set_point_light(size_t index, const PointLight &light);`
+- `bool set_point_light(size_t index, const PointLightD &light);`
+- `size_t get_point_light_count() const;`
+- `void clear_point_lights();`
+
+**Typical usage:**
+- On level load, add all static lights.
+- At runtime, animate or toggle lights based on gameplay events (e.g. explosions, flickering lamps).
+
+### Lighting - Spot Lights
+
+**Structs:**
+```cpp
+struct SpotLight
+{
+    glm::vec3 position{0.0f};
+    glm::vec3 direction{0.0f, -1.0f, 0.0f};
+    float radius{10.0f};
+    glm::vec3 color{1.0f};
+    float intensity{1.0f};
+    float inner_angle_deg{15.0f};
+    float outer_angle_deg{25.0f};
+};
+
+struct SpotLightD  // double-precision variant
+{
+    glm::dvec3 position{0.0};
+    glm::vec3 direction{0.0f, -1.0f, 0.0f};
+    float radius{10.0f};
+    glm::vec3 color{1.0f};
+    float intensity{1.0f};
+    float inner_angle_deg{15.0f};
+    float outer_angle_deg{25.0f};
+};
+```
+
+**API:**
+- `size_t add_spot_light(const SpotLight &light);`
+- `size_t add_spot_light(const SpotLightD &light);`
+- `bool remove_spot_light(size_t index);`
+- `bool get_spot_light(size_t index, SpotLight &out) const;`
+- `bool get_spot_light(size_t index, SpotLightD &out) const;`
+- `bool set_spot_light(size_t index, const SpotLight &light);`
+- `bool set_spot_light(size_t index, const SpotLightD &light);`
+- `size_t get_spot_light_count() const;`
+- `void clear_spot_lights();`
+
+**Usage Example:**
+```cpp
+GameAPI::Engine api(&engine);
+
+// Create a flashlight
+GameAPI::SpotLight flashlight;
+flashlight.position = glm::vec3(0.0f, 1.5f, 0.0f);
+flashlight.direction = glm::vec3(0.0f, 0.0f, -1.0f);
+flashlight.radius = 20.0f;
+flashlight.color = glm::vec3(1.0f, 0.95f, 0.8f);  // Warm white
+flashlight.intensity = 50.0f;
+flashlight.inner_angle_deg = 10.0f;
+flashlight.outer_angle_deg = 25.0f;
+
+size_t idx = api.add_spot_light(flashlight);
+
+// Later: update flashlight direction to follow camera
+flashlight.direction = camera_forward;
+api.set_spot_light(idx, flashlight);
+```
 
 ---
 
@@ -723,3 +1092,416 @@ void draw_gizmo(const PickingSystem::PickInfo& pick,
     }
 }
 ```
+
+---
+
+## Time and Statistics
+
+Header: `src/core/game_api.h`
+
+### Delta Time
+
+```cpp
+// Get delta time in seconds for the current frame (clamped to 0.0-0.1)
+float get_delta_time() const;
+```
+
+Use this for frame-rate independent movement and animation.
+
+### Engine Statistics
+
+```cpp
+struct Stats
+{
+    float frametime{0.0f};       // ms
+    float drawTime{0.0f};        // ms
+    float sceneUpdateTime{0.0f}; // ms
+    int triangleCount{0};
+    int drawCallCount{0};
+};
+
+Stats get_stats() const;
+```
+
+**Usage Example:**
+```cpp
+GameAPI::Engine api(&engine);
+
+// Frame-rate independent movement
+float dt = api.get_delta_time();
+player_position += velocity * dt;
+
+// Display performance stats
+GameAPI::Stats stats = api.get_stats();
+fmt::println("FPS: {:.1f} | Tris: {} | Draws: {}",
+             1000.0f / stats.frametime,
+             stats.triangleCount,
+             stats.drawCallCount);
+```
+
+---
+
+## Volumetrics (Clouds, Smoke, Flame)
+
+Header: `src/core/game_api.h`
+
+The engine supports GPU-based voxel volumetric rendering for clouds, smoke, and flame effects. Up to 4 independent volumes can be active simultaneously.
