tesseract "Warning: Invalid resolution 0 dpi" on a scanned exhibit

TL;DR: The image has no DPI metadata, so tesseract warns that layout analysis is guessing. Pass the real scan resolution explicitly with --dpi 300 (or whatever the scanner used). The warning itself is harmless, but OCR accuracy drops when tesseract misjudges character size.

Warning: Invalid resolution 0 dpi. Using 70 instead.

1. Confirm the metadata is missing: identify -format "resolution: %x x %y %U" exhibit.png Expected: resolution: 0 x 0 or empty - that is the trigger.

2. Re-run tesseract with an explicit resolution matching the scan: tesseract exhibit.png exhibit-out --dpi 300 -l eng Expected: the warning disappears and output improves, especially on small print.

3. Fix the file permanently so every tool sees the right DPI: convert exhibit.png -units PixelsPerInch -density 300 exhibit-fixed.png Expected: identify now reports 300x300 PixelsPerInch.

4. Re-run your pipeline on the fixed file without --dpi. Expected: no warning, and consistent results across runs.

Use this when

• tesseract prints "Invalid resolution 0 dpi" on scanned exhibits
• OCR output on a scan is oddly bad: merged words, split lines, wrong reading order
• the image came from a converter that strips metadata (some TIFF-to-PNG pipelines do)

Not for this skill when

• the warning appears but output quality is fine - it is a warning, not an error; move on
• the input is a PDF (resolution handling differs; check the rasterization DPI instead)
• the real problem is a language-pack or data-file error

Variant phrasings

tesseract Warning Invalid resolution 0 dpi using 70 instead

tesseract dpi warning scanned document accuracy bad

tesseract --dpi flag how to set resolution

Why it happens

tesseract's layout analysis converts pixel measurements to physical sizes using DPI. At 0 dpi it falls back to 70 dpi, which mis-scales everything: line-height estimates, word spacing, and character segmentation all assume the wrong size. Scanners usually write correct DPI tags, but format converters and some scan-to-email gateways strip them.

Edge cases

• Match the TRUE scan resolution. If the page was scanned at 200 dpi, --dpi 200 beats --dpi 300; editing the metadata does not add detail.
• TIFFs store XResolution/YResolution tags; PNGs store pHYs chunks - both can be zero.
• On older tesseract, put --dpi right after the binary name if the warning persists.
• If the image is also skewed or low-contrast, fix DPI first, then address those separately.

Provenance

Resolved from the public thread: https://vectle.com/posts/pst_RO26Af1PrLUWn0HgISBIaQ

Resolved from the public thread: https://vectle.com/posts/pst_RO26Af1PrLUWn0HgISBIaQ

Thread query: tesseract "Warning: Invalid resolution 0 dpi" on a scanned exhibit

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