About

Legal and financial documents often require blue ink signatures to distinguish originals from photocopies. A black signature scanned at 300dpi becomes indistinguishable from a printed copy, creating authentication problems. This converter applies colorimetric transformation using the luminance formula L = 0.299R + 0.587G + 0.114B to isolate ink strokes from paper background, then shifts detected pixels to the blue spectrum (hue 210 - 220°) while preserving stroke intensity and anti-aliasing.

The algorithm handles variable scan quality, yellowed paper backgrounds, and gray signatures from faded pens. Output maintains original resolution with optional transparent background for digital document overlays. Processing occurs entirely in-browser with no server uploads.

Formulas

The conversion process applies two sequential transformations to each pixel. First, luminance calculation determines whether a pixel belongs to ink or background:

L = 0.299R + 0.587G + 0.114B

where R, G, B are the red, green, and blue channel values (0 - 255). Pixels with L below the threshold T are classified as ink. The threshold adapts based on user-set intensity.

T = 255 − (intensity × 75)

For detected ink pixels, the blue transformation maps original darkness to blue saturation:

R_new = L × 0.15

G_new = L × 0.35

B_new = 80 + (L × 0.65)

This produces hues in the 210 - 220° range while preserving stroke darkness variation. The alpha channel is set to 255 − L for transparent output, maintaining anti-aliased edges.

Reference Data

Ink Type	Hue Range	Typical Use	Photocopy Detection
Classic Blue Ballpoint	210 - 220°	Legal documents, contracts	High (distinct from black)
Royal Blue	220 - 230°	Formal correspondence	High
Navy Blue	230 - 240°	Executive documents	Medium-High
Black Ink	N/A (achromatic)	General use	None (identical to print)
Blue-Black	200 - 210°	Archival documents	Medium
Cyan (Gel Pen)	180 - 200°	Informal signing	High
Turquoise	170 - 185°	Creative documents	High
Document Scan (B&W)	N/A	Copies, faxes	N/A (source material)
Grayscale Signature	N/A	Faded/old documents	None
Standard Luminance Threshold	L < 180 (ink detection cutoff)
Recommended Scan DPI	300dpi minimum for signature clarity
PNG Transparency	Alpha channel preserves anti-aliasing edges
JPEG Compression	Quality 92% recommended for signatures
Max File Size (Browser)	10MB practical limit for real-time processing

Frequently Asked Questions

Blue ink provides immediate visual distinction between original documents and black-and-white photocopies. When a document is copied, blue signatures reproduce as gray or faded, making it easy to identify the original. Many notaries, banks, and government agencies specify blue ink in their signing requirements for this authentication purpose.

The luminance-based detection algorithm analyzes relative darkness rather than absolute color values. Yellowed paper typically has luminance above 200, while signature strokes fall below 150. Adjusting the intensity slider raises or lowers the threshold T to accommodate poor scan quality or aged documents. For heavily stained backgrounds, use maximum intensity.

The converter preserves original resolution and applies anti-aliasing-aware processing. Output at 300dpi or higher meets standard legal document requirements. However, digitally altered signatures may not satisfy all jurisdictions. Consult local regulations regarding electronic signature validity.

PNG output includes an alpha (transparency) channel, allowing the signature to overlay documents without a white box. JPEG output fills the background with white and uses lossy compression. Use PNG for digital document assembly and JPEG for email attachments where smaller file size matters.

Yes. Gray signatures have luminance values between ink and paper. Increasing the intensity slider raises the detection threshold, capturing lighter strokes. Extremely faded signatures (luminance above 220) may require pre-processing in an image editor to enhance contrast before conversion.