# Image Extraction from PDF — ЦТ/ЦЭ Questions

## Tools available
- **pdftoppm** (poppler, via scoop): renders PDF pages to PNG
- **sharp** (npm, installed in `backend/`): crops images in Node.js
- Script: `backend/src/db/crop_images.js`

## Workflow for extracting figures from exam PDF

### Step 1 — Render pages at 200 DPI
```bash
pdftoppm -png -r 200 -f <first_page> -l <last_page> "path/to/file.pdf" "/tmp/prefix"
# Output: /tmp/prefix-06.png, /tmp/prefix-07.png ...
# Copy to: frontend/img/questions/pageN.png
```

### Step 2 — Calibrate coordinates using 72 DPI reference
```bash
pdftoppm -png -r 72 -f <first_page> -l <last_page> "path/to/file.pdf" "/tmp/pt"
# Output: /tmp/pt-06.png etc. (614×844 px for A4)
# Copy to: frontend/img/questions/ptN.png
```

At 72 DPI: A4 = 614×844 px. Scale to 200 DPI: **×2.777**
Measure coordinates visually on 72 DPI images, multiply by 2.777 to get 200 DPI coords.

### Step 3 — Test crops (Node.js)
```javascript
// Run from backend/ folder
const sharp = require('sharp');
sharp('../frontend/img/questions/pt6.png')
  .extract({ left: 220, top: 80, width: 200, height: 100 })
  .toFile('../frontend/img/questions/test.png');
```

### Step 4 — Run crop_images.js
```bash
cd backend && node src/db/crop_images.js
```

---

## ЦТ 2021 — Variant 1 crop coordinates (200 DPI, 1705×2344)

| File | Question | Source page | left | top | width | height |
|------|----------|-------------|------|-----|-------|--------|
| ct2021v1_a1.png  | A1 triangle     | page6.png | 611 | 222 | 556 | 292 |
| ct2021v1_a7.png  | A7 graph f(x)   | page6.png | 278 | 1222| 750 | 403 |
| ct2021v1_a15.png | A15 parabola    | page7.png | 556 | 917 | 695 | 278 |
| ct2021v1_a17.png | A17 grid A,B    | page7.png | 861 | 1439| 639 | 194 |
| ct2021v1_a18.png | A18 pyramid     | page7.png | 389 | 1656| 945 | 472 |
| ct2021v1_b1.png  | B1 bar chart    | page8.png | 28  | 83  | 1167| 556 |
| ct2021v1_b3.png  | B3 3D planes    | page8.png | 1015| 1717| 319 | 417 |
| ct2021v1_b4.png  | B4 enclosure    | page9.png | 945 | 14  | 542 | 208 |

PDF source: `ЦТ-ЦЭ/ЦТ 2021.pdf`
- Page 6 = Variant 1, Part A (A1–A11)
- Page 7 = Variant 1, Part A (A12–A18)
- Page 8 = Variant 1, Part B (B1–B3)
- Page 9 = Variant 1, Part B (B4–B14)

Images stored: `frontend/img/questions/ct2021v1_*.png`
Served at: `/img/questions/ct2021v1_*.png` (via express.static on frontendDir)

---

## DB: updating image field after seeding
```javascript
const upd = db.prepare('UPDATE questions SET image = ? WHERE text LIKE ? AND year = ?');
upd.run('/img/questions/ct2021v1_a1.png', '[ЦТ 2021 · A1]%', 2021);
```

---

## Notes
- PDF contains scanned raster images — no extractable vector graphics
- Each PDF page = one large bitmap scan
- `pdfimages` extracts only full-page bitmaps (not individual diagram crops)
- sharp must be required from `backend/` directory (installed there)
- Temp page renders NOT committed to git — regenerate with pdftoppm when needed
