OpenFCD

Fast Checkerboard Demodulation for free-surface reconstruction — desktop app + CLI.

Reconstructs the free-surface height field η(x, y, t) from high-speed video of a checkerboard pattern viewed through water, using the Wildeman / Moisy synthetic schlieren FCD algorithm. Designed for thin-film wave studies, sloshing experiments, mm-scale swimming-robot wakes, and any free-surface experiment that films a patterned backdrop through a transparent fluid.

Status

v0.0.2 — early release. Core pipeline and GUI are functional; expect rough edges. Contributions welcome.

What's new in 0.0.2

• Frame management — Shift multi-select + batch disable / enable / compute on frames; FramePickerDialog excludes disabled frames and the reference frame from selection; SimTree disabled-frame state resets on project switch.
• QC CLI — openfcd qc-summary / noise-floor / sensitivity subcommands for per-frame QC verdicts, flat-water noise-floor stats, and calibration-sensitivity analysis.
• GUI polish — HiDPI-aware preview colorbar at native resolution; profile-composite heatmap & waveform x-axes aligned; unified η color range across EtaMap / EtaHeatmap / ProfileScene / ProfileComposite; Qt palette theming; annotation flushed to disk before each run.

Features

• End-to-end desktop GUI (PyQt6) with a STAR-CCM+-style 3-pane layout: simulation tree · scene tabs · properties.
• First-class CLI (openfcd run path.ofcd) — GUI and CLI share the same pipeline, producing byte-identical results.h5.
• Per-frame debug mode — compute a single frame, tune parameters live with a slider, re-compute without re-running the whole sequence.
• Robust to imperfect reference frames:
  • Automatic carrier-period scale normalization when the reference was captured at a slightly different zoom
  • Optional spatial high-pass to remove non-physical low-frequency drift from cross-session ref/def pairs
• Project files on disk — *.ofcd/ directories contain a git-friendly project.yaml, snapshot runs under runs/{id}/, and annotations under annotations/default.json.
• HDF5 results with SWMR so scripts can tail the output while a run is in progress.
• Session resume on re-open — re-opening a previously-computed .ofcd lands directly on the last saved state: frames are filtered, reference is set, the most recent run's η overlay is restored, and the annotation ROI/mask is populated. Stale last_run_id self-heals when the result file is missing. Use File → Re-import Frames… to re-run the import flow (blocked while a run is active).
• Window/splitter geometry persisted across sessions via QSettings.

Installation

Requires Python 3.11+.

# CLI only
pip install openfcd

# CLI + desktop GUI (PyQt6)
pip install 'openfcd[gui]'

Development install

git clone https://github.com/xunhe730/OpenFCD.git
cd OpenFCD
pip install -e '.[gui,dev]'   # editable, with GUI + test/lint extras

Quick start

GUI

python -m openfcd.gui
# or
python -m openfcd

Workflow:

1. File → New Project → point at a folder of image frames, set the checker cell side length (mm), window thickness, fluid depth.
2. Click a frame in the tree → Set as Reference (pick a flat-water frame captured in the same session).
3. Draw an ROI + optional occlusion polygon on the preview.
4. Click Compute This Frame to preview one frame.
5. Tune the Highpass σ slider if the result shows large-scale drift.
6. Click Run All Frames when satisfied.

CLI

# Create a new project
openfcd new myproject --image-folder /path/to/frames --pattern 'Img*.jpg'

# Edit myproject.ofcd/project.yaml for geometry & process parameters,
# then run
openfcd run myproject.ofcd

# Results land under myproject.ofcd/runs/{timestamp}/results.h5

# Render QC panels for a frame or for all frames in a run
openfcd qc-summary myproject.ofcd --run run-YYYYMMDD-HHMMSS --frame 0 -o qc.png
openfcd qc-summary myproject.ofcd --run run-YYYYMMDD-HHMMSS --all --output-dir qc/

# Estimate flat-water noise floor and calibration sensitivity
openfcd noise-floor myproject.ofcd --from-run run-YYYYMMDD-HHMMSS -o noise_floor.json
openfcd sensitivity myproject.ofcd --run run-YYYYMMDD-HHMMSS --frame 0 -o sensitivity.json

Algorithm notes

FCD measures η by comparing the apparent displacement of a checkerboard pattern seen through the water surface. The algorithm steps:

1. Flatfield (openfcd.core.flatfield) — divide out smooth illumination variation using a Gaussian background.
2. Carrier detection (openfcd.core.fcd.calculate_carriers) — find the two dominant spatial-frequency peaks of the checkerboard in k-space.
3. Scale normalization (openfcd.core.registration) — if the reference and deformed frames were shot at slightly different zooms, rescale the reference to match the deformed frame's carrier period. Triggers at 0.5 % mismatch.
4. Occlusion inpaint — mask the robot/object and replace with a synthesized carrier field so the FCD demodulation sees continuous texture.
5. Cosine taper — soften image boundaries to suppress FFT edge artifacts.
6. Demodulation + phase unwrap (openfcd.core.fcd.fcd) — extract the pixel-displacement field (u, v) at each carrier.
7. Poisson integration (fftinvgrad) — integrate (u, v) to get the apparent depth, then calibrate to mm using the optical stack geometry.
8. Post-processing — plane detrend, filament suppression, optional high-pass filter, edge NaN margin.

geometry.pattern_period_mm is the legacy project-file key, but its physical meaning is checker_cell_mm: the side length of one single checkerboard cell, not the full black-white cycle. The carrier calibration assumes |k_phys| = π√2 / checker_cell_mm.

Each computed frame stores a QC verdict (PASS, WARN, or FAIL) plus the stats and QC maps used to reach it. The default warning/fail thresholds are kept under project.qc: saturated ratio 0.001, invalid ratio 0.20, carrier amplitude median minimum 1e-6, normalized Poisson/curl residual warning 0.10 and fail 0.20, and phase warning at 0.7π / fail at 0.9π. These are conservative starting points and should be calibrated against flat-water data for each optical setup.

Physical correctness depends on the reference frame. Best results come from a flat-water reference captured in the same acquisition session as the deformed frames. For cross-session references the high-pass filter (slider in the GUI, process.highpass_sigma_px in project.yaml) removes low-frequency drift.

Project structure

openfcd/
├── core/          # FCD algorithm (fcd, flatfield, inpaint, mask, …)
├── geometry/      # Optical-stack calibration
├── io/            # project.yaml schema, HDF5 results, annotations
├── pipeline/      # Stage protocol + orchestration
├── cli/           # Typer CLI (new / run / replay / project)
└── gui/           # PyQt6 GUI (main window, panels, scenes, renderers)

Development

# Run tests
pytest

# Lint
ruff check openfcd tests

Golden-comparison tests (tests/golden/) require a local BOS reference dataset. Set OPENFCD_BOS_ROOT to the dataset root to enable them; otherwise they are skipped.

License

MIT — see LICENSE for full text.

Acknowledgements

The FCD method is due to Moisy, Rabaud & Salsac (2009), "A synthetic Schlieren method for the measurement of the topography of a liquid interface", and Wildeman (2018), "Real-time quantitative Schlieren imaging by fast Fourier demodulation of a checkered backdrop". This project's reference implementation follows Wildeman's pyfcd approach, extended with a PyQt6 GUI, CLI, and engineering polish for experimental use.