PDF Patch

Scanned PDF Text Repair

Use this skill for image-only PDFs where text extraction returns empty or does not contain the target text. The goal is visual repair: rebuild the page image with a small localized patch, not semantic PDF text editing.

Workflow

1. Inspect the PDF before editing.

   • Count pages with PyMuPDF.
   • Extract text with page.get_text().
   • If the target text is absent and the page has one or more images, treat it as scanned/image-only.
   • Render the page at 2x or 3x scale and inspect the target area.
   • Prefer scripts/inspect_pdf.py for this step.

2. Locate the exact patch.

   • Use rendered pixel coordinates, not guessed PDF point coordinates.
   • Crop the target line and identify dark-pixel runs for each character.
   • Patch only the old character/phrase plus a tiny margin.
   • Do not repaint surrounding text unless the user explicitly wants phrase-level replacement.
   • Use scripts/find_dark_runs.py on the rendered page or crop when exact character bounds are unclear.

3. Rebuild from the original PDF every time.

   • Never edit on top of a previous repaired PDF during iterations.
   • Re-render the original page, apply one patch, and create the final PDF from that result.
   • This prevents cumulative blur, halos, and dirty paper blocks.

4. Match visual style.

   • Use nearby original characters as the reference for font family, size, baseline, weight, and gray level.
   • For Chinese official documents, test simfang.ttf, STFANGSO.TTF, simsun.ttc, STSONG.TTF, and simkai.ttf when available.
   • Prefer FangSong/simfang when the body text resembles Chinese official-document FangSong.
   • Match the replacement glyph's bottom alignment to adjacent characters before judging top alignment.
   • If the user says the glyph is too thin, increase stroke weight or opacity before changing font family.
   • If the user says the glyph is too small, increase font size and adjust baseline downward as needed.
   • Use scripts/font_variants.py to generate a contact sheet when visual matching is not obvious.

5. Preserve the paper background.

   • Build the cover patch from local bright pixels around the target area.
   • Add slight deterministic noise instead of a flat white rectangle.
   • Apply small Gaussian blur to replacement text only when needed to match scan softness.

6. Verify output.

   • Render the repaired PDF.
   • Crop the target line and inspect it visually.
   • Check page count and page size are unchanged.
   • For gray/weight checks, compare dark-pixel count and mean grayscale against adjacent original characters.
   • Report the output file and the specific text that changed.

Script

Use these scripts instead of rewriting one-off Python snippets:

• scripts/inspect_pdf.py: inspect text layer, image count, page size, render a page, and make a crop.
• scripts/find_dark_runs.py: locate dark-pixel runs inside a rendered image area to estimate character boxes.
• scripts/font_variants.py: generate font/size preview sheets on the real target crop.
• scripts/repair_scanned_pdf_text.py: apply the final localized repair. It preserves other PDF pages by default; pass --single-page only when a one-page output is desired.

Do not preserve every exploratory command as a script. Keep only reusable steps: inspect, locate, preview, repair, verify.

Repair Script

Typical use:

py -X utf8 C:\Users\zhouh\.codex\skills\pdf-patch\scripts\repair_scanned_pdf_text.py `
  --input .\source.pdf `
  --output .\source_已修改.pdf `
  --page 1 `
  --cover 651,518,688,561 `
  --text 为 `
  --position 654,525 `
  --font C:\Windows\Fonts\simfang.ttf `
  --size 30 `
  --fill 22,25,30,250 `
  --blur 0.16

Render and crop after writing:

py -X utf8 C:\Users\zhouh\.codex\skills\pdf-patch\scripts\repair_scanned_pdf_text.py `
  --input .\source.pdf `
  --output .\source_已修改.pdf `
  --page 1 `
  --cover 651,518,688,561 `
  --text 为 `
  --position 654,525 `
  --font C:\Windows\Fonts\simfang.ttf `
  --size 30 `
  --render-check .\check.png `
  --crop-check .\check-crop.png `
  --crop 620,505,1015,585

Inspect and crop before editing:

py -X utf8 C:\Users\zhouh\.codex\skills\pdf-patch\scripts\inspect_pdf.py `
  --input .\source.pdf `
  --page 1 `
  --render .\page-1.png `
  --crop 620,505,1015,585 `
  --crop-out .\target-crop.png

Find candidate character runs:

py -X utf8 C:\Users\zhouh\.codex\skills\pdf-patch\scripts\find_dark_runs.py `
  --image .\page-1.png `
  --box 620,505,1015,585 `
  --threshold 180

Generate font/size previews:

py -X utf8 C:\Users\zhouh\.codex\skills\pdf-patch\scripts\font_variants.py `
  --image .\page-1.png `
  --output .\font-variants.png `
  --preview-crop 620,505,1015,585 `
  --cover 651,518,688,561 `
  --text 为 `
  --position 654,525 `
  --fonts C:\Windows\Fonts\simfang.ttf C:\Windows\Fonts\STFANGSO.TTF C:\Windows\Fonts\simsun.ttc `
  --sizes 28 29 30 31

Coordinate Notes

• --cover, --position, and --crop are rendered image coordinates, not PDF points.
• The default render scale is 2, matching a standard A4-ish PDF render near 1191 x 1685.
• If the source page has a different resolution or orientation, render first and locate coordinates on that rendered image.
• For single-character Chinese repair, cover about the original character bounding box plus 2-5 pixels of margin.

Failure Modes

• Visible font mismatch: test nearby system Chinese fonts and use adjacent characters to score shape similarity.
• Flat white block: use local paper sampling and noise; reduce cover size.
• Glyph too high or low: adjust only --position y first.
• Glyph too thin or light: increase alpha, add --stroke-width 1, or use a darker fill.
• Glyph too bold: remove stroke, lower alpha, or add a small blur.
• Repeated artifacts after iterations: confirm the script input is the original PDF, not the previous output.