+
+### API
+
+```cpp
+// Enable/disable volumetrics system
+void set_volumetrics_enabled(bool enabled);
+bool get_volumetrics_enabled() const;
+
+// Get/set voxel volume settings by index (0-3)
+bool get_voxel_volume(size_t index, VoxelVolumeSettings& out) const;
+bool set_voxel_volume(size_t index, const VoxelVolumeSettings& settings);
+
+// Get maximum number of voxel volumes
+size_t get_max_voxel_volumes() const;  // Returns 4
+```
+
+### VoxelVolumeSettings Structure
+
+```cpp
+struct VoxelVolumeSettings
+{
+    bool enabled{false};
+    VoxelVolumeType type{VoxelVolumeType::Clouds};  // Clouds, Smoke, Flame
+
+    // Volume positioning
+    bool followCameraXZ{false};     // Follow camera in XZ, offset in Y
+    bool animateVoxels{true};       // Run voxel advection/update compute
+    glm::vec3 volumeCenterLocal{0.0f, 2.0f, 0.0f};
+    glm::vec3 volumeHalfExtents{8.0f, 8.0f, 8.0f};
+    glm::vec3 volumeVelocityLocal{0.0f, 0.0f, 0.0f};  // Drift when not following camera
+
+    // Raymarch/composite controls
+    float densityScale{1.0f};
+    float coverage{0.0f};        // 0..1 threshold (higher = emptier)
+    float extinction{1.0f};      // Absorption/extinction scale
+    int stepCount{48};           // Raymarch steps
+
+    // Voxel grid resolution (cubic)
+    uint32_t gridResolution{48};
+
+    // Voxel animation (advection + injection) parameters
+    glm::vec3 windVelocityLocal{0.0f, 2.0f, 0.0f};  // Local units/sec (buoyancy)
+    float dissipation{1.25f};    // Density decay rate (1/sec)
+    float noiseStrength{1.0f};   // Injection rate
+    float noiseScale{8.0f};      // Noise frequency in UVW space
+    float noiseSpeed{1.0f};      // Time scale for injection noise
+
+    // Smoke/flame source in normalized volume UVW space
+    glm::vec3 emitterUVW{0.5f, 0.05f, 0.5f};
+    float emitterRadius{0.18f};  // Normalized (0..1)
+
+    // Shading
+    glm::vec3 albedo{1.0f, 1.0f, 1.0f};  // Scattering tint (cloud/smoke)
+    float scatterStrength{1.0f};
+    glm::vec3 emissionColor{1.0f, 0.6f, 0.25f};  // Flame emissive tint
+    float emissionStrength{0.0f};
+};
+```
+
+### Usage Example
+
+```cpp
+GameAPI::Engine api(&engine);
+
+// Enable volumetrics
+api.set_volumetrics_enabled(true);
+
+// Create a flame effect
+GameAPI::VoxelVolumeSettings flame;
+flame.enabled = true;
+flame.type = GameAPI::VoxelVolumeType::Flame;
+flame.volumeCenterLocal = glm::vec3(0.0f, 1.0f, -5.0f);
+flame.volumeHalfExtents = glm::vec3(2.0f, 3.0f, 2.0f);
+flame.gridResolution = 64;
+flame.densityScale = 1.5f;
+flame.coverage = 0.3f;
+flame.windVelocityLocal = glm::vec3(0.0f, 5.0f, 0.0f);  // Upward
+flame.emitterUVW = glm::vec3(0.5f, 0.1f, 0.5f);
+flame.emitterRadius = 0.2f;
+flame.emissionStrength = 2.0f;
+flame.emissionColor = glm::vec3(1.0f, 0.5f, 0.1f);
+
+api.set_voxel_volume(0, flame);
+
+// Create cloud layer that follows camera
+GameAPI::VoxelVolumeSettings clouds;
+clouds.enabled = true;
+clouds.type = GameAPI::VoxelVolumeType::Clouds;
+clouds.followCameraXZ = true;
+clouds.volumeCenterLocal = glm::vec3(0.0f, 50.0f, 0.0f);  // Offset in Y
+clouds.volumeHalfExtents = glm::vec3(100.0f, 20.0f, 100.0f);
+clouds.gridResolution = 128;
+clouds.densityScale = 0.8f;
+clouds.coverage = 0.5f;
+clouds.albedo = glm::vec3(0.9f, 0.95f, 1.0f);  // Bluish tint
+
+api.set_voxel_volume(1, clouds);
+```
+
+---
+
+## Particle Systems
+
+Header: `src/core/game_api.h`
+
+GPU-accelerated particle systems with flipbook animation, soft particles, and flexible spawning.
