Custom Crosstalk Matrices

The Separation control in the Lab sidebar runs spectral crosstalk correction. NegPy ships with one built-in matrix (Default), but you can drop in your own — calibrated per film stock or scanner — without touching any code.

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What it does

A color negative's three dye layers are not spectrally pure: the cyan, magenta and yellow dyes each leak a little density into the channels they shouldn't affect. The result is muddy, low-separation color. Crosstalk correction unmixes the channels by multiplying the per-pixel density vector by a 3×3 matrix.

The math, per pixel:

density      = -log10(rgb)
density_out  = M · density
rgb_out      = 10^(-density_out)

M is your 3×3 matrix. The Separation slider blends it with the identity matrix and row-normalizes the result, so Separation = 1.0 is a no-op and higher values push toward the full matrix:

M_applied = I · (1 - strength) + M · strength      # strength = Separation - 1.0
M_applied = M_applied / row_sums(M_applied)        # each row normalized to sum 1

Because every row is renormalized to sum to 1, a uniform gray stays gray — the matrix only redistributes color differences between channels.

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Reading the matrix

The matrix is row-major. Rows are output channels, columns are input channels:

	in R	in G	in B
out R	1.00	-0.05	-0.02
out G	-0.04	1.00	-0.08
out B	-0.01	-0.10	1.00

• The diagonal stays near 1.0 (each channel keeps its own density).
• Off-diagonal terms are usually small and negative — they subtract the contamination one layer leaks into another.
• Keep rows roughly summing near 1.0; large deviations are fine (they get normalized) but make the effect harder to reason about.

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File format (TOML)

Put .toml files in your user folder:

<Documents>/NegPy/crosstalk/

On first run NegPy copies the bundled gallery (crosstalk/ in the repo) here, so you start with some ready-made profiles. Each file is one matrix:

# my_film.toml
name = "Kodak Gold 200"        # optional display name; falls back to the filename
matrix = [                     # 3x3, row-major (out R/G/B × in R/G/B)
  [ 1.00, -0.05, -0.02],
  [-0.04,  1.00, -0.08],
  [-0.01, -0.10,  1.00],
]

• matrix is required: exactly 3 rows of 3 numbers.
• name is optional. If omitted, the dropdown shows the filename (without .toml).
• Malformed files (wrong shape, non-numbers, bad TOML) are silently skipped.
• The name Default is reserved for the built-in matrix and ignored if reused.

The chosen matrix is baked into the edit when you select it, so saved edits and presets stay reproducible even if you later move or delete the file.

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Using it

1. Drop your .toml into <Documents>/NegPy/crosstalk/.
2. Open the Lab sidebar → the crosstalk dropdown under COLOR. New files appear the next time the panel syncs (e.g. switching photos); restart if you don't see it.
3. Pick your profile and raise Separation above 1.0 to apply it.
4. Pick Default to revert to the built-in matrix.

Crosstalk is a color operation and is hidden in B&W mode.

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Contributing a matrix

Calibrated a film stock or scanner? Add your .toml to the repo's crosstalk/ folder and open a PR — bundled matrices ship to all users on the next release. See crosstalk/README.md.