CGI & Visual EffectsParticles, Simulations & Houdini

// ── Houdini VEX: Custom Particle Forces ──────────────
// VEX is Houdini's high-performance scripting language.
// It runs per-point/particle and is FAST.
//
// Apply this in a Point Wrangle SOP.

// ── 1. Curl Noise Force (Organic Swirling Motion) ─────
// Creates beautiful, non-repeating turbulent motion.
// Used for: smoke trails, magic effects, energy swirls.

float freq = chf("noise_frequency");  // User slider (0.5-3.0)
float amp  = chf("noise_amplitude");  // User slider (0.1-2.0)
float time_offset = @Time * chf("noise_speed");

// 3D Curl Noise — divergence-free (no convergence points)
vector noise_pos = @P * freq + set(0, time_offset, 0);
vector curl = curlnoise(noise_pos);

// Apply as force (multiply by amplitude)
@v += curl * amp * @TimeInc;

// ── 2. Age-Based Particle Behavior ────────────────────
// Make particles change behavior as they age.

float life_ratio = @age / @life;  // 0.0 (birth) → 1.0 (death)

// Fade out opacity as particle dies
@Alpha = fit(life_ratio, 0.7, 1.0, 1.0, 0.0);

// Grow particle size over lifetime
@pscale = fit(life_ratio, 0.0, 0.3, 0.01, 0.1) 
        * fit(life_ratio, 0.7, 1.0, 1.0, 0.0);

// Color shift: blue (birth) → orange (mid) → red (death)
vector color_young = set(0.2, 0.5, 1.0);  // Blue
vector color_mid   = set(1.0, 0.5, 0.1);  // Orange
vector color_old   = set(1.0, 0.1, 0.05); // Red

if (life_ratio < 0.5) {
    @Cd = lerp(color_young, color_mid, life_ratio * 2);
} else {
    @Cd = lerp(color_mid, color_old, (life_ratio - 0.5) * 2);
}

// ── 3. Attraction to Target ───────────────────────────
// Particles gravitate toward a target point.
// Used for: energy gathering, implosion effects.

vector target = chv("target_position");  // User input
float attract_strength = chf("attraction");

vector dir_to_target = normalize(target - @P);
float dist = length(target - @P);

// Strength increases as particle gets closer (inverse square)
float force = attract_strength / (dist * dist + 0.01);
@v += dir_to_target * force * @TimeInc;

// ── 4. Ground Collision (Simple) ──────────────────────
float ground_y = 0.0;
float bounce = chf("bounciness");  // 0.0-1.0

if (@P.y < ground_y) {
    @P.y = ground_y;
    @v.y = abs(@v.y) * bounce;  // Reflect Y velocity
    @v.x *= 0.9;  // Friction
    @v.z *= 0.9;
}

// ── Houdini Pyro Simulation Setup ─────────────────────
// Pyro uses volume grids to simulate fire, smoke, and
// explosions. This is the setup used on major films.

// ── STEP 1: Source Setup (Point Wrangle on emitter) ───
// Define what the source emits into the pyro sim.

// Temperature drives fire (combustion)
@temperature = 1.5;          // Higher = hotter flames
@temperature *= rand(@id);   // Randomise per-point

// Density = smoke thickness
@density = 0.8;

// Fuel = what burns (consumed by combustion)
@fuel = 1.0;

// Emission velocity (upward + outward burst)
@v = set(0, 3.0, 0);                    // Base upward velocity
@v += normalize(@P) * 2.0;              // Outward burst
@v += curlnoise(@P * 0.5 + @Time) * 1.0; // Turbulence

// ── STEP 2: Pyro Solver Settings (DOP Network) ───────
// These are the key parameters to art-direct fire/smoke.
// Set these on the Pyro Solver DOP node.

// COMBUSTION:
//   burn_rate = 0.8         High = fast burn, Low = lingering
//   flame_height = 1.5      How tall flames reach
//   temperature_diffusion = 0.02   How fast heat spreads

// DISSIPATION (smoke fading):
//   density_dissipation = 0.01    Low = thick smoke, High = wispy
//   temperature_cooling = 0.5     How fast fire cools to smoke

// TURBULENCE (organic detail):
//   turbulence_strength = 0.5     Swirling motion intensity
//   turbulence_scale = 0.3        Size of swirl features
//   turbulence_speed = 0.2        How fast swirls evolve

// BUOYANCY:
//   buoyancy_direction = {0, 1, 0}   Upward
//   buoyancy_strength = 1.0          How strongly heat rises