+
+### API
+
+```cpp
+// Create/destroy particle systems
+uint32_t create_particle_system(uint32_t particle_count);
+bool destroy_particle_system(uint32_t id);
+bool resize_particle_system(uint32_t id, uint32_t new_count);
+
+// Get/set particle system settings
+bool get_particle_system(uint32_t id, ParticleSystem& out) const;
+bool set_particle_system(uint32_t id, const ParticleSystem& system);
+
+// Query particle systems
+std::vector<uint32_t> get_particle_system_ids() const;
+uint32_t get_allocated_particles() const;
+uint32_t get_free_particles() const;
+uint32_t get_max_particles() const;
+
+// Preload VFX textures (e.g., "vfx/flame.ktx2")
+void preload_particle_texture(const std::string& assetPath);
+```
+
+### ParticleSystem Structure
+
+```cpp
+struct ParticleSystem
+{
+    uint32_t id{0};
+    uint32_t particleCount{0};
+    bool enabled{true};
+    bool reset{true};
+    ParticleBlendMode blendMode{ParticleBlendMode::Additive};  // Additive or Alpha
+    ParticleParams params{};
+
+    // Asset-relative texture paths (e.g., "vfx/flame.ktx2")
+    std::string flipbookTexture{"vfx/flame.ktx2"};
+    std::string noiseTexture{"vfx/simplex.ktx2"};
+};
+
+struct ParticleParams
+{
+    glm::vec3 emitterPosLocal{0.0f, 0.0f, 0.0f};
+    float spawnRadius{0.1f};
+    glm::vec3 emitterDirLocal{0.0f, 1.0f, 0.0f};
+    float coneAngleDegrees{20.0f};
+
+    float minSpeed{2.0f};
+    float maxSpeed{8.0f};
+    float minLife{0.5f};
+    float maxLife{1.5f};
+    float minSize{0.05f};
+    float maxSize{0.15f};
+
+    float drag{1.0f};
+    float gravity{0.0f};  // Positive pulls down -Y in local space
+
+    glm::vec4 color{1.0f, 0.5f, 0.1f, 1.0f};
+
+    // Soft particles (fade near opaque geometry)
+    float softDepthDistance{0.15f};
+
+    // Flipbook animation (atlas layout)
+    uint32_t flipbookCols{16};
+    uint32_t flipbookRows{4};
+    float flipbookFps{30.0f};
+    float flipbookIntensity{1.0f};
+
+    // Noise UV distortion
+    float noiseScale{6.0f};
+    float noiseStrength{0.05f};
+    glm::vec2 noiseScroll{0.0f, 0.0f};
+};
+```
+
+### Usage Example
+
+```cpp
+GameAPI::Engine api(&engine);
+
+// Create fire particle system
+uint32_t fireId = api.create_particle_system(4096);
+
+GameAPI::ParticleSystem fire;
+fire.id = fireId;
+fire.particleCount = 4096;
+fire.enabled = true;
+fire.blendMode = GameAPI::ParticleBlendMode::Additive;
+fire.flipbookTexture = "vfx/flame.ktx2";
+fire.noiseTexture = "vfx/simplex.ktx2";
+
+fire.params.emitterPosLocal = glm::vec3(0.0f, 0.0f, -5.0f);
+fire.params.spawnRadius = 0.5f;
+fire.params.emitterDirLocal = glm::vec3(0.0f, 1.0f, 0.0f);
+fire.params.coneAngleDegrees = 15.0f;
+fire.params.minSpeed = 2.0f;
+fire.params.maxSpeed = 4.0f;
+fire.params.minLife = 0.8f;
+fire.params.maxLife = 1.5f;
+fire.params.minSize = 0.3f;
+fire.params.maxSize = 0.6f;
+fire.params.gravity = -2.0f;  // Upward buoyancy
+fire.params.color = glm::vec4(1.0f, 0.7f, 0.3f, 1.0f);
+fire.params.flipbookCols = 16;
+fire.params.flipbookRows = 4;
+fire.params.flipbookFps = 24.0f;
+
+api.set_particle_system(fireId, fire);
+
+// Later: move emitter to follow player
+fire.params.emitterPosLocal = player_position;
+api.set_particle_system(fireId, fire);
+
+// Reset particles (trigger burst)
+fire.reset = true;
+api.set_particle_system(fireId, fire);
+```
+
+---
+
+## Debug Drawing
+
+Header: `src/core/game_api.h`
+
+Runtime debug visualization for primitives (lines, spheres, boxes, etc.) with optional depth testing and duration.
