trichrome

Merge red/green/blue-light RAW triplets into 16-bit linear TIFFs, then (optionally) delete the source RAWs.

You shoot one static scene three times — once under a pure red light, once green, once blue. This tool takes every consecutive triplet of RAWs and builds one full-colour image from them by keeping each frame's own colour channel and throwing the other two away. The result is written as an archival linear TIFF: the raw channel combination and nothing else, ready for whatever converter you grade in.

Extracted from FreeCCR's 3-way RGB merge, stripped of the GUI, catalog, crop and colour pipeline. Standalone — it is not intended to merge back.

Install

pip install -e .            # needs numpy, rawpy, tifffile, imagecodecs

Python 3.9+.

Use

trichrome list ./shoot                        # show the triplet grouping first
trichrome merge ./shoot                       # write TIFFs, keep the RAWs
trichrome merge ./shoot --out ./merged        # write them somewhere else
trichrome merge ./shoot --dry-run             # plan only, decode nothing
trichrome merge ./shoot --delete-originals    # destructive; asks first

$ trichrome merge ./shoot
2 triplet(s) · light order RGB · demosaic (full resolution)
[1/2] img001.arw + img002.arw + img003.arw  ->  img001_RGB.tif
[2/2] img004.arw + img005.arw + img006.arw  ->  img004_RGB.tif

wrote /shoot/img001_RGB.tif  (6024x4024, uint16)
wrote /shoot/img004_RGB.tif  (6024x4024, uint16)

2 merged, 0 failed

Options

Flag	Meaning
-r, --recursive	descend into subfolders of an input folder
-o, --out DIR	write TIFFs here instead of beside each triplet's first frame
--suffix S	output name is <first frame><S>.tif (default _RGB)
--order RGB	which light each frame of a triplet was shot under, in filename order — BGR if you shot blue first
--demosaic / --photosite	see Two ways to extract a channel below (default --demosaic)
--delete-originals	permanently delete each triplet's RAWs once its TIFF verifies
-y, --yes	skip the confirmation prompt
-n, --dry-run	show what would happen; decode, write and delete nothing

Files are ordered by filename (directory ignored, case-insensitive) and taken three at a time. The batch must be a multiple of three and every file must be a supported RAW, or nothing runs.

Supported RAW: .cr3 .cr2 .nef .arw .dng .rw2 .orf .raf .srw .pef .3fr.

How the merge works

Each frame contributes exactly one channel — R from the red-light frame, G from green, B from blue — scaled to 16-bit by 65535 / white_level. No white balance, no colour matrix, no gamma, no tone curve, no inversion. The output is camera-native linear RGB.

Bayer (RGGB) sensors — two ways to extract a channel:

• --demosaic (default): a LINEAR (bilinear) demosaic at full sensor resolution, then take this frame's channel. Bilinear interpolates each colour plane only from its own photosites, so there is still no inter-channel crosstalk.
• --photosite: no demosaic at all. The RAW mosaic is read directly and only this colour's photosites are taken, at half sensor resolution (one site per 2×2 quad — that is this mode's full resolution). R and B use their single site per quad; green averages its two same-colour sites, which keeps the green SNR. Black levels are subtracted per site.

Monochrome sensors have no CFA, so there is nothing to demosaic: the whole grayscale frame is that frame's channel, at full resolution. Both modes decode identically. A monochrome sensor is in fact the ideal trichrome sensor — no wasted photosites, full resolution, zero crosstalk by construction.

X-Trans and 4-colour (CYGM/RGBE) sensors are rejected. All three frames of a triplet must be the same sensor type.

The output file

A 16-bit RGB TIFF, deflate-compressed with Predictor 2 (lossless, universally readable, ~1.3–2× smaller than plain deflate on 16-bit continuous-tone data), no ICC profile, linear.

Its Software tag carries FreeCCR's merge marker (FreeCCR:3-way-RGB-merge-linear-v1), so a file this tool writes opens in FreeCCR as a normal image even while FreeCCR's own 3-way merge mode is on.

Deleting the originals

--delete-originals is destructive and off by default. When it is on:

• A RAW is deleted only after its replacement TIFF exists on disk and reads back as a valid uint16 RGB image of the expected size.
• If a triplet fails, none of its sources are deleted — and neither are those same files if another triplet referenced them. A frame's only copy is never orphaned.
• A failed triplet leaves no partial TIFF behind.
• Interrupting deletes nothing and removes the TIFFs written so far.
• An existing file is never overwritten; a numeric suffix is added instead.
• Without a terminal (a script, a pipe) the confirmation prompt declines rather than guessing — pass --yes to mean it.

As a library

from trichrome import collect_raw_files, plan_jobs, run_jobs

jobs = plan_jobs(collect_raw_files(["./shoot"]), out_dir="./merged")
summary = run_jobs(jobs, demosaic=True, delete_originals=False)
print(len(summary.written), "merged;", len(summary.failures), "failed")

merge_raw_channels(sources, demosaic=True, light_order="RGB") returns (uint16 HxWx3 array, (H, W)) if you just want the pixels.

Tests

pip install -e ".[dev]"
pytest

The decode is monkeypatched, so the suite needs no RAW files and runs in about a second. It covers the pure merge maths (CFA phase slicing, white-level scaling, light order) and every deletion-safety rule above.

Contributing

main takes no direct pushes; changes land through a pull request, and the tracked hooks/ directory needs git config core.hooksPath hooks once per clone. See CONTRIBUTING.md.