webSMLM

A browser-based tool for single-molecule localization microscopy (SMLM). It loads a raw image stack, detects and localizes single emitters, and reconstructs a super-resolution image — entirely in the browser. Nothing is uploaded; all computation runs client-side.

Status: proof-of-concept. Not a validated replacement for established SMLM packages, but a fast, zero-install way to try localization on your own data.

Quick start

Option A — just run it. Download webSMLM.html and open it in any modern browser (double-click works; no internet, no install, no server). Everything needed is inside that one file.

Option B — hosted. Open the published version, served via GitHub Pages: https://hohlbeinlab.github.io/webSMLM/webSMLM.html.

Then: click Simulate movie to try it immediately, or Load movie for your own .tif/.tiff stack. Set the pixel size (nm), pick a fit method, and press Localize. Open Help & guide in the app for a full walk-through of every step.

What it does

• Loads multi-frame TIFF stacks (8/16/32-bit, little- or big-endian, uncompressed or deflate/LZW-compressed). 16-bit depth is preserved.
• Handles very large stacks. Files too big to hold in memory are read frame by frame with File.slice(), so the file is never fully loaded. Contiguous ImageJ stacks (single directory entry, frames laid out after it — as written above ~4 GB) are indexed arithmetically; multi-IFD stacks (e.g. multi-GB Micro-Manager MMStacks) are indexed by walking the IFD chain. A 4.89 GB / 40,000-frame stack processes in ~12 s.
• Memory-aware loading: caches frames in RAM within a configurable budget, or streams large stacks in bounded heaps (the pattern that also enables real-time processing of a live camera buffer). Falls back to streaming automatically if an in-memory load hits the browser's memory ceiling.
• Detects ROIs by band-passing each frame, with a choice of three filters: an à trous B-spline wavelet (the default, as in ThunderSTORM; no σ, the fastest to filter), a Difference-of-Gaussians filter, or a uniform box filter (Huang et al. 2011). Wavelet and DoG threshold on strict local maxima above mean + k·σ; the box filter thresholds on a plain intensity value instead. The three respond differently, so re-tune the threshold when you switch. A Real-time update toggle re-detects/re-fits the scrubbed frame live as you change detection or fit settings, without a full Run.
• Localizes with phasor fitting (very fast, no iteration), a least-squares 2D Gaussian fit, or a Poisson maximum-likelihood fit (Gaussian MLE 2D, the default — integrated-Gaussian, Smith et al. 2010 / Picasso gaussmle). MLE is statistically optimal at low photon counts and reports a proper per-localization CRLB uncertainty.
• 3D astigmatism, two independent ways: Phasor 3D (z from the phasor magnitude ratio) and Gaussian MLE 3D (z from the elliptical σ_x/σ_y widths) — a built-in cross-check. Calibrate a bead z-stack — every spot is fit both by LS (real σ_x/σ_y curves) and phasor (magnitude ratio); the calibration carries both models, tagged, and a guard stops a 3D fit running against the wrong one. A "Fix bead x,y" option freezes each bead's lateral position from a composite of the calibration range before fitting widths per frame, so the fit can't wander to a wrong lobe when the PSF flattens, rings or splits at large defocus. Calibrations save/load as JSON (with their source file), and the reconstruction can be depth-coded (hue = z, brightness = density) with an adjustable z range.
• Localizations table (View data + filtering): sortable, with cumulative filtering (e.g. intensity > 1000 and uncertainty < 20, Enter to apply, removable chips, Reset) that drives the reconstruction live; any column can be histogrammed in the raw panel (with x-zoom/pan). Consecutive-frame detections of the same molecule can also be merged into temporal-clustering events (tempClusteringXY/tempClusteringZ filters), reducing noise at the cost of localization density.
• Renders a super-resolution image with adjustable magnification and blur, a choice of colour maps (Fire, Inferno, Viridis, Turbo, Grey) and percentile-based display scaling. All render settings apply instantly without refitting.
• Builds up live: the raw frame refreshes during a run with detected ROIs (green) and accepted localizations (magenta sub-pixel crosshairs); the reconstruction previews on a time budget. A Stop button ends the analysis early while keeping the localizations gathered so far.
• Navigate both panels: independent zoom/pan (wheel or pinch, drag, double-click to reset) on the raw frame and the reconstruction, plus a frame slider to scrub the stack. Both panels take the loaded stack's data aspect ratio, so they stay equal-sized with no letterboxing. The controls panel is collapsible — on desktop, re-opening it after a collapse floats it as an overlay so toggling never resizes the display windows.
• Measures distances / line profiles: click two points in the reconstruction to plot the intensity profile along the line (averaged over a 3-pixel band), with the length in nm and an x-zoomable/pannable plot. A crop tool next to it adds an x/y-range filter from two clicked corners, restricting the reconstruction, export, NeNA and FRC to that region like any other table filter.
• Save/Load settings as JSON to reproduce an analysis configuration.
• Works on small screens: single-column layout on phones and tablets, with drag/pinch-to-zoom navigation of the reconstruction (plus a scale bar).
• Corrects drift with AIM (adaptive intersection maximization) — a point-based estimator that needs no image rendering and no FFT, so it fits the single-file design and works natively in 3D (x/y grid search + a separate z search on the Phasor-3D output). It re-estimates from the raw localizations on every run, so segment size and search radius can be swept and compared; corrected coordinates drive the render and CSV, the raw ones are kept, and the drift-vs-frame curve can be plotted.
• Measures precision & resolution (experimental, new in 0.8.0): NeNA — the mean per-localization precision from the nearest-neighbour distance distribution (with a fitted histogram plotted); and FRC — image resolution at the 1/7 threshold by Fourier ring correlation of two independent halves (a compact inline FFT, no dependency), with the curve plotted. Both report an error estimate. NeNA needs a static structure (a diffusing probe inflates it); FRC folds in labelling density and drift, so run it after drift correction.
• Exports localizations as ThunderSTORM-compatible CSV, including background-subtracted intensity, background level and a Thompson/Larson/Webb uncertainty estimate. See the caveat on ADU-to-photon conversion below.