// RESOLUTION:
//   division_size = 0.05    Grid cell size (smaller = more detail)
//   substeps = 2            Temporal accuracy (increase for fast motion)

// ── STEP 3: Volume Shader (Material) ─────────────────
// RenderMan / Mantra / Arnold volume shader settings:
//
// Fire: Emit light based on temperature field
//   emission_color = blackbody(temperature * 6500)
//   emission_intensity = fit(temperature, 0.1, 1.5, 0, 50)
//
// Smoke: Scatter light through density field
//   scatter_color = {0.3, 0.3, 0.35}
//   density_multiplier = 2.0
//   shadow_density = 4.0 (smoke casts dense shadows)

// ── PRO TIPS ─────────────────────────────────────────
// 1. Low-res sim first (division_size=0.2), then increase
// 2. Cache to disk ALWAYS (.vdb format)
// 3. Use "Gas Resize Fluid Dynamic" to auto-grow bounds
// 4. Upres pass: sim low-res, add detail in post
// 5. Real fire reference is ESSENTIAL — study slow-mo videos

# ── Nuke/After Effects Compositing Workflow ───────────
# This script demonstrates the compositing pipeline
# for integrating CG renders over live-action plates.

# ── Render Passes (AOVs) Required ─────────────────────
# Modern VFX compositing uses SEPARATE render passes
# (called AOVs — Arbitrary Output Variables) that give
# the compositor complete control in post.

RENDER_PASSES = {
    "beauty":       "Final combined render (fallback)",
    "diffuse":      "Flat color without reflections/shadows",
    "specular":     "Reflections and highlights only",
    "emission":     "Self-illuminating elements (fire, screens)",
    "shadow":       "Shadow contribution (multiply over plate)",
    "ambient_occ":  "Contact shadows in crevices",
    "normal":       "Surface direction (for relighting in comp)",
    "depth":        "Z-depth (for depth-of-field, fog)",
    "motion":       "Per-pixel motion vectors (motion blur)",
    "matte":        "Object isolation masks (ID mattes)",
    "crypto":       "Cryptomatte (per-object/material masks)",
}

print("📦 Required Render Passes for Film-Quality VFX:")
print("=" * 55)
for pass_name, description in RENDER_PASSES.items():
    print(f"  {pass_name:15s} → {description}")

# ── Compositing Layer Order ───────────────────────────
COMP_STACK = """
┌─────────────────────────────────────────────────────┐
│           COMPOSITING STACK (Bottom → Top)          │
├─────────────────────────────────────────────────────┤
│                                                     │
│  Layer 1: PLATE (Live-action footage)              │
│    ├── Lens distortion correction                  │
│    ├── De-grain (noise removal)                    │
│    └── Color correction to linear colorspace       │
│                                                     │
│  Layer 2: SHADOW PASS (Multiply blend)             │
│    └── CG shadows cast onto real environment       │
│                                                     │
│  Layer 3: CG BEAUTY (Over blend with alpha)        │
│    ├── Color match to plate (exposure, white bal.)  │
│    ├── Edge integration (light wrap / spill)       │
│    └── Contact shadows (AO pass)                   │
│                                                     │
│  Layer 4: ATMOSPHERE (Add/Screen blend)            │
│    ├── Depth fog (from Z-depth pass)               │
│    ├── Volumetric light rays                       │
│    └── Lens flares (if applicable)                 │
│                                                     │
│  Layer 5: EFFECTS (Add blend)                      │
│    ├── Fire/sparks emission                        │
│    ├── Light interaction on plate (re-light)       │
│    └── Motion blur (from motion vectors)           │
│                                                     │
│  Layer 6: FINAL (Output)                           │
│    ├── Re-grain (match original film grain)        │
│    ├── Lens distortion (re-apply)                  │
│    ├── Color grade                                 │
│    └── Output: 16-bit EXR or DPX                   │
│                                                     │
└─────────────────────────────────────────────────────┘
"""
print(COMP_STACK)

# ── Color Space Management ────────────────────────────
print("🎨 Color Space Rules (ACES Pipeline):")
print("  • Work in Linear (scene-referred) colorspace")
print("  • Textures: Convert sRGB → ACEScg on import")
print("  • Renders: Output as 32-bit EXR (full dynamic range)")
print("  • Display: Apply ACES Output Transform for viewing")
print("  • Delivery: Rec.709 (web/TV) or DCI-P3 (cinema)")