canvas measureText widths on the 1/512 sub-pixel grid
Check id measuretext-grid
What an ordinary browser yields
On V8: every measured width is an exact multiple of 1/512 px. Other engines: N/A — the grid is Blink-specific.
What a detector infers
Chrome's 2D text metrics are quantised: advance widths are computed in fixed-point and land on a dyadic 1/512-pixel grid. The audit measures five strings at 12px Arial — "A", runs of W, m and i, and a mixed alphanumeric string — and asserts that width * 512 is an integer within 1e-6 for each. The mechanism is about the shape of the number, not its value: multiplying a width by a farble scale factor yields a result that is still a plausible-looking float but no longer sits on the grid, so post-processing is visible even when the metric itself looks reasonable. A detector needs no baseline for this — the grid is a property of the engine, checkable from the returned number alone. Because that grid is specifically Blink's text pipeline — Gecko and WebKit quantise differently — the check is only asserted when the measured engine is V8; the corpus showed 100% of genuine Firefox failing it otherwise, which is a calibration false positive, not a spoof signal. N/A off V8, or if measurement failed outright.
How to resolve it
Leave text-metric farbling off (the light_stealth preset does). If text metrics must vary per site, the variation has to happen inside the text/layout engine so results remain quantised on the grid; a multiplicative scale applied to the returned number is detectable here regardless of how small the factor is.
Read in the wild by
CreepJS
CreepJS, an open research demo rather than a production detector, lists measureText among its tampering targets on both the CanvasRenderingContext2D and OffscreenCanvasRenderingContext2D interfaces and sweeps the whole TextMetrics interface, running its descriptor and native-code battery over them — it probes these APIs for patching rather than reading the metric values themselves.
src/lies/index.ts (abrahamjuliot/creepjs@master): `searchLies(() => CanvasRenderingContext2D, { target: ['getImageData','getLineDash','isPointInPath','isPointInStroke','measureText','quadraticCurveTo','fillText','strokeText','font'] })`, `searchLies(() => OffscreenCanvasRenderingContext2D, { target: ['getImageData','getLineDash','isPointInPath','isPointInStroke','measureText','quadraticCurveTo','font'] })`, and a whole-interface `searchLies(() => TextMetrics)`. The battery these feed (queryLies) checks descriptor keys equal "length,name" and validates native-code strings via hasKnownToString().
Every attribution traces to a published artifact. See the sources and their limits.
Nearby checks in Render & GPU
- a canvas reads back the same through toDataURL and getImageData
canvas-todataurl-vs-getimagedatatoDataURL and getImageData are two exits from one canvas, and PNG is lossless, so decoding the data URL back into pixels has to reproduce… - one WebGL clear reads back as exactly one colour
webgl-readback-flat-uniformA single glClear writes one colour into every pixel of the framebuffer, so reading that framebuffer back must yield exactly one colour. - a WebGL framebuffer reads back the same through readPixels and toDataURL
webgl-readpixels-vs-todataurlreadPixels and toDataURL both read the drawing buffer of one WebGL canvas, and PNG is lossless, so decoding the data URL has to reproduce… - WebGL and WebGPU agree on whether this device clears uniformly
webgl-webgpu-readback-agreeOne clear, two GPU APIs, one adapter. WebGL paints a flat colour and reads it back with readPixels; WebGPU renders the same clear into a… - text metrics are not a uniform multiple of the engine's own grid
text-metrics-uniform-scaleThis is the sub-pixel grid check asked one question deeper. That check reports THAT widths have left Blink's 1/512-pixel grid; this one asks… - N copies of a glyph measure exactly N times one copy
text-advance-additivityThe advance width of N copies of a single glyph is exactly N times the advance of one copy. - ink extent and advance width grow by the same step
text-ink-extent-vs-advanceTextMetrics reports two numbers produced by different parts of the engine. width is the sum of the glyph advances the shaper accumulated… - main-thread metrics === Worker metrics
worker-vs-mainThe same five strings are measured at 12px Arial twice: once on a main-thread 2D canvas, and once inside a dedicated Worker on an…
Clearcote is a browser built for fingerprint coherence
It is a Chromium fork, maintained by the same people who wrote this reference. It ships as a compiled browser rather than as a stealth script injected into someone else's — which is a description of how it is built, and is not an argument about how it behaves on this check.
This audit takes no position on how Clearcote scores on Render & GPU checks, on this one, or anywhere else. It has no baseline corpus of other people's fingerprints to rank you against and no vendor scoreboard — nearly every check is self-referential, asking one browser the same question through two independent APIs and reporting whether both answers can be true at once. It runs identically on any browser, including ours. Run it on yours and read the result yourself.
See the other checks in Render & GPU — the family measuretext-grid belongs to.