| name | add-watermark |
| description | Overlay a diagonal text watermark (DRAFT, PRIVATE, CONFIDENTIAL, etc.) at 45° across every page in light red (or user-specified color), at low opacity so body text remains readable. Triggers on phrases like "add a DRAFT watermark", "stamp PRIVATE on every page", "watermark this PDF". |
Add Diagonal Watermark
Stamp a large rotated text watermark across every page of a PDF.
Inputs
- Input PDF path (required).
- Watermark text (required) —
DRAFT, PRIVATE, CONFIDENTIAL, or arbitrary string.
- Color (optional) — default
light red (RGB 0.85, 0.2, 0.2 at 25% opacity). Accept named colors or hex.
- Opacity (optional) — default 0.20 (20%).
- Angle (optional) — default 45°.
- Font (optional) — default Helvetica Bold.
- Size (optional) — default auto-fit (≈ page diagonal length / text length × 0.6).
- Output path (optional) — defaults to
<input>-<watermark-slug>.pdf.
Tooling
Generate a single-page overlay PDF the same size as the input, then stamp it onto every page using qpdf --overlay.
Overlay generation (Python + ReportLab)
from reportlab.pdfgen import canvas
from reportlab.lib.colors import Color
c = canvas.Canvas("overlay.pdf", pagesize=(width_pt, height_pt))
c.saveState()
c.translate(width_pt/2, height_pt/2)
c.rotate(45)
c.setFillColor(Color(0.85, 0.2, 0.2, alpha=0.20))
c.setFont("Helvetica-Bold", font_size)
c.drawCentredString(0, 0, "DRAFT")
c.restoreState()
c.save()
Stamp onto input
qpdf input.pdf --overlay overlay.pdf -- output.pdf
Workflow
- Probe page size with
pdfinfo.
- Compute font size to fit (text width ≈ 60% of page diagonal).
- Generate the overlay.
- Verification — render page 1 of the output to PNG and inspect:
gs -sDEVICE=png16m -r100 -o /tmp/wm-check.png -dFirstPage=1 -dLastPage=1 output.pdf
Confirm watermark is visible, centered, at the right angle, body text still readable.
- Stamp onto all pages.
- Report and offer to show the user the preview PNG.
Output
Watermarked PDF.
Caveats
- Some viewers (older Acrobat) struggle with PDF transparency; if printing is critical and the printer is old, consider
flatten-transparency after watermarking.
