OpenPIV

Overview

OpenPIV (Open Particle Image Velocimetry) analyzes fluid flow from PIV image pairs. It covers preprocessing, cross-correlation, vector validation, outlier replacement, smoothing, and scaling to physical units.

Targets openpiv 0.26.1. Synthetic checks ran on Python 3.13, NumPy 2.5.3, SciPy 1.18.1, scikit-image 0.26.0, and Matplotlib 3.11.2. Rust was not installed; use backend="scipy" for the tested path. See review evidence.

When to use

Use this skill when working with experimental PIV or flow-visualization image pairs: measuring 2D velocity fields, tuning interrogation-window parameters, validating vectors, or deriving vorticity, strain rate, and turbulence statistics. For simulating flow rather than measuring it, use a CFD skill instead.

Quick Start

Install OpenPIV:

uv pip install openpiv

# Pin it when the analysis needs to be reproducible -- this is the version every
# snippet below was checked against.
uv pip install "openpiv==0.26.1"

Run PIV analysis on an image pair:

import numpy as np
from openpiv import tools, pyprocess, validation, filters, scaling

frame_a = tools.imread("image_a.bmp")
frame_b = tools.imread("image_b.bmp")

# Always returns (u, v, s2n); s2n is all NaN if sig2noise_method=None.
u, v, s2n = pyprocess.extended_search_area_piv(
    frame_a.astype(np.float32),
    frame_b.astype(np.float32),
    window_size=32,
    overlap=12,
    dt=0.02,
    search_area_size=38,
    correlation_method="linear",   # circular also supports extended search
    normalized_correlation=True,
    backend="scipy",
    sig2noise_method="peak2peak",
)

x, y = pyprocess.get_coordinates(
    image_size=frame_a.shape,
    search_area_size=38,
    overlap=12,
    center_on_field=False,         # matches sliding-window positions
)

# flags is a boolean array: True marks a spurious vector.
flags = validation.sig2noise_val(s2n, threshold=1.05) | ~np.isfinite(s2n)
flags |= ~np.isfinite(u) | ~np.isfinite(v)
u, v = filters.replace_outliers(u, v, flags, method="localmean", max_iter=3, kernel_size=2)

# Scale, then convert to right-handed image-boundary coordinates.
x, y, u, v = scaling.uniform(x, y, u, v, scaling_factor=96.52)
y, v = frame_a.shape[0] / 96.52 - y, -v

tools.save("vectors.txt", x, y, u, v, flags)

Or use the bundled CLI, which also checks inputs and preserves masks and processing parameters:

python skills/openpiv/scripts/runner.py \
    --image frame_a.bmp --image frame_b.bmp --output_dir results --verbose

Core Concepts

PIV Fundamentals

Particle Image Velocimetry is an optical method for measuring fluid velocity by tracking illuminated tracer particles between two images.

Process flow:

1. Capture an image pair (frame_a, frame_b) separated by a known time dt.
2. Divide the images into interrogation windows.
3. Cross-correlate matching windows to find peak displacement.
4. Validate vectors (signal-to-noise, global range, local median).
5. Replace spurious vectors with interpolated values.
6. Scale pixel displacements to physical units.

Interrogation Window Parameters

**window_size** — correlation window in pixels (typically 16–128). Larger windows give better correlation but coarser spatial resolution.

**overlap** — pixels shared between adjacent windows (typically 50–75% of window_size). Higher overlap raises vector density and cost, but adjacent vectors become correlated rather than independent.

**search_area_size** — the window searched in the second frame. Must be ≥ window_size; a few pixels larger accommodates larger displacements. Both "circular" and "linear" support extended search. The CLI chooses zero-padded "linear" with normalized_correlation=True for it; this does not recover arbitrary displacement or eliminate spurious peaks. Grid stride is search_area_size - overlap, so overlap must be smaller than the search area.

Rules of thumb: keep the largest displacement under about a quarter of window_size, and aim for 5–10 particles per window.

Signal-to-Noise Ratio

s2n measures how distinct the correlation peak is. sig2noise_method controls how it is computed — "peak2mean" (the function default) or "peak2peak". The two are on different scales, so a threshold tuned for one is meaningless for the other. Typical peak2peak thresholds are 1.05–1.3.

flags = validation.sig2noise_val(s2n, threshold=1.05)
# flags is bool: True == spurious. `~flags` selects the good vectors.

