Tutorial · Stage 3 of 6
Deskew in ScanTailor Advanced: Straighten Tilted Text — Including Oblique Deskew
Orientation fixed the quarter-turns. Deskew fixes the small angles: the one-or-two-degree tilt that makes text lines slope across the page and quietly sabotages OCR. This stage also hosts v1.1.1’s headline addition — oblique deskew for pages that are physically tilted, not just rotated.
What Deskew Actually Corrects
Deskew determines the angle by which a page must be rotated so its text sits properly horizontal, and applies that rotation. The angle is detected automatically, based on the text and image content of the page.
Know the stage’s limit, stated plainly in the project documentation: deskew is a simple rotation. It cannot correct keystone distortion (a page photographed at an angle, so one side is larger than the other) and it cannot correct curling. Those are geometric distortions, and they belong to the Dewarping step at the Output stage. If a page looks trapezoidal rather than merely tilted, deskew is not your tool — leave its angle at the automatic value and handle the shape later.
Automatic Detection, and When to Override It
For most pages you will do nothing here: run the stage’s batch button, let the angle be detected for every page, and move on. Override manually when a page fools the detector — typical offenders are pages with very little text, pages dominated by a rotated illustration, and title pages with decorative layouts.
Manual adjustment works two ways:
- Drag the round handles at the edges of the page in the stage’s working area to rotate it by eye, lining text up against the horizontal.
- Type the angle explicitly in the stage’s panel, in degrees. Positive values rotate clockwise, negative values counter-clockwise. A fine-adjustment trick from the documentation: click into the angle field, then use the mouse wheel to nudge the value in small steps while watching the text lines.
As everywhere in the pipeline, a hand-tuned angle can be propagated with the stage’s Apply to options when the same tilt repeats across a run of pages — a misaligned scanner lid produces exactly that pattern. Pages whose detected skew disagrees wildly with the rest of the book are worth a look: ScanTailor Advanced’s deviation highlighting flags pages with unusually high skew in the Deskew stage, which turns “find the crooked pages” into a sorting exercise instead of a page-by-page hunt.
Oblique Deskew: For Pages That Are Tilted, Not Just Rotated
v1.1.1 (April 2026) added the feature this stage was missing. The official changelog describes it directly: pages that are not just horizontally skewed but also physically tilted can now be corrected automatically. A new vertical lever in the Deskew stage allows fine-tuned manual adjustment of that vertical component.
The distinction matters in practice:
- Ordinary skew — the whole page rotated a degree or two on the scanner glass. Text lines slope, but they slope uniformly, and a single rotation fixes them.
- Oblique tilt — the page itself sat at a physical tilt during capture (a book page lifting toward the spine, a sheet fed at a slant), so the text is sheared: lines are not merely rotated, the page’s geometry leans. A plain rotation can make one edge horizontal only by leaving the rest wrong. The oblique correction addresses that vertical component.
Two version notes, both from the official release notes. In v1.2.1 the oblique model was extended with separate automatic-oblique gating and selective Apply To paths — and automatic oblique correction is off by default on new projects, so on the Linux v1.2.x line you enable it deliberately rather than having it act on every page. The Windows build (v1.1.1) is where most desktop users meet the feature first; its lever is there for the pages where the automatic result needs a nudge.
How Deskew Errors Propagate
Deskew sits early in the pipeline, so its mistakes are inherited: the content box at Select Content is fitted to text that should be level, and dewarping builds its distortion model from text lines it assumes are horizontal on the flat page. A page left a degree out will still process — but its content detection and dewarping both start from a small lie. The cheap insurance is the thumbnail pass: after batch deskew, flip through the thumbnails quickly. Crooked pages are easy to spot at thumbnail size precisely because every other page is straight.
Frequently Asked Questions
What is the difference between Fix Orientation and Deskew?
Fix Orientation rotates in 90° steps to get a sideways or upside-down scan upright, and it is manual. Deskew then applies the small fractional angle that makes text lines horizontal, detected automatically per page. One fixes which way is up; the other fixes how level “up” is.
Can Deskew fix a page photographed at an angle (keystoning)?
No. Deskew is a simple rotation, and the project documentation states it cannot correct keystone distortion or curling. Geometric distortion is handled by Dewarping at the Output stage, which builds a distortion model of the page instead of merely rotating it.
What is Oblique Deskew and which version has it?
Oblique Deskew corrects pages that are physically tilted, not just rotated in the plane — a vertical shear component that plain rotation cannot fix. It arrived in v1.1.1 with automatic correction plus a vertical lever for manual fine-tuning; v1.2.1 extended the model but leaves automatic oblique correction off by default on new projects.