Perspective Corrector

How to Correct Perspective

Features

Every Phone Photo of a Document Is a Little Bit Wrong

Unless a camera is perfectly parallel to the page, a photographed rectangle never comes out as a rectangle — one edge is closer to the lens than the other, and the whole shape comes out as a trapezoid. Most people don't notice until they try to read text near the edges, or line the photo up against something else and see it doesn't quite match. This tool fixes exactly that: not by trimming the photo down, but by mapping the four corners you place back onto a true rectangle, mathematically removing the skew.

How Auto-Detect Output Size Actually Picks a Width and Height

With Auto-Detect Output Size on, the tool measures all four edges of the quadrilateral you've drawn — top, bottom, left and right — and takes the longer of each opposite pair: the greater of the top and bottom edge lengths becomes the output width, the greater of the left and right edge lengths becomes the output height. That's a deliberate choice rather than an average or a guess, because perspective distortion by definition makes the near edge of a photographed rectangle measure longer in the source photo than the far edge — using the longer measurement each time means the output dimensions reflect the subject's true, undistorted size rather than a foreshortened one. The result is capped at 3500 pixels on its longest edge and scaled down proportionally if needed, which keeps processing fast without ever stretching or squashing the final aspect ratio.

Why the Loupe Sits Offset From Your Finger, Not Under It

A magnifier that appears directly under whatever you're touching solves nothing on a touchscreen — your own finger is still covering the exact pixel you're trying to place precisely. This tool's zoom loupe deliberately renders about 20 pixels away from your actual touch or cursor position instead, staying just clear of the fingertip while still tracking the same point you're dragging, and it automatically stays clamped within the visible screen area so it never gets clipped off the edge of the window on a smaller device. That small offset is the entire difference between a loupe that's decorative and one that actually makes pixel-precise corner placement possible on a phone.

Who Actually Reaches for This

• Students and researchers — photographing textbook pages, handwritten notes or whiteboard summaries flat and legible
• Freelancers and small businesses — straightening a photographed invoice, receipt or signed contract before filing it
• Anyone digitizing old documents — photos, certificates or letters that can't go through a flatbed scanner
• Designers — correcting a photographed poster, artwork or sign taken at an unavoidable angle
• Real estate and resale listings — straightening a photographed floor plan or printed spec sheet

What's Actually Happening When You Drag a Corner

The four points you place, together with the rectangle they're meant to become, are enough information to solve for a specific kind of transform called a homography — the same underlying math a document scanning app uses. Rather than moving pixels from the old position to the new one directly (which leaves gaps), the tool works backwards: for every pixel in the final straightened image, it calculates exactly where that content originally sat in your photo and samples it from there. That's why the result stays sharp and gap-free instead of looking stretched or torn.

A Contract Photographed on a Desk

Someone signs a printed contract, photographs it on a desk rather than walking to a scanner, and the resulting image is angled just enough that the page looks like a trapezoid — technically legible, but unprofessional to forward as-is. Dragging the four corners onto the actual paper edges and downloading the result takes under a minute and produces something that looks scanned, not photographed. If the desk or background behind the document also needs to disappear entirely, run the result through our Background Remover afterward.

Choosing Between JPG and PNG for the Output

JPG with the quality slider set high — 90% or above — is the right default for anything photographic: a whiteboard photo, a business card, a signed document with a signature and printed text together, where some lossy compression is invisible to the eye but keeps the file small enough to email or upload without friction. PNG is worth switching to specifically when the corrected image needs to stay lossless for further editing, or when it contains flat graphic elements like a logo or clean line art where JPG's compression artifacts would actually show up as visible noise around hard edges. The quality slider only applies to JPG output, since PNG compression is lossless by definition and has no equivalent quality trade-off to adjust.

Getting a Cleaner Result

• Zoom the loupe in on each corner individually before moving to the next — placing all four roughly first and refining after tends to be slower than getting each one right the first time.
• Turn on the alignment grid if the document itself has straight lines (a printed page, a ruled whiteboard) — comparing the grid against those lines makes it obvious when a corner is off.
• Try Document Enhance mode specifically for text-heavy photos — the contrast boost tends to make printed or handwritten text noticeably easier to read afterward.
• If your photo came out sideways, rotate it first before placing corners — it's much easier to align a quadrilateral against a document that's right-side up.
• For a photo that's just tilted, not trapezoidal, our simpler Rotate & Flip tool's single-angle straighten is faster than placing four corners.

Where This Has Real Limits

This corrects planar (flat) perspective distortion — it can't do anything for a subject that's actually curved, like a photographed page from a thick book that bows away from the spine, since no four-corner transform can un-bend a curve. It also can't recover detail that motion blur or poor focus destroyed in the original photo; a soft photo produces a soft — if properly straightened — result. For genuinely curved-page scanning, dedicated book-scanning software with multi-point mesh correction is built for that specific problem in a way this tool isn't trying to be.