Common Operations

Dynamic Masking

Masking lives in openpiv.preprocess and returns (image, mask). Use floating-point copies to avoid unsigned subtraction artifacts. Inspect masks on both frames before analysis.

from openpiv import preprocess

# Intensity masking uses Otsu; the threshold argument is for edges.
frame_a_masked, mask_a = preprocess.dynamic_masking(
    frame_a.astype(np.float64), method="intensity", filter_size=7, threshold=0.005
)
frame_b_masked, mask_b = preprocess.dynamic_masking(
    frame_b.astype(np.float64), method="intensity", filter_size=7, threshold=0.005
)

Preserve the union mask_a | mask_b as excluded physical regions. The CLI samples it at interrogation centers, saves it, and keeps those velocities NaN after replacement. Windows straddling an object can still be biased: inspect/dilate masks for the experiment. In 0.26.1 method="edges" indexes with uint8 instead of bool, potentially corrupting the image or failing. The CLI refuses it; use intensity masking or a verified boolean mask in a custom workflow.

Multi-Pass Processing

Multi-pass (window deformation) lives in openpiv.windef, driven by a PIVSettings dataclass. pyprocess has no multi-pass entry point.

import numpy as np
from openpiv import scaling, windef

settings = windef.PIVSettings(
    windowsizes=(64, 32, 16), overlap=(32, 16, 8), num_iterations=3,
    backend="scipy", sig2noise_method="peak2peak", sig2noise_threshold=1.05,
)

x, y, u, v, flags = windef.simple_multipass(
    frame_a.astype(np.float32), frame_b.astype(np.float32), settings
)

# Output is in PIXELS PER FRAME -- convert yourself. scaling.uniform only divides
# by scaling_factor, so apply dt separately.
dt = 0.02
x, y, u, v = scaling.uniform(x, y, u, v, scaling_factor=96.52)
u, v = u / dt, v / dt

simple_multipass validates, replaces outliers, fills remaining NaNs with zeros, and transforms coordinates. Keep its flags: repaired/zero-filled output is not an independent measurement. Always pass settings: its no-settings path has two window sizes but three iterations in 0.26.1.

Units trap: simple_multipass and its alias multigrid_windef ignore settings.dt and settings.scaling_factor; convert their arrays as above. Batch windef.piv(settings) applies both before saving — do not scale its files again. The simple wrapper does not honor all batch preprocessing/output switches.

For control over individual passes, windef.first_pass and windef.multipass_img_deform are the lower-level building blocks.

Validation and Post-Processing

Validation Methods

Every validator returns a boolean array where True marks a spurious vector. Also reject nonfinite u, v, and s2n: sig2noise_val alone does not flag NaN.

# Signal-to-noise
flags = validation.sig2noise_val(s2n, threshold=1.05)

# Global range -- takes (min, max) TUPLES, positionally or as u_thresholds/v_thresholds.
flags = validation.global_val(u, v, (-300, 300), (-300, 300))

# Local median -- u_threshold and v_threshold are REQUIRED; size is the neighbourhood half-width.
flags = validation.local_median_val(u, v, u_threshold=30.0, v_threshold=30.0, size=1)

# Combine with boolean OR (not np.maximum -- these are bool arrays).
flags = (
    validation.sig2noise_val(s2n, threshold=1.05)
    | validation.global_val(u, v, (-300, 300), (-300, 300))
    | validation.local_median_val(u, v, u_threshold=30.0, v_threshold=30.0)
)

**Set these thresholds in the units of u and v, not in pixels per frame.** extended_search_area_piv divides by dt, so with dt=0.02 a 3 px/frame displacement arrives as 150 px/s. The thresholds above suit that case; the (-30, 30) figure that PIV literature and PIVSettings.min_max_u_disp use is a px/frame limit, and applying it to px/s output rejects the entire field. Correlate with dt=1 to validate displacements before conversion, or adjust thresholds to the actual velocity units (including both time and calibration factors).

