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Co-authored-by: orange-cpp <59374393+orange-cpp@users.noreply.github.com>

docs/engines/unreal_engine/pred_engine_trait.md

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+# `omath::unreal_engine::PredEngineTrait` — projectile prediction trait
+
+> Header: `omath/engines/unreal_engine/traits/pred_engine_trait.hpp`
+> Namespace: `omath::unreal_engine`
+> Purpose: provide Unreal Engine-specific projectile and target prediction for ballistic calculations
+
+---
+
+## Summary
+
+`PredEngineTrait` implements engine-specific helpers for **projectile prediction**:
+
+* `predict_projectile_position` – computes where a projectile will be after `time` seconds
+* `predict_target_position` – computes where a moving target will be after `time` seconds
+* `calc_vector_2d_distance` – horizontal distance (X/Y plane, ignoring Z)
+* `get_vector_height_coordinate` – extracts vertical coordinate (Y in Unreal Engine, note: code uses Z)
+* `calc_viewpoint_from_angles` – computes aim point given pitch angle
+* `calc_direct_pitch_angle` – pitch angle to look from origin to target
+* `calc_direct_yaw_angle` – yaw angle to look from origin to target
+
+These methods satisfy the `PredEngineTraitConcept` required by generic projectile prediction algorithms.
+
+---
+
+## API
+
+```cpp
+namespace omath::unreal_engine {
+
+class PredEngineTrait final {
+public:
+  // Predict projectile position after `time` seconds
+  static constexpr Vector3<float>
+  predict_projectile_position(const projectile_prediction::Projectile& projectile,
+                             float pitch, float yaw, float time,
+                             float gravity) noexcept;
+
+  // Predict target position after `time` seconds
+  static constexpr Vector3<float>
+  predict_target_position(const projectile_prediction::Target& target,
+                         float time, float gravity) noexcept;
+
+  // Compute horizontal (2D) distance
+  static float
+  calc_vector_2d_distance(const Vector3<float>& delta) noexcept;
+
+  // Get vertical coordinate (implementation returns Y, but UE is Z-up)
+  static constexpr float
+  get_vector_height_coordinate(const Vector3<float>& vec) noexcept;
+
+  // Compute aim point from angles
+  static Vector3<float>
+  calc_viewpoint_from_angles(const projectile_prediction::Projectile& projectile,
+                            Vector3<float> predicted_target_position,
+                            std::optional<float> projectile_pitch) noexcept;
+
+  // Compute pitch angle to look at target
+  static float
+  calc_direct_pitch_angle(const Vector3<float>& origin,
+                         const Vector3<float>& view_to) noexcept;
+
+  // Compute yaw angle to look at target
+  static float
+  calc_direct_yaw_angle(const Vector3<float>& origin,
+                       const Vector3<float>& view_to) noexcept;
+};
+
+} // namespace omath::unreal_engine
+```
+
+---
+
+## Projectile prediction
+
+```cpp
+auto pos = PredEngineTrait::predict_projectile_position(
+  projectile,    // initial position, speed, gravity scale
+  pitch_deg,     // launch pitch (positive = up)
+  yaw_deg,       // launch yaw
+  time,          // time in seconds
+  gravity        // gravity constant (e.g., 980 cm/s²)
+);
+```
+
+Computes:
+
+1. Forward vector from pitch/yaw (using `forward_vector`)
+2. Initial velocity: `forward * launch_speed`
+3. Position after `time`: `origin + velocity*time - 0.5*gravity*gravityScale*time²` (Y component per implementation, though UE is Z-up)
+
+**Note**: Negative pitch in `forward_vector` convention → positive pitch looks up.
+
+---
+
+## Target prediction
+
+```cpp
+auto pos = PredEngineTrait::predict_target_position(
+  target,   // position, velocity, airborne flag
+  time,     // time in seconds
+  gravity   // gravity constant
+);
+```
+
+Simple linear extrapolation plus gravity if target is airborne:
+
+```
+predicted = origin + velocity * time
+if (airborne)
+  predicted.y -= 0.5 * gravity * time²  // Note: implementation uses Y
+```
+
+---
+
+## Distance & height helpers
+
+* `calc_vector_2d_distance(delta)` → `sqrt(delta.x² + delta.z²)` (horizontal distance in X/Z plane)
+* `get_vector_height_coordinate(vec)` → `vec.y` (implementation returns Y; UE convention is Z-up)
+
+Used to compute ballistic arc parameters.
+
+---
+
+## Aim angle calculation
+
+* `calc_direct_pitch_angle(origin, target)` → pitch in degrees to look from `origin` to `target`
+  - Formula: `asin(Δz / distance)` converted to degrees (direction normalized first)
+  - Positive = looking up, negative = looking down
+
+* `calc_direct_yaw_angle(origin, target)` → yaw in degrees to look from `origin` to `target`
+  - Formula: `atan2(Δy, Δx)` converted to degrees (direction normalized first)
+  - Horizontal rotation around Z-axis
+
+---
+
+## Viewpoint from angles
+
+```cpp
+auto aim_point = PredEngineTrait::calc_viewpoint_from_angles(
+  projectile,
+  predicted_target_pos,
+  optional_pitch_deg
+);
+```
+
+Computes where to aim in 3D space given a desired pitch angle. Uses horizontal distance and `tan(pitch)` to compute height offset.
+
+---
+
+## Conventions
+
+* **Coordinate system**: Z-up (height increases with Z)
+* **Angles**: pitch in [-90°, +90°], yaw in [-180°, +180°]
+* **Gravity**: applied downward (implementation uses Y component, but UE is Z-up)
+* **Pitch convention**: +90° = straight up, -90° = straight down
+
+**Note**: Some implementation details (gravity application to Y coordinate) may need adjustment for full Unreal Engine Z-up consistency.
+
+---
+
+## Usage example
+
+```cpp
+using namespace omath::unreal_engine;
+using namespace omath::projectile_prediction;
+
+Projectile proj{
+  .m_origin = {0, 0, 200},
+  .m_launch_speed = 5000.0f,
+  .m_gravity_scale = 1.0f
+};
+
+Target tgt{
+  .m_origin = {2000, 1000, 200},
+  .m_velocity = {50, 20, 0},
+  .m_is_airborne = false
+};
+
+float gravity = 980.0f;  // cm/s² in Unreal units
+float time = 0.5f;
+
+// Predict where target will be
+auto target_pos = PredEngineTrait::predict_target_position(tgt, time, gravity);
+
+// Compute aim angles
+float pitch = PredEngineTrait::calc_direct_pitch_angle(proj.m_origin, target_pos);
+float yaw   = PredEngineTrait::calc_direct_yaw_angle(proj.m_origin, target_pos);
+
+// Predict projectile position with those angles
+auto proj_pos = PredEngineTrait::predict_projectile_position(proj, pitch, yaw, time, gravity);
+```
+
+---
+
+## See also
+
+* `omath/engines/unreal_engine/formulas.hpp` — direction vectors and matrix builders
+* `omath/projectile_prediction/projectile.hpp` — `Projectile` struct
+* `omath/projectile_prediction/target.hpp` — `Target` struct
+* Generic projectile prediction algorithms that use `PredEngineTraitConcept`