claudes_game/docs/session_summaries/2026-01-05-soi-bug-and-dynamic-orbits.md

SOI Bug Fix and Dynamic Orbital Rendering (2026-01-05)

Changes Made

Core Bug Fixes

• Fixed SOI calculation bug: was using current distance instead of orbital semi_major_axis
  • Caused incorrect SOI radii for eccentric orbits (e.g., comet at perihelion vs aphelion)
  • Now properly uses stored orbital parameter from config file
• Added parent change tracking to headless simulation mode for debugging SOI transitions

Test Refactoring

• Refactored test to use same code path as simulation via load_system_config()
  • Eliminated ~30 lines of manual body setup in test
  • Ensures test and simulation behave identically
• Added config_loader.cpp to test build dependencies
• Refactored verbose test output into separate functions with compile-time flag
  • Added COMET_TEST_VERBOSE_OUTPUT flag (default: 0)
  • Moved output to print_orbital_snapshots() and print_parent_changes()

Graphics Enhancements

• Implemented dynamic orbital rendering with real-time element calculation
  • Orbits now update automatically when parent body changes
  • Calculates orbital elements from current position/velocity relative to parent
• Added support for parabolic trajectories (e ≈ 1)
  • Limited to ±86° from periapsis to avoid infinite vertices
• Added support for hyperbolic trajectories (e > 1)
  • Limited to asymptotic angle to avoid infinite vertices
• Properly orients orbits in 3D space using eccentricity vector and angular momentum

Code Structure

Refactored renderer for separation of concerns:

• OrbitalBasis struct - Holds orbital reference frame
• calculate_orbital_basis() - Computes periapsis direction, normal, and perpendicular vectors
• orbital_to_cartesian() - Transforms 2D orbital coordinates to 3D space
• draw_orbit_segment() - Handles rendering of individual line segments
• render_elliptical_orbit() - Draws closed elliptical paths (100 segments)
• render_hyperbolic_orbit() - Draws open hyperbolic/parabolic paths (60 segments)

Test Configuration

Enhanced configs/test_simple.txt with diverse orbit examples:

• Parabolic object (e=1.0 at 0.5 AU, exactly at escape velocity)
• Inclined asteroid (circular orbit with 30° inclination)
• Hyperbolic visitor (e=1.5, interstellar trajectory)

Commits

• 89931c3 Fix SOI calculation and orbit direction bugs
• 8e21f71 Refactor test to use same code path as simulation
• 232614d Revert cross product direction in config_loader
• 4a313f8 remove extra debugging not added by claude (user commit)
• c613d21 Refactor verbose test output to separate functions
• 060ff40 Add dynamic orbital rendering with hyperbolic/parabolic support
• e1df0b4 Add diverse orbit examples to test_simple config

Results

7 files changed, +209/-106 (net +103 lines)

Key Findings

• Simulation and test now show identical behavior (parent change Sun→Mars at day 1532)
• Comet enters Mars's SOI at 0.003787 AU (just within 0.003792 AU SOI radius)
• Comet is ejected from solar system after Mars encounter (ends at 58.7 AU with e=1.0)
• Renderer now correctly displays elliptical, parabolic, and hyperbolic trajectories
• Orbits update dynamically when gravitational parent changes

Test Results

All tests passing (21 assertions in 5 test cases) ✓

Outstanding Issues

• Comet remains Mars's child after leaving Mars's SOI
  • find_dominant_body() doesn't check if body is still within current parent's SOI
  • Should switch back to grandparent (Sun) when exiting child SOI
  • Results in comet staying as Mars-relative even after ejection