Outlier Replacement

u, v = filters.replace_outliers(
    u, v, flags, method="localmean", max_iter=3, tol=1e-3, kernel_size=2
)

method accepts "localmean", "disk", or "distance"; an invalid name raises ValueError when filling missing cells. Use kernel_size>=2 for "distance": size 1 truncates all neighbor weights to zero in 0.26.1 and leaves holes. Replacement fills the flagged positions with interpolated values — if you then overwrite them with NaN, the replacement was wasted. Choose one or the other:

# Keep flagged vectors out of the analysis entirely, instead of interpolating them.
u = np.where(flags, np.nan, u)
v = np.where(flags, np.nan, v)

Smoothing

Smoothing is openpiv.smoothn.smoothn; there is no openpiv.smooth module. It returns a tuple whose first element is the smoothed field. NaN/Inf are missing observations; supply a copy because it can modify input. Zero-filling holes first incorrectly treats them as measured zeros.

from openpiv.smoothn import smoothn

u_smooth, *_ = smoothn(np.asarray(u, dtype=float).copy(), s=0.5)  # larger == smoother
v_smooth, *_ = smoothn(np.asarray(v, dtype=float).copy(), s=0.5)
u_smooth = np.asarray(u_smooth)

Visualization

Vector Field Plotting

display_vector_field reads a saved vectors file and calls plt.show() internally, so select a non-interactive backend for batch runs.

import matplotlib
matplotlib.use("Agg")
import matplotlib.pyplot as plt
from openpiv import tools

fig, ax = plt.subplots(figsize=(8, 8))
tools.display_vector_field(
    "vectors.txt",
    ax=ax,
    scaling_factor=96.52,   # same factor used in scaling.uniform, to map back onto the image
    scale=50,
    width=0.0035,
    on_img=False,
)
image = tools.imread("frame_a.bmp")
height, width = image.shape
ax.imshow(image, cmap="gray", origin="upper", zorder=-1,
          extent=(0, width / 96.52, 0, height / 96.52))
fig.savefig("vector_field.png", dpi=150, bbox_inches="tight")
plt.close(fig)

Set image extents explicitly: the helper's on_img=True infers them from the last vector and can stretch overlays when windows leave unused margins. The CLI plots with the actual image bounds.

Custom Visualization

import numpy as np
import matplotlib.pyplot as plt

fig, axes = plt.subplots(1, 3, figsize=(15, 5))

mag = np.sqrt(u**2 + v**2)
for ax, field, title, cmap in [
    (axes[0], mag, "Velocity Magnitude", "viridis"),
    (axes[1], u, "U Velocity", "RdBu_r"),
    (axes[2], v, "V Velocity", "RdBu_r"),
]:
    im = ax.imshow(field, cmap=cmap)
    ax.set_title(title)
    plt.colorbar(im, ax=ax)

fig.tight_layout()
fig.savefig("velocity_components.png")
plt.close(fig)

Analysis Functions

scripts/analyze.py bundles these against a params.npz written by runner.py. It infers the physical grid spacing from the saved coordinates, so the derivatives come out per unit length:

import sys
sys.path.insert(0, "skills/openpiv/scripts")
from analyze import PIVAnalyzer

piv = PIVAnalyzer("results/params.npz")
vorticity = piv.compute_vorticity()          # dv/dx - du/dy
exx, eyy, exy = piv.compute_strain()
stats = piv.compute_statistics()             # excludes flagged and masked vectors
piv.plot_vector_field(save_path="quiver.png")

The standalone forms, if you would rather compute them inline:

Vorticity

def compute_vorticity(u, v, dx=1.0, dy=None):
    """Out-of-plane vorticity dv/dx - du/dy. Pass the physical grid spacing, not 1.0."""
    dy = dx if dy is None else dy
    return np.gradient(v, dx, axis=1) - np.gradient(u, dy, axis=0)

For the single-pass extended-search grid, spacing is (search_area_size - overlap) / scaling_factor in physical units; it reduces to (window_size - overlap) / scaling_factor only when the two window sizes match. Prefer differences of the saved x and y coordinates, especially after multipass processing. Leaving dx=1.0 yields vorticity per grid cell, not per unit length. See OpenPIV coordinate generation.

Sign convention: the CLI writes y = image_height / scaling_factor - y and negates v, keeping rows in image order. Saved y decreases down the array. The standalone forms need signed dy (negative here), or use PIVAnalyzer, which reads orientation. Positive dy changes the y-derivative contribution and can even cancel real rotation.