+
+### Settings API
+
+```cpp
+// Enable/disable debug drawing system
+void set_debug_draw_enabled(bool enabled);
+bool get_debug_draw_enabled() const;
+
+// Control which debug layers are visible (bitmask)
+void set_debug_layer_mask(uint32_t mask);
+uint32_t get_debug_layer_mask() const;
+
+// Show/hide depth-tested primitives
+void set_debug_show_depth_tested(bool show);
+bool get_debug_show_depth_tested() const;
+
+// Show/hide overlay (always-on-top) primitives
+void set_debug_show_overlay(bool show);
+bool get_debug_show_overlay() const;
+
+// Set tessellation quality (segments for circles/spheres)
+void set_debug_segments(int segments);
+int get_debug_segments() const;
+
+// Clear all debug draw commands
+void debug_draw_clear();
+```
+
+### Drawing API
+
+All drawing functions support both single and double-precision variants:
+
+```cpp
+// Line
+void debug_draw_line(const glm::vec3& a, const glm::vec3& b,
+                     const glm::vec4& color = glm::vec4(1.0f),
+                     float duration_seconds = 0.0f,
+                     bool depth_tested = true);
+void debug_draw_line(const glm::dvec3& a, const glm::dvec3& b, ...);
+
+// Ray (origin + direction + length)
+void debug_draw_ray(const glm::vec3& origin, const glm::vec3& direction, float length,
+                    const glm::vec4& color = glm::vec4(1.0f),
+                    float duration_seconds = 0.0f,
+                    bool depth_tested = true);
+void debug_draw_ray(const glm::dvec3& origin, const glm::dvec3& direction, double length, ...);
+
+// AABB (axis-aligned bounding box)
+void debug_draw_aabb(const glm::vec3& center, const glm::vec3& half_extents,
+                     const glm::vec4& color = glm::vec4(1.0f),
+                     float duration_seconds = 0.0f,
+                     bool depth_tested = true);
+void debug_draw_aabb(const glm::dvec3& center, const glm::vec3& half_extents, ...);
+
+// Sphere
+void debug_draw_sphere(const glm::vec3& center, float radius,
+                       const glm::vec4& color = glm::vec4(1.0f),
+                       float duration_seconds = 0.0f,
+                       bool depth_tested = true);
+void debug_draw_sphere(const glm::dvec3& center, float radius, ...);
+
+// Capsule (line segment + radius)
+void debug_draw_capsule(const glm::vec3& p0, const glm::vec3& p1, float radius,
+                        const glm::vec4& color = glm::vec4(1.0f),
+                        float duration_seconds = 0.0f,
+                        bool depth_tested = true);
+void debug_draw_capsule(const glm::dvec3& p0, const glm::dvec3& p1, float radius, ...);
+
+// Circle (center + normal + radius)
+void debug_draw_circle(const glm::vec3& center, const glm::vec3& normal, float radius,
+                       const glm::vec4& color = glm::vec4(1.0f),
+                       float duration_seconds = 0.0f,
+                       bool depth_tested = true);
+void debug_draw_circle(const glm::dvec3& center, const glm::dvec3& normal, float radius, ...);
+
+// Cone (apex + direction + length + angle)
+void debug_draw_cone(const glm::vec3& apex, const glm::vec3& direction,
+                     float length, float angle_degrees,
+                     const glm::vec4& color = glm::vec4(1.0f),
+                     float duration_seconds = 0.0f,
+                     bool depth_tested = true);
+void debug_draw_cone(const glm::dvec3& apex, const glm::dvec3& direction,
+                     float length, float angle_degrees, ...);
+```
+
+### Usage Example
+
+```cpp
+GameAPI::Engine api(&engine);
+
+// Enable debug drawing
+api.set_debug_draw_enabled(true);
+api.set_debug_segments(32);  // Smooth circles/spheres
+
+// Visualize player bounds (persistent, depth-tested)
+api.debug_draw_aabb(player_pos, glm::vec3(0.5f, 1.0f, 0.5f),
+                    glm::vec4(0.0f, 1.0f, 0.0f, 1.0f),
+                    0.0f,    // Duration 0 = single frame
+                    true);   // Depth tested
+
+// Visualize raycast (red ray, always on top, 2 seconds)
+api.debug_draw_ray(ray_origin, ray_dir, 100.0f,
+                   glm::vec4(1.0f, 0.0f, 0.0f, 1.0f),
+                   2.0f,     // Show for 2 seconds
+                   false);   // Always on top
+
+// Visualize trigger volume (transparent sphere)
+api.debug_draw_sphere(trigger_pos, trigger_radius,
+                      glm::vec4(1.0f, 1.0f, 0.0f, 0.3f),  // Yellow, 30% alpha
+                      0.0f,
+                      true);
+
+// Visualize spot light cone
+api.debug_draw_cone(light_pos, light_dir, light_radius, light_angle_deg,
+                    glm::vec4(1.0f, 0.9f, 0.7f, 0.5f),
+                    0.0f,
+                    true);
+
+// One-shot clear (useful for clearing persistent debug viz)
+api.debug_draw_clear();
+```
+
+**Notes:**
+- `duration_seconds = 0.0f`: Draw for a single frame (re-submit each frame for persistent viz).
