RH Hybrid Dye Engine

🎞️ Spectral Film Chemistry & Additive Digital Fusion

RH Hybrid Dye Engine is an advanced film emulation and color grading tool that runs two completely distinct image processing pipelines in parallel and intelligently blends them based on scene exposure. By fusing a rigorous spectral film dye simulation with a pristine digital tone curve, it delivers the thick, rich colors of celluloid in the shadows and the clean, unbound highlights of modern digital sensors.

It features Multi-Layer Emulsion Modeling and DIR Coupler Interimage Effects, giving you authentic color separation and organic density without ever muddying your highlights or fighting your camera's dynamic range.

---

📖 Table of Contents

• 1. Core Concept
• 2. Multi-Layer & Blend Controls
• 3. Color & Physical Characteristics
• 4. Dye Tuning
• 5. Visual Aids
• 6. Under the Hood — Dual-Path Architecture
• 7. Node Placement

---

1. Core Concept 💡

The engine splits the incoming scene-referred log signal into two parallel paths:

• Path A (Subtractive Spectral Dye Model): Simulates the physical absorption of Cyan, Magenta, Yellow, and Silver dyes across 36 distinct wavelengths of light (380nm–730nm), exactly how physical film absorbs photons.
• Path B (Additive Digital Tone Curve): Processes the image using a mathematically pristine, additive digital curve that preserves exact RGB ratios and applies a clean highlight roll-off without color twisting.

A Density-Dependent Blend Controller seamlessly crossfades between them: as density increases (shadows and midtones), the image is driven by the rich Spectral Dye model; as density decreases (bright highlights), it smoothly transitions into the clean digital tone curve.

---

2. Multi-Layer & Blend Controls 🎛️

🌐 Global Blend

Master mix slider between the incoming untouched log signal and the processed hybrid output.

• Default: 1.0

🎞️ Fast-Slow Ratio

Balances the response between the dual sublayers inside the emulsion:

• Default: 0.3
• Fast Sublayer: Simulates large, highly sensitive grains for soft, smooth highlight roll-offs.
• Slow Sublayer: Simulates fine, low-sensitivity grains for thick, punchy shadow contrast.
• Higher values favor softer highlights; lower values produce denser, crunchier shadows.

🧪 DIR Strength

Controls the intensity of the Development Inhibitor Releasing (DIR) couplers.

• Default: 0.5
• As one dye layer builds density during development, it chemically inhibits adjacent layers. This cross-channel subtraction creates the legendary color separation of cinema (e.g., preventing reds from becoming muddy).

🌐 Film-Digital Blend

Determines the global maximum dominance of the film chemistry path versus the clean digital path.

• Default: 0.6
• 1.0 = 100% subtractive film chemistry; 0.0 = 100% additive digital curve.

🎚️ Blend Transition

Shapes the steepness and pivot of the crossfade curve between the filmic shadows and digital highlights.

• Default: 1.0

📉 Film Speed EV

Push or pull the effective exposure of the signal before it hits the spectral dye layers.

• Default: 0.0
• Range: -3.0 to +3.0 EV

📈 Contrast & D-Max

• Contrast: Adjusts the global contrast curve across both pipelines.
• D-Max: Sets the maximum density ceiling for the physical dye absorption model.

---

3. Color & Physical Characteristics 🎨

🎨 Color Amount

Global saturation and dye intensity scale.

• Default: 1.0

🌫️ Fog Level

Raises the base density of the film emulsion (D-Min).

• Default: 0.02
• Lifts deep shadows and reduces overall macro-contrast to simulate vintage or un-blipped film stock.

🩸 Silver Retention

True physical bleach-bypass emulation.

• Default: 0.0
• Retains spectrally accurate neutral silver halide density in the emulsion before integration, delivering gritty contrast and organic desaturation.

🔴 Halation

Simulates light scattering on film backing layers.

• Default: 0.0
• Physically reduces cyan dye generation in bright high-contrast boundaries, producing a natural red/orange halation glow without artificial blurring.

---

4. Dye Tuning 🧪

These sliders allow you to physically tune the spectral absorption multipliers for each individual dye layer before CIE color matching integration.

---

5. Visual Aids 👁️

📊 Show Curve

Overlays a live graph showing the physical transfer curves directly on your viewer.

• Orange Line: Subtractive Path A (Physical Dye density curve).
• Green Line: Additive Path B (Digital tone curve).
• Gray Line: Linear 1:1 reference.
• Use this diagnostic tool to visually balance your Fast-Slow Ratio, Contrast, and Blend Transition right in the viewer!

---

6. Under the Hood — Dual-Path Architecture 🔬

Multi-Layer Density & DIR Couplers

Instead of simple RGB matrices, the signal is converted into density via dual-sigmoid functions across Fast and Slow sublayers. The engine then applies chemical inhibition equations where heavy dye development in one primary layer subtracts development in adjacent layers, preventing color mud and expanding color separation organically.

36-Band Spectral Integration

The CMY densities are evaluated across 36 distinct spectral wavelengths (380nm to 730nm in 10nm increments). The engine multiplies standard Illuminant D65 lighting by the combined spectral transmittance of the dyes (and silver density), collapsing the result back to RGB using standard CIE 1931 color matching functions.

Von Kries Adaptation

Because thick physical dyes naturally shift the white balance of shadows and midtones, a Von Kries chromatic adaptation is applied after spectral integration. This anchors the neutral axis, ensuring that pure greys and deep blacks remain perfectly balanced without unwanted color casts.

Density-Weighted Crossfade

To protect highlights from the cyan twists and saturation compression common in subtractive film emulation, the engine evaluates pixel luminance and smoothly crossfades into a Pegtop additive sigmoid curve in the highlights.

---

7. Node Placement 🗂️

Expected pipeline:

• Scene-referred log input (e.g., DaVinci Wide Gamut / Intermediate or Arri LogC4).

Recommended placement:

• Primary Look Anchor: Place near the end of your grading node tree, after primary balance and exposure corrections, but before any display output transform (e.g., Kodak 2383 Print LUT or Rec.709 ODT).
• Because the blending between Film and Digital paths is density-dependent, feeding a properly exposed signal into this node ensures the shadows receive thick film density while highlights remain clean and modern.