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Resolution Limitations for Technical Drawings

Overview

Werk24 requires a minimum resolution of 200 DPI to accurately process and extract information from technical drawings. This ensures that details such as dimensions, annotations, and tolerances are legible and correctly interpreted by the system.

To validate resolution, Werk24 infers the paper size by analyzing the content and then calculates an approximate resolution based on the detected dimensions. If the resolution is too low, the API will return a DRAWING_RESOLUTION_TOO_LOW exception.

Minimum Resolution Requirement

  • 📌 Required Resolution: ≥ 200 DPI (Dots Per Inch)
  • ⚠️ Below 200 DPI: processing fails with DRAWING_RESOLUTION_TOO_LOW.

Rule of thumb: If your smallest text (e.g., 2.5–3.5 mm nominal height) looks fuzzy to the eye at 100% zoom, it will likely fail automated reading.

How Resolution is Determined

  1. Paper Size Inference Werk24 analyzes the drawing (title block, borders, scale, frame marks) to estimate the intended ISO 216 paper size (e.g., A4, A3, A2). We do not trust file metadata alone, because export/scan pipelines can scale or crop.

  2. Resolution Calculation The inferred paper size’s physical dimensions are compared to the image’s pixel dimensions to estimate effective DPI:

DPI_x = pixel_width  / width_in_inches
DPI_y = pixel_height / height_in_inches
effective_DPI = min(DPI_x, DPI_y)

If effective_DPI < 200, the file is rejected.

Note: If a drawing is exported at a print scale (e.g., “fit to page”, 1:2, 2:1), the apparent DPI at the drawing’s intended scale changes. See Scaling & Plot Ratios below.

Minimum Pixel Dimensions by Paper Size (at 200 DPI)

ISO Size mm (W × H) inches (W × H) Min pixels @ 200 DPI (W × H)
A4 210 × 297 8.27 × 11.69 1654 × 2338
A3 297 × 420 11.69 × 16.54 2338 × 3307
A2 420 × 594 16.54 × 23.39 3307 × 4677
A1 594 × 841 23.39 × 33.11 4677 × 6622
A0 841 × 1189 33.11 × 46.81 6622 × 9362

These are minimums. For best OCR of fine text, symbols, and tolerances, aim for 300–400 DPI (e.g., A3 ≈ 3507×4961 at 300 DPI).

CAD → PDF (Vector Preferred)

  • ✅ Preferred: Vector PDFs directly from CAD (no rasterization).
  • Set line weights ≥ 0.1 mm, text height ≥ 2.5–3.5 mm at print scale.
  • Avoid “flatten to image” options. Keep embedded rasters (logos, photos) ≥ 300 DPI at placed scale.
  • Disable “fit to page” if it changes intended plot scale unintentionally.
  • Use black on white or high-contrast monochrome; avoid colored construction layers.

Raster Exports (PNG/TIFF)

  • Use PNG or TIFF (lossless). Avoid JPEG (compression artifacts hinder OCR).
  • Resolution: 300 DPI recommended (200 DPI minimum).
  • Color mode: Grayscale or bilevel (with good thresholding).
  • Post-processing: Deskew, crop borders, remove heavy dithering; do not over-sharpen.

Scanning Paper Drawings

  • Flatbed preferred; auto-feed can skew.
  • 300 DPI grayscale; avoid auto “photo” filters.
  • Disable “auto-deskew” if it causes warping; basic deskew is fine.
  • Keep the entire border/title block visible (Werk24 uses them for size inference).
  • Scan multi-sheet sets consistently (same DPI and orientation).

Scaling & Plot Ratios (Important!)

  • If you downscale the drawing at print/export (e.g., plotting an A1 to an A3), the effective DPI at the original design scale drops proportionally.
  • Similarly, camera photos that are “zoomed out” reduce the effective DPI of the content of interest.

Examples

  • A1 printed to A3 at 300 DPI → effectively \~150 DPI w.r.t the original A1 geometry → likely to fail.
  • A3 exported at 400 DPI but scaled to 50% → effective ~200 DPI → bare minimum.

Best practice: Export/scan at the intended paper size (no scaling) and ≥300 DPI.

Vector PDFs vs. Raster Files

  • Vector PDF: DPI requirement does not apply to vector primitives (lines, text). However, any embedded images (e.g., raster logos, stamps, pasted sketches) must still be legible.
  • Raster-only PDFs: Treated like images; the 200 DPI minimum applies to the page raster.
  • Hybrid PDFs: If your export flattens to raster, you must meet the 200 DPI rule. Prefer keeping vector content intact.

Common Failure Patterns (and Fixes)

  • “Fit to page” exports that unintentionally shrink content → Re-export at 1:1 for the intended paper size.
  • Low-bitrate JPEG scans with ringing/noise → Re-scan as PNG/TIFF.
  • Phone photos (keystone distortion, glare) → Avoid; use a scanner/export from CAD.
  • Clipped title block/borders → Ensure full frame is included.
  • Colored or low-contrast layers → Use monochrome/high contrast.

FAQ

Q: My PDF is vector; do I still need 200 DPI? A: Not for the vector geometry itself. But any embedded raster regions must still be legible; if your export flattened to an image, the 200 DPI rule applies.

Q: Does color matter? A: No, but contrast does. Monochrome or grayscale with crisp edges yields the best results.

Q: Can I upscale a low-res image to pass the check? A: No. Upscaling increases pixel count but not true detail; OCR accuracy will remain poor.

Q: Multi-sheet PDFs? A: Each sheet is evaluated. Use consistent orientation and resolution across all pages.

Q: Minimum readable text? A: As guidance, keep nominal text heights ≥ 2.5–3.5 mm at the intended paper size/scale.

Best-Practice Checklist

  • [ ] Export vector PDF from CAD whenever possible.
  • [ ] If raster is required, use PNG/TIFF, ≥300 DPI, no “fit to page.”
  • [ ] Maintain intended paper size (A4/A3/A2/…).
  • [ ] Ensure full borders/title block are visible.
  • [ ] Keep line weights/text sufficiently thick/large.
  • [ ] Avoid JPEG compression and phone photos.

If you still see DRAWING_RESOLUTION_TOO_LOW after following this guide, please share a sample (with redactions if needed) and the export/scan settings so we can help diagnose.