# Session Summary: Config Display, Compiler Warnings, Orbit Rendering Fix, and Camera Initialization

## Date
2026-01-18

## Overview
Fixed multiple UI and rendering issues: added config file name display, resolved compiler warnings, corrected orbit rendering for nested bodies (moons), and initialized camera to follow root body on startup.

## Key Features Implemented

### 1. Config File Name Display
- Added `char config_name[256]` field to `SimulationState` struct
- Config file path now stored when loaded via `load_system_config()`
- Displayed in simulation info panel under "Config:" label
- Eliminates hardcoded "solar_system.txt" reference

**Before**: Info panel always showed "solar_system.txt" regardless of actual config
**After**: Shows actual loaded config path (e.g., "tests/configs/solar_system.toml")

### 2. Compiler Warning Fixes
- Removed unused `departure_period` variable from `calculate_hohmann_transfer()`
- Removed unused `sun` variable from `spawn_spacecraft_on_transfer()`
- Increased `temp_buffer` size from 256 to 512 in `render_info()` to prevent truncation warning

**Result**: Clean build with zero compiler warnings

### 3. Orbit Rendering Fix for Nested Bodies
- Changed `render_orbit()` to use `local_velocity` instead of global `velocity`
- Critical fix for correct moon and satellite orbit paths

**Technical Details**:
- Planets: `local_velocity ≈ global_velocity` (orbits around Sun)
- Moons: `local_velocity` is velocity relative to parent planet
- Prior fix: `local_velocity` used for physics integration, but `global_velocity` used for orbit rendering
- Current fix: Both physics and rendering now use `local_velocity`

**Impact**:
- Moon orbits now render correctly around their parent planets
- Future satellites will render correctly around moons
- Parabolic and hyperbolic orbit types also fixed

### 4. Camera Initialization on Root Body
- Camera now automatically selects root body (star) on startup
- Camera follow mode enabled by default for root body
- `body_list_active` synchronized with `selected_body_index`
- Default camera offset (0, 50, 100) applied relative to root body

**Before**: No body selected, camera in neutral position
**After**: Camera follows root body (e.g., Sun) immediately on startup

## Technical Details

### SimulationState Structure Changes
```cpp
struct SimulationState {
    CelestialBody* bodies;
    int body_count;
    int max_bodies;
    double time;
    double dt;
    char config_name[256];    // NEW: name of the config file loaded
};
```

### Config Loading
```cpp
// In load_system_config()
strncpy(sim->config_name, filepath, sizeof(sim->config_name) - 1);
sim->config_name[sizeof(sim->config_name) - 1] = '\0';
```

### Orbit Rendering Fix
```cpp
// Before (incorrect for moons)
Vec3 r_vec = vec3_sub(body->position, parent->position);
double v = vec3_magnitude(body->velocity);  // Global velocity
double r_dot_v = r_vec.x * body->velocity.x + ...

// After (correct for all bodies)
Vec3 r_vec = vec3_sub(body->position, parent->position);
Vec3 v_vec = body->local_velocity;  // Local velocity relative to parent
double v = vec3_magnitude(v_vec);
double r_dot_v = r_vec.x * v_vec.x + ...
```

### Camera Initialization
```cpp
// Find root body
int root_body_index = -1;
for (int i = 0; i < sim->body_count; i++) {
    if (sim->bodies[i].parent_index == -1) {
        root_body_index = i;
        break;
    }
}

// Initialize with root body selected
render_state.selected_body_index = root_body_index >= 0 ? root_body_index : -1;
render_state.body_list_active = root_body_index >= 0 ? root_body_index : -1;
render_state.camera_follow_body = root_body_index >= 0;
```

## Files Modified

### Core Implementation
- `src/simulation.h` - Added `config_name` field to `SimulationState`
- `src/simulation.cpp` - Initialize `config_name` to empty string in `create_simulation()`
- `src/config_loader.cpp` - Store config filepath in `sim->config_name`
- `src/renderer.h` - Removed `config_name` parameter from `render_info()`
- `src/renderer.cpp` - Display config name, use `local_velocity` for orbit rendering, increase buffer size
- `src/main.cpp` - Find root body and initialize camera selection/follow state

## Commits Summary

1. `c4262ab` - Store and display config file name in simulation state
2. `8f60ba5` - Fix compiler warnings: remove unused variables, increase buffer size
3. `8278cb0` - Fix orbit rendering to use local_velocity for nested orbits
4. `c3a4606` - Initialize camera to follow root body on startup

## Testing Status

All changes compiled and tested:
- ✅ Config file name displayed correctly in UI
- ✅ Zero compiler warnings in full rebuild
- ✅ Moon orbit paths render correctly around parent planets
- ✅ Planet orbit paths remain correct
- ✅ Camera follows root body on startup
- ✅ Tests pass (23/24, with 1 pre-existing Hohmann transfer test failure)

## Branch Status

Working branch: `main`
Clean working tree ready for next session.

## Future Enhancements

### Potential Improvements
- Visual highlighting of selected body in 3D view
- Camera focus indicator (show which body is being followed)
- Search/filter functionality in body list
- Reference frame switching (view from body's perspective)

### Bug Follow-ups
- Investigate Hohmann transfer test failure (energy drift > 200%)