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Interpolation is one of Chimera’s most significant features, solving a fundamental issue in Halo PC where object movement is tied to the server tick rate.

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The Problem

Halo: Combat Evolved has a critical limitation inherited from the original Xbox version:
Object movement is tied to tick rate, meaning objects will never move faster than 30 frames per second, regardless of your actual frame rate.
This creates a jarring visual experience on modern displays running at 60 Hz, 144 Hz, or higher refresh rates. Even if you’re rendering at 144 FPS, players and objects will appear to stutter because their positions only update 30 times per second.

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The Solution

Chimera adds interpolation to smooth object movement between ticks. The system intelligently predicts and smooths object positions, creating fluid motion that matches your frame rate.

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What Gets Interpolated

Chimera interpolates multiple game elements:
  • Objects - Players, vehicles, weapons, and all dynamic objects
  • Camera - First-person and third-person camera movement
  • Particles - Effects, explosions, and particle systems
  • Lights - Dynamic lighting effects
  • Flags - CTF flags and similar objects
  • Antennas - Warthog antennas and similar animated models
  • First Person - Weapon models and first-person animations

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Technical Implementation

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Tick Progress Calculation

The interpolation system tracks progress between ticks: 
// From interpolate.cpp
float interpolation_tick_progress = 0;

static void on_preframe() noexcept {
    if(game_paused()) {
        return;
    }
    
    interpolation_tick_progress = get_tick_progress();
    
    interpolate_antenna_before();
    interpolate_flag_before();
    interpolate_light_before();
    interpolate_object_before();
    interpolate_particle();
}

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Frame Synchronization

Interpolation hooks into Chimera’s event system:
  1. On Tick - Store previous state and reset progress
  2. Pre-frame - Calculate interpolation progress (0.0 to 1.0)
  3. Pre-camera - Apply camera interpolation
  4. Frame - Restore original values after rendering
static void on_tick() noexcept {
    if(game_paused()) {
        return;
    }
    
    interpolate_antenna_on_tick();
    interpolate_flag_on_tick();
    interpolate_fp_on_tick();
    interpolate_light_on_tick();
    interpolate_object_on_tick();
    interpolate_camera_on_tick();
    interpolate_particle_on_tick();
    interpolation_tick_progress = 0;
}

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Always Enabled

Interpolation is always enabled in Chimera and cannot be disabled. This is a fundamental enhancement that all players benefit from.
The system is designed to:
  • Work transparently without configuration
  • Maintain compatibility with all game modes
  • Preserve gameplay accuracy (only visual smoothing)
  • Handle game state changes automatically

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Game State Handling

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Pause Detection

Interpolation automatically pauses when the game is paused: 
if(game_paused()) {
    return;
}

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Buffer Clearing

Interpolation buffers are cleared on major game state changes: 
void clear_buffers() noexcept {
    interpolate_object_clear();
    interpolate_particle_clear();
    interpolate_light_clear();
    interpolate_flag_clear();
    interpolate_camera_clear();
    interpolate_fp_clear();
}
This prevents visual glitches when:
  • Loading a new map
  • Reverting to checkpoint
  • Respawning
  • Major game events

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Performance Characteristics

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Optimized for Modern Hardware

Interpolation is highly optimized:
  • Minimal CPU overhead - Simple linear interpolation
  • No memory leaks - Buffers cleared on state changes
  • Cache-friendly - Sequential memory access patterns
  • Frame-rate independent - Scales with any FPS

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No Gameplay Impact

Interpolation is purely visual. All game logic, physics, and netcode still run at the server’s tick rate (typically 30 ticks per second).
This means:
  • Hit detection is unaffected
  • Physics simulation remains identical
  • Network synchronization stays the same
  • Demo playback compatibility preserved

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Visual Comparison

At 144 FPS without interpolation:
  • Objects update position 30 times per second
  • Visible stuttering on smooth camera pans
  • Choppy vehicle movement
  • Jarring player animations

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First Person Camera

Chimera also handles first-person camera interpolation: 
float *first_person_camera_tick_rate = nullptr;

// Update FP camera tick rate dynamically
float current_tick_rate = effective_tick_rate();
if(*first_person_camera_tick_rate != current_tick_rate) {
    overwrite(first_person_camera_tick_rate, current_tick_rate);
}
This ensures weapon bob, recoil, and view movement remain smooth.

Uncap Cinematic

Uncap cinematic frame rate from 30 FPS

Throttle FPS

Limit frame rate for consistent interpolation

Bug Fixes

Other frame-rate related fixes

Auto Center Fix

Fixes vehicle handling at high FPS

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Technical Notes

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Implementation Files

The interpolation system is implemented across multiple files:
  • src/chimera/fix/interpolate/interpolate.cpp - Core system
  • src/chimera/fix/interpolate/object.cpp - Object interpolation
  • src/chimera/fix/interpolate/camera.cpp - Camera interpolation
  • src/chimera/fix/interpolate/particle.cpp - Particle effects
  • src/chimera/fix/interpolate/light.cpp - Dynamic lights
  • src/chimera/fix/interpolate/flag.cpp - Flag objects
  • src/chimera/fix/interpolate/fp.cpp - First person view
  • src/chimera/fix/interpolate/antenna.cpp - Antenna animations

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Built-in Interpolation Override

Chimera disables Halo’s built-in first-person camera interpolation: 
// Block built-in fp camera interpolation. Let Chimera do it instead.
overwrite(get_chimera().get_signature("camera_interpolation_sig").data() + 0xF, 
          static_cast<unsigned char>(0xEB));
This provides more consistent and higher-quality interpolation.

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