Perspective Correction Online
Drag the four corners onto the thing that should be rectangular. Get it straight, at full resolution, without uploading anything.
You have the perspective. Now draw into it.
Drop any of 46 grids onto a layer in Procreate, Photoshop, Clip Studio or Krita: transparent PNG, SVG and PSD, no setting up. Or draw over 252 finished scenes with the lens, camera height and tilt already written on them. The same 46 grids print as vector sheets if you work on paper, and thirty worked exercises teach the geometry behind all of it. Every one has a free starter, so you can try the real files first.
Free starters: seven layers, twenty scenes, five exercises. The three grid-and-reference packs together are $24, seven less than the parts. The printable templates and the reference library stay free whatever you do — a pack sold is what keeps them that way.
How to fix perspective in a photo
Point-and-shoot photos of buildings lean backwards, whiteboards photographed from a seat are trapezoids, paintings snapped in a gallery are never square. All of these are the same problem: a flat rectangle seen from an angle. Give the tool the four corners of that rectangle and it computes the exact transform that puts it back.
Two things to know before you drag a corner. Correction always resamples, so a heavily leaning building comes out softer than the original and there is no way round that; correct once from the largest file you have rather than repeatedly. And if the edges are curved rather than leaning, that is lens distortion, not perspective, and no four-corner tool will fix it: run a lens profile first in Lightroom or Affinity, then come back.
Open your photo, then drag TL, TR, BR and BL onto the corners. Zoom the browser if the corners are small. The result preview updates as you drag; when it looks right, download at full resolution.
Choosing the output shape
Auto estimates the rectangle's real proportions from how the four corners are arranged in perspective. For documents you know the size of, set it directly: A4 or Letter for pages, 16:9 for screens and TVs, square for album covers and tiles. A wrong ratio produces a result that is straight but stretched, which is easy to spot on text.
Where the pixels come from
Straightening stretches the far side of the object, so the part that was small in the photo is enlarged and loses sharpness. Shoot as square-on as you can and use the correction to finish the job rather than to rescue a photo taken from a sharp angle. For scanned documents, add a light sharpen afterwards.
The same thing inside your app
Photoshop: the Perspective Crop tool, or Edit → Perspective Warp. Affinity Photo: the Perspective tool in Source/Destination mode (lesson). Lightroom: Transform → Guided. GIMP: the Perspective tool in Corrective mode. This page does it without installing anything, and at full resolution.
Other image tools
If you are correcting a photo to draw over it, the 3-point grid can match the camera angle first, and the perspective warp tool does the reverse job: putting a flat image back into a photo.
Questions
How does perspective correction work?
Four corner correspondences define a projective transform (a homography) exactly. The tool computes it from where you put the handles and where a rectangle of the chosen shape would be, then resamples every output pixel from the photo with bilinear filtering. It is the same mathematics Photoshop's Perspective Crop and Lightroom's Guided Upright use.
Is my photo uploaded anywhere?
No. The image is read into your browser's memory and processed there. Nothing is sent to a server; the site has no upload endpoint.
Why does the result look stretched?
Auto estimates the real proportions from the shape of the quad, which works well for moderate angles. For extreme angles or objects with known proportions, pick the ratio explicitly (A4, 16:9, square).
Can it straighten a whole building, not just one face?
It straightens one plane at a time. Put the handles on the corners of the main façade; the rest of the photo is transformed consistently. For two faces at once, use the 3-point grid to plan and Photoshop's Perspective Warp with two planes.
What is the maximum image size?
There is no fixed limit; large phone photos (12–48 MP) work but take a few seconds to render at export. The output width is capped at 4,000 px.