Strain Rate

def compute_strain(u, v, dx=1.0, dy=None):
    """Return (exx, eyy, exy) of the 2D strain-rate tensor."""
    dy = dx if dy is None else dy
    du_dx = np.gradient(u, dx, axis=1)
    du_dy = np.gradient(u, dy, axis=0)
    dv_dx = np.gradient(v, dx, axis=1)
    dv_dy = np.gradient(v, dy, axis=0)
    return du_dx, dv_dy, 0.5 * (du_dy + dv_dx)

Turbulence Statistics

def compute_statistics(u, v):
    """Single-frame spatial statistics. NOT Reynolds decomposition."""
    u_prime = u - np.nanmean(u)
    v_prime = v - np.nanmean(v)
    rms_u, rms_v = np.nanstd(u_prime), np.nanstd(v_prime)
    return {
        "u_mean": np.nanmean(u),
        "v_mean": np.nanmean(v),
        "rms_u": rms_u,
        "rms_v": rms_v,
        "tke": 0.5 * (rms_u**2 + rms_v**2),
    }

Caveat: subtracting the spatial mean of one frame measures spatial variance, which equals turbulent intensity only under justified homogeneity/ergodicity assumptions. Reynolds decomposition needs an ensemble: subtract its mean field from each realization. The legacy tke key is only two-component spatial variance energy, not full turbulent kinetic energy. The analyzer excludes flagged vectors by default; compute_statistics(include_interpolated=True) includes repaired ones.

CLI Usage

# Basic run
python skills/openpiv/scripts/runner.py \
    --image img1.bmp --image img2.bmp --output_dir results --verbose

# Tuned parameters with dynamic masking
python skills/openpiv/scripts/runner.py \
    --image frame_a.bmp \
    --image frame_b.bmp \
    --output_dir results \
    --window_size 32 \
    --overlap 12 \
    --search_area 38 \
    --dt 0.02 \
    --scaling 96.52 \
    --threshold 1.05 \
    --mask dynamic \
    --mask_method intensity \
    --verbose

CLI Options

Verify an install end to end against OpenPIV's own bundled image pair:

python skills/openpiv/scripts/run_example.py --output_dir /tmp/openpiv-demo

Output Files

• vectors.txt — tab-delimited, %.4e formatted, with a # x y u v flags mask comment header
• params.npz — x, y, u, v, flags, mask, s2n, timing/calibration/window parameters, requested backend, and OpenPIV version. Coordinates use physical units; velocity uses units/s.
• vector_field.png — vector field drawn over the first frame

# x	y	u	v	flags	mask
2.1757e-01	3.5226e+00	-6.2220e-02	-2.7081e+00	0.0000e+00	0.0000e+00
4.8695e-01	3.5226e+00	-3.1587e-01	-2.9800e+00	0.0000e+00	0.0000e+00

flags is float, 0 for valid and 1 for flagged. mask=1 marks an excluded object region, independently of flags. Preserve both with the measured field.

Best Practices

Parameter Selection

1. Window size — 32×32 suits most cases. 64/128 for better correlation at coarser resolution; 16/24 for finer resolution at the cost of noise.
2. Overlap — 50–75% of window size.
3. Threshold — raise it to reject more vectors; always re-tune after switching sig2noise_method.
4. Scaling factor — calibrate against a known reference such as a calibration grid, and keep the units straight (96.52 in OpenPIV's test1 tutorial data is px/mm).

Image Quality

• Particles visible and evenly distributed, 5–10 per interrogation window
• No saturated or overexposed regions
• Minimal background noise; consider background subtraction across a run

Processing Tips

1. Start from the defaults, then tune against the vector field you get.
2. Inspect the s2n distribution — a low median means poor correlation, not a bad threshold.
3. Visualize early; obvious problems (uniform vectors, edge artifacts) show up immediately.
4. Use multi-pass (windef) for flows with large velocity gradients or displacements.
5. Mask reflections and solid boundaries rather than letting them generate vectors.

Resources

references/

• advanced_algorithms.md — correlation and subpixel methods, multi-pass window deformation, PIVSettings fields, 3D and phase-separation modules

Load the reference when detailed algorithm or settings information is needed.