Performance

Detection and fitting run in parallel across a pool of Web Workers (one per logical core). Measured on a laptop, running the single HTML file directly from file:// with the default wavelet detector:

Dataset	Fit method	Frames	Frame size	Time	Rate
3D STORM (4.89 GB) [Leterrier]	Phasor 3D	40000	256 × 256	~12 s	~350,000 loc/s

Small stacks finish faster than can be timed reliably (well under a second — JIT warm-up and timer resolution dominate), so only a stack large enough to run for several seconds gives a stable throughput figure: the 4.89 GB stack above — never held in memory, streamed frame by frame — completes in ~12 s (~350k loc/s) with the Phasor fitter. Notes:

• Rate is fitter-dependent. Phasor (above) is non-iterative and by far the fastest; Gaussian MLE 2D/3D — the statistically-preferred default — costs several-fold more per localization for its ~5-iteration Newton fit, and on very large stacks frame I/O/decode can dominate wall time as much as fitting itself, so the figure above is a ceiling, not a typical number across methods.

• Browser matters. On macOS the numeric path runs fastest in Safari, then Chrome, then Firefox (JS-engine differences). Expect run-to-run variation too (JIT warm-up, GC, thermal, disk cache) — a repeat of the same stack is usually quicker.

• Detector choice changes results, not just speed: the three filters find slightly different spot sets, so counts differ between them.

• Workers are probed before use, with a self-test that exercises the whole numeric path. If they are unavailable — some browsers restrict workers on file:// — the app falls back to single-threaded automatically and says so in the log.

• Small, short stacks stay single-threaded on purpose: below ~20,000 pixels per frame and below ~30 million total pixels (frames × width × height), the cost of handing work to a worker outweighs the work itself. Either condition alone is enough to parallelize — a stack with many small frames (e.g. thousands of 64×64 frames) still crosses the total-volume threshold and runs on the worker pool.

• The DoG filter approximates its background term with a box filter by default (~2× faster than a true Gaussian, changes ~0.4% of its detections); tick Exact band-pass for the true Gaussian. The wavelet filter has no such option (and no σ).

• The run log reports a timing breakdown (frame access / detect / fit) so you can see where time goes on your own data.

Data & privacy

The application is a single static HTML file. Your image data is read locally by the browser and never leaves your machine — there is no server and no upload.

How it works & references

The in-app Help & guide documents each stage and lists references. Key ones:

• Phasor localization (the fast fitter here implements this): K. J. A. Martens, A. N. Bader, S. Baas, B. Rieger, J. Hohlbein, Phasor based single-molecule localization microscopy in 3D (pSMLM-3D), J. Chem. Phys. 148, 123311 (2018). https://doi.org/10.1063/1.5005899
• Detection & thresholding (both the default à trous B-spline wavelet filter and the DoG band-pass, plus the std-based threshold, follow ThunderSTORM): M. Ovesný et al., Bioinformatics 30(16), 2389–2390 (2014). https://doi.org/10.1093/bioinformatics/btu202
• Uniform box filter (the third detection filter, thresholded on a plain intensity value instead): F. Huang, S. L. Schwartz, J. M. Byars, K. A. Lidke, Simultaneous multiple-emitter fitting for single molecule super-resolution imaging, Biomed. Opt. Express 2(5), 1377–1393 (2011). https://doi.org/10.1364/BOE.2.001377
• LS vs MLE fitting: K. I. Mortensen et al., Nat. Methods 7, 377–381 (2010). https://doi.org/10.1038/nmeth.1447 — and localization-precision theory: R. E. Thompson, D. R. Larson, W. W. Webb, Biophys. J. 82, 2775–2783 (2002). https://doi.org/10.1016/S0006-3495(02)75618-X
• Drift correction (AIM) (the estimator implemented here): H. Ma, M. Chen, P. Nguyen, Y. Liu, Toward drift-free high-throughput nanoscopy through adaptive intersection maximization, Sci. Adv. 10(21), eadm7765 (2024). https://doi.org/10.1126/sciadv.adm7765 — adapted from the reference implementation in Picasso (picasso/aim.py, https://github.com/jungmannlab/picasso; J. Schnitzbauer et al., Super-resolution microscopy with DNA-PAINT, Nat. Protoc. 12, 1198–1228, 2017, https://doi.org/10.1038/nprot.2017.024): a parabolic sub-pixel peak fit replaces the FFT phase refinement and linear interpolation replaces the spline.
• Localization precision (NeNA) (the estimator implemented here): U. Endesfelder, S. Malkusch, F. Fricke, M. Heilemann, A simple method to estimate the average localization precision of a single-molecule localization microscopy experiment, Histochem. Cell Biol. 141, 629–638 (2014). https://doi.org/10.1007/s00418-014-1192-3
• Image resolution (FRC) (the measure implemented here): R. P. J. Nieuwenhuizen, K. A. Lidke, M. Bates, D. L. Puig, D. Grünwald, S. Stallinga, B. Rieger, Measuring image resolution in optical nanoscopy, Nat. Methods 10, 557–562 (2013). https://doi.org/10.1038/nmeth.2448
• Overview: M. Lelek et al., Nat. Rev. Methods Primers 1, 39 (2021). https://doi.org/10.1038/s43586-021-00038-x

Known limitations

• 3D astigmatism is 2D-vs-3D by two independent, still-unreconciled methods (Phasor 3D from the magnitude ratio, Gaussian MLE 3D from the elliptical σ_x/σ_y widths) rather than a single validated 3D model. They are cross-checked against each other and against synthetic ground truth, but not yet against real bead data. The astigmatism calibration itself fits a vertex-quadratic per axis — a local approximation, best over a cropped z-range around focus.
• NeNA and FRC are new and not yet cross-validated against established tools (ThunderSTORM, Picasso, FRCbar) or previously-analysed datasets — the numbers they report should not yet be treated as authoritative. 3D FSC (the spherical-shell counterpart to 2D FRC) is not implemented.
• Intensities are in ADU unless a camera gain is entered, in which case the exported intensity [photon] and uncertainty [nm] columns are not on a physical scale. A single scalar gain also suits EMCCD better than sCMOS, where gain, offset and read noise vary per pixel — see docs/REFACTOR_PLAN.md.
• Dense samples with overlapping PSFs are fitted with a single-emitter model, which biases positions where emitters overlap. Multi-emitter fitting would be the real fix.
• The mean + k·σ threshold assumes roughly stationary noise, computed over the whole frame including signal — so at high blink density the threshold rises and dim localizations get silently dropped. Strong background gradients also favour a local threshold over a global one.
• σ_PSF is a fixed, user-supplied value, not estimated or calibrated from the data.

Roadmap

Past releases — including implementation detail and notable rejected approaches — are logged in CHANGELOG.md; forward-looking notes are kept in docs/REFACTOR_PLAN.md.

Distribution & citation

This project is distributed as a single file. It lives at github.com/HohlbeinLab/webSMLM, is served via GitHub Pages at https://hohlbeinlab.github.io/webSMLM/, and is archived on Zenodo with a citable DOI (10.5281/zenodo.21445041).

To cite webSMLM, use the concept DOI above (it always resolves to the latest version) or the metadata in CITATION.cff — GitHub's Cite this repository button reads it automatically. Please also cite the phasor SMLM paper it implements (Martens et al., 2018; see below).

Each new GitHub release is picked up by Zenodo automatically and gets its own version DOI; pushing to main redeploys the Pages site.

License

© 2026 Hohlbein et al., licensed under CC BY 4.0 — see LICENSE. You may share and adapt this work, including commercially, with attribution.

Bundled third-party decoders retain their own MIT licenses: UTIF.js and pako.