Fix Perspective in Photos Free – Auto Corner Detection

Why Fix Perspective Distortion?

A rectangle photographed from anywhere but straight on stops being a rectangle. A page becomes a trapezium, a building's walls lean toward each other, and a painting on a gallery wall comes out as a kite. Correcting perspective maps that shape back to the rectangle it really is, which is what makes a phone photo of a document usable as a document.

What Correction Gives You

A readable page:: An angled photo of a contract or a receipt becomes a flat, rectangular scan that text recognition and printers can both handle.
Honest architecture:: Converging verticals are a side effect of pointing a camera upward, not a property of the building. Correcting them shows the walls as they are.
The true proportions:: The tool measures the rectangle's real width-to-height ratio from the photo itself, so a corrected A4 page comes out A4-shaped rather than stretched.
Clean edges:: Trim to a clean rectangle removes the transparent wedges that every correction leaves at the corners.
Full resolution:: The output is sized from the longest edge of the region you marked, so nothing is resampled smaller than the detail the photo actually holds.

How to Fix Perspective Distortion in a Photo

Mark the rectangle you photographed, check the corrected preview beside it, then export.

Add your photo: Drop in a JPG, PNG, WebP, GIF, AVIF or BMP.
Find the edges: Press Find the edges. The four handles jump to the page, whiteboard or painting.
Refine the corners: Drag any handle onto the exact corner. A magnifier follows your finger, and arrow keys nudge a pixel at a time.
Choose the output shape: Measured recovers the rectangle's true proportions from the photo. A4, Letter, square and 16:9 are there when you want to force one.
Correct and download: Press Correct perspective, then save as PNG, JPEG or WebP.

How Perspective Correction Works

Four points in the photo and four points in the output define exactly one projective transform — an eight-number map that turns any quadrilateral into any other. Every output pixel asks that map where it came from and reads the photo there, which is what keeps straight lines straight instead of leaving holes wherever the correction stretches.

The Steps It Takes

Edge detection:: Gradients across the whole picture are grouped into candidate lines, and every candidate rectangle is scored by how much of its outline sits on real edges — so four strong lines that happen to cross do not win.
Sub-pixel refinement:: Each of the four sides is re-fitted to the pixels beside it at full resolution, which is the difference between a corner that is close and a corner that is right.
Shape measurement:: The two pairs of opposite edges meet at two points that reveal the camera's angle, and from those the rectangle's true proportions follow.
Resampling:: Nearest, bilinear, bicubic and Lanczos-3 are all available; the wider kernels keep fine detail through a steep correction.
Hardware acceleration:: The correction runs on your device's graphics hardware, so the preview beside the handles updates while you drag rather than after you let go.

Three Ways to Describe the Correction

Not every photo has four visible corners, and not every correction is about a rectangle. The tool offers three routes to the same transform, and you can move between them without losing your place.

Find the edges:: One press. Best for a document, a whiteboard, a screen or a framed picture — anything with a boundary that stands out from what is behind it. The confidence is shown, so you know when it was a guess.
Four corners:: Drag each handle onto a real corner. The magnifier makes a one-pixel target reachable with a finger, and arrow keys nudge a pixel at a time for a mouse.
Guide lines:: For architecture, where there is rarely a visible rectangle but always a vertical — a door frame, a downpipe, a window column. Auto Upright finds those lines and levels them, on either axis or both.

Common Use Cases

Anywhere a flat, rectangular thing was photographed from an angle instead of scanned.

Document capture

A contract on a desk, a receipt in a hand, a book page under a lamp. Mark the page, and the output is a rectangular scan ready for filing, printing or text recognition.

Whiteboards and slides

Meeting notes photographed from a seat at the side of the room come back square, legible and croppable.

Architecture

Point a camera up at a building and the verticals converge. Guide lines put them back upright without touching the rest of the frame.

Artwork and reproductions

A painting or a print photographed off-axis is squared up, and the measured output shape keeps the artwork's real proportions instead of guessing them.

Product photography

Packaging, labels and flat lays straighten so a batch of shots share one geometry.

Speed and Privacy

Everything happens on your device — the photo is never uploaded. Corrections run on your graphics hardware, which is what makes the preview move with the handles: a 12-megapixel correction takes a few milliseconds, and dragging a corner re-renders without touching the file again.

Frequently Asked Questions

How do I fix a leaning building in a photo?

Switch to Guide lines and press Vertical. The tool finds the strongest near-vertical edges — door frames, downpipes, window columns — and works out the correction that makes them parallel. If the automatic guides land on the wrong features you can pick Level for horizontals or Full for both, and the fine adjustment sliders let you take the correction further or pull it back.

What does the Measured output shape do?

It recovers the real width-to-height ratio of the rectangle you photographed. A quadrilateral in a photo carries enough information to work out the angle the camera was at, and from that the true proportions of the rectangle follow. It means a photo of an A4 page corrects to an A4-shaped image without you telling the tool it was A4 — and if the subject is not really a rectangle, the other output shapes are there to override it.

What is the difference between perspective correction and keystone correction?

Keystone correction is one particular case: a rectangular subject that looks wider at the top than the bottom, or the reverse. This tool performs full four-point correction, which covers keystone along with any other converging-line distortion, on either axis or both at once.

Can I correct a document or book page photographed at an angle?

Yes, and it is the case the automatic detection is tuned for. Press Find the edges and the four handles jump to the page; refine any corner that landed short, then export. The result is a flat rectangular image suitable for text recognition, archiving or printing, and it works the same on a phone snapshot as on a photo from a camera.

Does correcting perspective crop my image?

In four-corner mode, yes by design: the output is the region you marked, and pixels outside it are not part of the rectangle you asked for. In guide-line mode the whole frame is corrected, which leaves transparent wedges at the corners — Trim to a clean rectangle removes them by taking the largest rectangle that fits inside the corrected shape. Both are optional and both are shown in the preview before you export.

Why does my result look softer than the original?

A correction resamples every pixel to a new position, and a steep correction stretches one side of the image much more than the other. Start from the highest-resolution version you have, and leave the resampling filter on Lanczos, which is the widest of the four and holds fine detail best. If a corner still looks wrong, check that the handles are on the true corners rather than a shadow or a border.

Can I correct several photos at once?

Pro queues up to 100 images and downloads them as a single ZIP. Because each photo has the page in a different place, the batch re-runs edge detection per image rather than copying one set of corners across all of them. Free corrects one image at a time, with every correction mode and every adjustment available.

Which file types can I use?

JPG, PNG, WebP, GIF, AVIF and BMP go in; PNG, JPEG and WebP come out. HEIC and TIFF cannot be opened on the device, so those are refused with a link to the converter rather than a failure part-way through.