how to deskew rotated scan pages before invoice OCR

TL;DR

Even a few degrees of skew collapses OCR accuracy because character segmentation assumes horizontal baselines. Detect skew with a Hough-transform or projection-profile method, rotate the page to level, and only then run OCR. Measure the win: accuracy on a skewed page typically jumps back to near-level performance.

Steps

1. Estimate the skew angle per page using projection profiles or Hough lines.

Expected: An angle in degrees per page.

1. If the absolute angle exceeds ~0.5 degrees, rotate the page by the negative angle.

Expected: A level page image.

1. Also detect 90/180/270-degree rotations via orientation classification.

Expected: Correctly oriented pages.

1. Re-run OCR and compare field confidence before and after.

Expected: Measurable accuracy improvement.

1. Cache the deskewed image so downstream steps reuse it.

Expected: No repeated preprocessing cost.

When to use

• Visibly rotated or skewed scans
• OCR confidence is low across a whole page
• Preprocessing stage of the intake pipeline

When not to use

• Digital PDFs
• Single-character misreads on level pages
• Table structure problems

Compatibility

OpenCV for skew detection and rotation; works upstream of any OCR engine.

Variant phrasings

deskew invoice scan

rotated page OCR fix

correct scan skew before OCR

Root cause

OCR line segmentation projects pixels onto horizontal axes; skew smears characters across lines. A small rotation moves enough pixels to break segmentation while remaining hard for a human to notice.

Edge cases

• Pages with large dark borders confuse skew detectors; crop borders first
• Mixed content (a rotated photo inside a level page) should not trigger page rotation
• 90-degree detection needs text-orientation signals, not just lines

Provenance

Resolved from the public thread: https://vectle.com/posts/pst7lZwjg58OZor-hR6SHZ0w