Resolution Limitations for Technical Drawings
Overview
The guidance below applies to any raster. That means plain images (PNG, JPEG, TIFF) and drawings that were scanned or flattened into a PDF, which are rasters wearing a PDF extension. A pure-vector PDF straight from CAD has no fixed resolution at all: we rasterise it ourselves at a resolution we choose.
⚠️ The automatic check, however, runs on image uploads only. A resolution is measured for PNG, JPEG, TIFF and HPGL inputs. No PDF is ever rejected for resolution, including a scan saved as a PDF, because a PDF page states its size rather than a scan resolution and we never infer one. So a coarse scan wrapped in a PDF is accepted and read as well as its pixels allow, with no warning that it was too coarse. Size it yourself from the table below; passing means only that nothing checked.
If a raster input is too coarse, the API returns DRAWING_RESOLUTION_TOO_LOW.
The number that predicts success is the height, in pixels, of the smallest text on the sheet. Below roughly 10 pixels extraction accuracy drops sharply; above it, it is stable and flat. Everything else on this page is a way of getting there.
Why DPI on its own is misleading
A file's stated DPI is a claim about how large it is meant to be printed. It is not a claim about how much detail it holds, and on large sheets the two come apart badly:
- An A0 drawing scanned to 3000 pixels on the long edge is about 64 DPI at its true size, however its header is labelled.
- The same 3000 pixels on an A4 is about 257 DPI, and reads comfortably.
Same pixel count, very different outcomes. Lettering on a technical drawing is roughly 2.5 to 3.5 mm whatever the sheet size, so on the A0 that same text lands on a quarter as many pixels.
⚠️ A current limitation, stated plainly. Our check today derives an effective DPI by comparing the long edge against A4's height, whatever the sheet actually is. On anything larger than A4 that over-estimates resolution, so some large-format scans pass the check and still read poorly. We are replacing it with a direct measurement of text height. Until then, do not treat passing the check as confirmation that a large-format scan is detailed enough. Size it from the table below instead.
Minimum Pixel Dimensions by Paper Size
Sized so the smallest text clears the ~10 pixel mark where accuracy stabilises.
| ISO Size | mm (W x H) | Minimum (W x H) | Recommended (W x H) |
|---|---|---|---|
| A4 | 210 x 297 | 1654 x 2338 | 2480 x 3508 |
| A3 | 297 x 420 | 2338 x 3307 | 3508 x 4961 |
| A2 | 420 x 594 | 3307 x 4677 | 4961 x 7016 |
| A1 | 594 x 841 | 4677 x 6622 | 7016 x 9933 |
| A0 | 841 x 1189 | 6622 x 9362 | 9933 x 14043 |
These are sizing targets, not the API's rejection threshold: they are 200 and 300 DPI at the sheet's real size, chosen so the smallest text clears ~10 px. The enforced check is separate and looser, and passing it does not mean a scan is detailed enough.
There is also a ceiling, but every row above now fits under it. An image upload whose long edge exceeds roughly 15,000 pixels is rejected with DRAWING_PAPER_SIZE_TOO_LARGE, whatever sheet it depicts. That is an A0 at just over 300 DPI, so scanning any supported sheet at the recommended resolution stays clear of it.
The ceiling was previously about 7,500 px, which refused ordinary A0 and A1 scans. If you retiled large masters to work around that, you no longer need to; see paper size.
⚠️ It can still surprise you on a sheet scanned very finely. An image carries no physical size, so the sheet is inferred from pixel count, and every sheet has a resolution at which it trips the limit: 1,284 DPI for A4, 908 for A3, 642 for A2, 454 for A1 and 321 for A0. The practical one is A1, where a 600 DPI scan is rejected even though the sheet is within the size limit. If you see DRAWING_PAPER_SIZE_TOO_LARGE on a sheet you know is supported, that is why. Re-scan at 300 DPI.
No PDF is subject to this ceiling, vector or scanned. It is applied when an image is imported, and a PDF page is never imported that way.
Scan at the sheet's real size. An A0 scanned "to A4 dimensions" is an A0 at a quarter of the detail, whatever the resulting file claims about DPI.
Recommended Export & Scan Settings
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: cropping to the drawing area loses the title block, and with it the general tolerances that apply to every unmarked dimension.
- 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.
Which case is my PDF?
A .pdf extension says nothing about how the page was made, and the two cases behave completely differently. We classify each page by how much of it is covered by embedded images:
| The page is | Treated as | Resolution check |
|---|---|---|
| Vector geometry, at most a logo or stamp | Vector | Does not apply. We rasterise it ourselves at whatever resolution we need. |
| One image filling the sheet (a scan, or a "flatten to image" export) | Raster | Applies in full. It is a photograph of a drawing. |
| A drawing with a large pasted image, or a scan with vector markup over it | Mixed | Applies to the raster parts. Prefer splitting these if you can. |
How to tell: open the PDF and zoom to 400%. Vector text stays crisp at any zoom; raster text turns into visible pixels. That is the whole distinction, and it decides whether any of the pixel guidance above applies to you.
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: No. Vector geometry has no resolution, so there is nothing to check. Embedded raster regions (logos, stamps, pasted sketches) still need to be legible in themselves.
Q: My PDF is a scan and the extraction is poor, but nothing was rejected. Why? A: Because the resolution check does not run on PDFs. The page is a fixed grid of pixels rather than geometry, so it is a raster and every pixel target above applies to it, but nothing measures that and nothing warns you. Zoom to 400%: if the text pixelates, this is your case. The fix is to re-export as vector from CAD, or re-scan at the sheet's real size and at 300 DPI.
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 at the sheet's real size, 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.