Image straighten tool
Choose a photo, turn it by a small angle with the slider while you watch a grid over the picture, and line up the horizon or a wall with a grid line. The empty corners are cropped away automatically, or you can keep the full canvas and fill it with a colour.
Runs in your browser: nothing you enter is sent anywhere.
How to use it
- Choose or drop a photo.
- Move the Angle slider until the horizon or a vertical line follows the grid.
- Choose to crop the corners or keep the original size.
- Download the straightened image.
How it works
The picture is rotated around its centre by the chosen angle θ. To crop, the tool finds the largest scale s such that a centred copy of the original rectangle, scaled by s, stays inside the turned picture: s = the smaller of w / (w cos θ + h sin θ) and h / (w sin θ + h cos θ).
Example
A 3000 × 2000 photo turned by 2 degrees has a width limit of 0.9778 and a height limit of 0.9508. The smaller value, s = 0.9508, is used, so the saved picture is 2852 × 1901 pixels.
Tips
- Use a long, clearly straight line such as the sea horizon; short lines are harder to judge.
- If the subject is near the edge, choose Keep original size so nothing is cropped, then crop yourself.
Frequently asked questions
How much can I turn the picture?
Up to 15 degrees either way, in steps of 0.1 degree. That is for fixing a slightly tilted horizon; to turn by 90 or 180 degrees use the rotate and flip tool.
What happens to the corners?
Turning a picture leaves empty triangles at the edges. With the crop option the tool cuts the largest centred rectangle of the original proportions that fits inside the turned picture, so no empty corners remain. The result is a bit smaller. With Keep original size the canvas stays the same and the corners are filled with your colour.
Is the grid saved in the picture?
No. The grid is only drawn in the preview to help you align; the downloaded file has none.
Does turning reduce quality?
The picture is resampled with the browser's smooth interpolation, which softens very fine detail slightly. Work from the original photo, not from an already compressed copy, and avoid turning the same picture repeatedly.