Full-Spectrum Simulation Engine

Overview

filmr 0.7.0 introduces a physics-based full-spectrum simulation engine that propagates light wavelength-by-wavelength through the film's multi-layer structure. This replaces the 3×3 matrix approximation used in Fast mode with a physically accurate model.

Dual Simulation Modes

Mode	Method	Use Case
Fast	3×3 spectral matrix	Real-time preview
Accurate	Per-wavelength per-layer propagation	Final develop (Develop button)

Physical Model

Light Propagation

Light enters the film from the top and traverses each layer sequentially:

Fresnel reflection at each interface: $R = \left(\frac{n_1 - n_2}{n_1 + n_2}\right)^2$
Beer-Lambert absorption within each layer: $I_{out} = I_{in} \cdot e^{-\alpha \cdot d}$
Scattering loss: modeled as additional attenuation coefficient
Emulsion exposure: absorbed energy (not scattered) is recorded per channel

Backward Pass (Halation)

After traversing all layers, light reflects off the film base (Fresnel at base/air interface) and propagates back up through the stack. This physically models halation — the reddish glow around highlights caused by base reflection.

Layer Structure

A typical colour negative film stack:

Incident light →
  [Overcoat]           n=1.50, transparent
  [Blue Emulsion]      n=1.53, absorbs ~450nm → Yellow dye
  [Yellow Filter]      n=1.52, blocks blue from lower layers
  [Green Emulsion]     n=1.53, absorbs ~545nm → Magenta dye
  [Interlayer]         n=1.50, spacer
  [Red Emulsion]       n=1.53, absorbs ~640nm → Cyan dye
  [Anti-Halation]      n=1.50, absorbs residual light
  [Base (PET)]         n=1.65, 125µm substrate

Each layer has:

Thickness (µm)
Refractive index (for Fresnel calculation)
Spectral absorption (81 bins, 380–780nm, 5nm steps)
Scattering coefficient (Mie/Rayleigh in emulsion)

Interlayer Interimage Effect

During development, one layer's chemical byproducts inhibit adjacent layers (DIR coupler effect). This is modeled as a 3×3 inhibition matrix applied after H-D curve density calculation:

$$D_i' = D_i + \sum_j \text{inhibition}[i][j] \cdot D_j$$

Off-diagonal values are negative (suppression), improving colour separation.

Spectral Data

CIE 1931 2° Standard Observer (x̄, ȳ, z̄) — ISO 11664-1
CIE Standard Illuminant D65 — ISO 11664-2
sRGB camera sensitivities derived from XYZ CMF × XYZ→sRGB matrix, D65-balanced

All data: 380–780nm, 5nm steps, 81 bins.

Pipeline (Accurate Mode)

sRGB input
  → Linearize
  → Light Leak (additive)
  → Halation (threshold + blur)
  → Per-pixel spectral propagation:
      uplift(R,G,B) → spectrum × D65 → propagate(layer_stack)
      → integrate → normalize → × exposure_time
  → Scattering spatial diffusion (Gaussian blur from layer scatter)
  → White balance + warmth
  → log₁₀ → H-D curves + colour matrix
  → Interlayer inhibition
  → MTF blur
  → Grain
  → Density → Transmission → sRGB output

Verification

69 tests verify the engine:

8 physical property tests (linearity, energy conservation, layer order, etc.)
6 strict physics tests (exact Beer-Lambert, Fresnel, scattering, energy budget)
17 per-stage model correctness tests
4 end-to-end integration tests
1 Fast/Accurate consistency test

Filmr Knowledge Base