How is this different from just raising contrast?

Global contrast darkens paper along with ink. CLAHE equalizes luminance locally, so faint strokes in stained areas get lifted without the paper going gray — then an ink mask darkens strokes while the paper is bleached white.

Which ink color setting should I pick?

Use 'Blue and black' for mixed or unknown documents, 'Blue ink' to favor ballpoint signatures, and 'Black ink' for pencil, gel, or printed text where colored marks should be damped.

What does the adaptation grid do?

It sets the CLAHE tile size. A finer grid (12–16) adapts to localized stains and shadows; a coarser grid (4–8) is smoother on evenly aged paper.

Will it preserve red stamps or seals?

Yes — enable 'Preserve colored marks' and saturated non-ink colors pass through untouched while the paper whitens around them.

Does it work on printed text too?

It is tuned for handwriting, but printed scans benefit from the same paper bleaching. For pure printed pages, PDF to Grayscale or Adjust Colors may be simpler.

Is my document uploaded?

No. All processing happens locally in your browser — the file never leaves your device.

增强手写墨迹

通过自适应对比度增强褪色的手写笔迹，漂白泛黄纸张，并返回干净的 PDF。

1. Upload a scanned document

Contracts, journals, ledgers — anything with faded ink on tinted paper.

2. Tune the boost

Pick the ink color, set the adaptation grid and strength, and watch the live preview.

3. Download the cleaned PDF

Faded strokes come back dark and crisp; the paper reads as clean white.

Bring faded handwriting back to readable

Why plain contrast is not enough

A global brightness or contrast slider treats the whole page identically: push it far enough to rescue a faint signature and the paper's yellow tint darkens with it, while already-dark strokes clip to blobs. This tool uses CLAHE — contrast-limited adaptive histogram equalization — which equalizes luminance in small local tiles instead. Faded ink in a stained corner gets the same lift as ink in a clean corner, and the clip limit stops noise from being amplified into speckles.

After equalization, a per-pixel ink mask separates strokes from paper. Strokes are darkened toward their own hue — blue ink stays blue, black stays black — while the paper is bleached toward white. The result is a purified PDF, not a transparent asset: the page is intact, just readable.

Controls that map to real problems

Each setting answers a specific scan defect.

Ink color profile

Blue ink, black ink, or both — the hue-aware mask favors the strokes you actually want to keep.

Adaptation grid

CLAHE tile size from 4×4 to 16×16. Finer grids follow coffee stains and shadows; coarser grids are gentler on clean scans.

Preserve colored marks

Keeps red seals, stamps and colored annotations vivid while the paper around them whitens.

Strength and sharpening

Clip limit controls how aggressively faint ink is lifted; an unsharp mask restores crisp stroke edges.

Made for contracts, journals and archives

Executed contracts photographed in bad light, grandparents' letters, historical ledgers, field notebooks — the common failure is the same: ink that faded faster than the paper aged. Because the pipeline preserves pen-pressure texture instead of thresholding to pure black, signatures keep their character and stay legible rather than turning into uniform blobs.

The output also makes a better input for OCR: run the boosted PDF through OCR PDF and the cleaner separation between ink and paper measurably improves recognition on handwriting-adjacent material like printed forms filled in by hand.

Private by construction

Everything runs on Canvas in your browser. A signed contract or a personal journal never leaves your device — there is no upload, no queue, and no server-side processing.