Documentation Index
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When It Runs
Watermark removal runs during the document preparation step. If watermarks are detected, the cleaned PDF is uploaded to storage alongside the original; otherwise the original passes through unchanged. Two complementary strategies are applied sequentially — if either detects watermarks, the cleaned document is used for subsequent processing.Strategy 1: Annotation Watermarks
Some PDF producers (especially office-suite “confidential” or “draft” stamp features) add watermarks as annotation overlays on top of page content. The PDF specification defines a dedicated watermark annotation subtype for this purpose. How it works: The system scans each page for annotations marked as watermarks and removes them. Because these are explicitly declared as watermarks by the producing application, no heuristics are needed — the annotation type is a definitive signal. Limitation: Many real-world watermarks are not implemented as annotations. Text-based watermarks embedded directly in the page content (common in documents from Chinese recruitment platforms, for example) are invisible to this strategy, which is why Strategy 2 exists.Strategy 2: Content-Stream Watermarks
This strategy targets watermarks embedded directly in the PDF page content as rotated, semi-transparent text. These are common in documents where the watermark was “baked in” during PDF generation rather than added as an overlay.Detection Heuristics
A text group on the page is classified as a watermark when both conditions are met:| Condition | Threshold |
|---|---|
| Rotated text | Rotation angle between 25 and 75 degrees |
| Semi-transparent | Fill opacity below approximately 29% |
- Rotated but fully opaque text (e.g., decorative headings) is left untouched
- Horizontal but transparent text (e.g., background labels) is left untouched
- Only text that is both significantly rotated AND significantly transparent is removed
How It Works
- Each page’s content stream is normalized into a single, well-formed structure
- Text groups (graphics-state blocks) are analyzed for rotation angle and transparency level
- Groups matching both criteria are removed from the content stream
- The modified page content is written back
Why This Approach?
The system operates on raw PDF drawing operations (rotation matrices and transparency settings) rather than trying to match specific watermark text. This makes it:- Encoding-independent — works regardless of font, language, or text encoding
- Low false-positive risk — the combination of non-trivial rotation AND significant transparency is highly specific to watermarks; normal document text is almost always horizontal and fully opaque
- Non-destructive — only the identified watermark elements are removed; all other page content is preserved
Safety
- False-positive risk is low: Normal document text is horizontal (0 degrees) and opaque. The dual-condition requirement (rotated + transparent) is highly specific to watermarks.
- Clean removal: Watermark text groups are self-contained in the PDF structure, so removing them does not affect surrounding content.
- Original preserved: The original uploaded PDF is always kept in storage. The cleaned version is stored separately.