add session summary

docs/session_summaries/2026-01-18-config-display-and-rendering-fixes.md

@@ -0,0 +1,144 @@
+# 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%)