+- `duration_seconds > 0.0f`: Draw for N seconds, then automatically expire.
+- `depth_tested = true`: Primitive is occluded by scene geometry.
+- `depth_tested = false`: Always on top (overlay mode).
+- All primitives support alpha blending via the color's alpha channel.

src/core/game_api.cpp

@@ -16,9 +16,12 @@
 
 #include <glm/gtx/matrix_decompose.hpp>
 #include <glm/gtx/quaternion.hpp>
-#include <cstdint>
 #include <filesystem>
 
+// ImGui integration for texture display
+#include "imgui.h"
+#include "imgui_impl_vulkan.h"
+
 namespace GameAPI
 {
 
@@ -332,6 +335,49 @@ void Engine::unload_texture(TextureHandle handle)
     _engine->_textureCache->unload(cacheHandle);
 }
 
+void* Engine::create_imgui_texture(TextureHandle handle, void* sampler)
+{
+    if (!_engine || !_engine->_textureCache)
+    {
+        return nullptr;
+    }
+
+    auto cacheHandle = static_cast<TextureCache::TextureHandle>(handle);
+    VkImageView imageView = _engine->_textureCache->image_view(cacheHandle);
+
+    if (imageView == VK_NULL_HANDLE)
+    {
+        return nullptr;
+    }
+
+    // Use provided sampler or default linear sampler
+    VkSampler vkSampler = reinterpret_cast<VkSampler>(sampler);
+    if (vkSampler == VK_NULL_HANDLE && _engine->_context && _engine->_context->samplers)
+    {
+        vkSampler = _engine->_context->samplers->defaultLinear();
+    }
+
+    // Create ImGui descriptor set using ImGui_ImplVulkan
+    VkDescriptorSet descriptorSet = ImGui_ImplVulkan_AddTexture(
+        vkSampler,
+        imageView,
+        VK_IMAGE_LAYOUT_SHADER_READ_ONLY_OPTIMAL
+    );
+
+    return reinterpret_cast<void*>(descriptorSet);
+}
+
+void Engine::free_imgui_texture(void* imgui_texture_id)
+{
+    if (imgui_texture_id == nullptr)
+    {
+        return;
+    }
+
+    VkDescriptorSet descriptorSet = reinterpret_cast<VkDescriptorSet>(imgui_texture_id);
+    ImGui_ImplVulkan_RemoveTexture(descriptorSet);
+}
+
 // ----------------------------------------------------------------------------
 // Shadows
 // ----------------------------------------------------------------------------

src/core/game_api.h

@@ -383,6 +383,15 @@ public:
     // This is optional - the cache manages memory automatically
     void unload_texture(TextureHandle handle);
 
+    // Create an ImGui descriptor set for a texture (for use with ImGui::Image())
+    // Returns ImTextureID (actually VkDescriptorSet) that can be used in ImGui
+    // The returned descriptor set is managed by ImGui and valid until cleanup
+    // sampler: VK_NULL_HANDLE uses default linear sampler
+    void* create_imgui_texture(TextureHandle handle, void* sampler = nullptr);
+
+    // Free an ImGui descriptor set created by create_imgui_texture()
+    void free_imgui_texture(void* imgui_texture_id);
+
     // ------------------------------------------------------------------------
     // Shadows
     // ------------------------------------------------------------------------