How to straighten a crooked document photo

Take a photo of a page on a desk and it rarely comes out as a neat rectangle. It's tilted, or the top is narrower than the bottom, or there's desk and coffee cup all around it. Rotating the photo doesn't fix that. What fixes it is perspective correction — the thing a scanner app does when it "finds the corners".

Why pages come out wedge-shaped

When the phone isn't perfectly parallel to the page, the edge of the paper closer to the lens looks bigger than the edge farther away. A rectangle photographed at an angle becomes a trapezoid: the far end narrows and the text there shrinks. On top of that, the page is usually rotated a few degrees and surrounded by background.

Rotating fixes the tilt but not the wedge. Cropping removes the background but leaves the page skewed. To get a flat, square page you need to map the page's four corners back to the corners of a rectangle — that's perspective correction.

How automatic corner detection works

A scanner app has to figure out where the page actually is. In CrunchFile this happens in two steps:

1. Find the sheet. An AI vision model looks at the photo and identifies the main sheet of paper — the one you're clearly trying to scan — even when there's a second piece of paper, a phone or a hand in the shot. It also recognizes when the image is already just the page (a normal PDF page, an earlier scan) and leaves it alone.
2. Pin the corners. Edge detection then finds the exact paper edges inside that area and where they meet, so the corners land on the actual page edge rather than roughly near it.

The page is then warped back into a rectangle: the far, narrow end is stretched out, the tilt is removed, and the background is cut away.

When you open a document in the Scan tool, this runs on every page automatically. Pages that were straightened are marked, and if the app isn't confident about a page, it marks it Check corners instead of guessing.

Fixing the corners by hand

Sometimes a corner is hidden under a thumb, or the page edge disappears into a white tablecloth.

1. Select the page and tap Crop corners.
2. Drag each corner handle to where the paper's corner actually is. Use the page's edges as your guide, not the text.
3. If a corner is cut off by the photo frame, put the handle where the two edges would meet at the border.
4. Tap Apply crop. If it looks worse, Undo puts the original back.

Things straightening can't fix

• Curled or folded pages. Perspective correction assumes the page is flat. A deep curl or a fold stays curved after correction. Flatten the page and retake it.
• Missing parts. If part of the page was outside the photo, it's gone. Leave background on all four sides when you shoot.
• Blur. Straightening a blurry photo gives you a straight, blurry page.

Take photos that need less fixing

• Shoot from directly above, phone parallel to the page. The closer to parallel, the less the image has to be stretched — and stretching is what softens text at the far end.
• Leave a margin of background around all four edges.
• Use a background that contrasts with the paper: a dark desk for white paper.
• Keep fingers off the corners. Hold the page down at the middle of an edge instead.

Get those right and the corners are found instantly; get them mostly right and a quick drag of one handle finishes the job.