Why scans look skewed — and how straightening actually works

“It's crooked” bundles three different geometry problems. Untangling them explains why some crooked photos come out perfectly straight and others need a re-shoot.

The three crookeds

• Rotation: the page is turned relative to the frame. Trivial — cropping to the detected corners removes it entirely.
• Perspective (keystone): you shot at an angle, so the far edge looks shorter — the page is a trapezoid. Fixed mathematically: knowing the four corners, the image is re-projected into a true rectangle. (How that works.)
• Curl/warp: the paper itself isn't flat, so text lines bend. Not fixable by corner math — the lines really are curved in 3D. Flatten before shooting.

Helping the straightener

1. Get all four corners in frame — correction is computed from the corners.
2. Keep angles moderate: correction from 15° is invisible; from 60° the far text was captured tiny and enlarging it costs sharpness.
3. If detection picked a wrong corner (clutter, folds), fix it in the editor — a correct quadrilateral in, a perfect rectangle out.

Why your result is a clean rectangle

After correction, ToPDF renders the page onto a fresh, axis-aligned canvas — so the PDF page is the document, edge to edge, no tilted photo inside a white frame. That's also why filling the frame at capture time translates directly into resolution in the final page.

Frequently asked questions

Is there a maximum angle that can be corrected?

Geometrically no, practically yes: past ~45° the far side’s pixels get too sparse to enlarge nicely. Shoot as square-on as the situation allows.

My text lines are straight but the page edges look bowed. What’s that?

Slight lens distortion or paper bow. Mild cases are harmless; strong bow means the paper wasn’t flat — see curled pages.