How to straighten a crooked photo
Paint by Zaweba straightens photos with the Crop tool's Straighten line, its Auto straighten option, or Image > Auto Straighten for tilted pictures.
The quickest way is the Crop tool's Straighten option. In Paint by Zaweba you press C for Crop, choose Straighten and draw a line along something that should be level, such as the horizon or the edge of a table, and the picture is rotated to match. The Crop tool also has Auto straighten, and Image > Auto Straighten levels a tilted picture without any line at all.
A crooked horizon is one of the most common faults in photos, and one of the easiest to fix. Even a tilt of a degree or two makes a picture feel uneasy, and the eye notices it on sea views and buildings in particular.
Drawing a straighten line
This is the most reliable method, because you choose what is meant to be level. Open the photo, pick the Crop tool and choose Straighten. Then drag along a line you know should be horizontal. A distant horizon is best, since it is truly flat. Rooflines and pavements can mislead, because perspective tilts them.
You can do the same with a vertical edge, such as the corner of a building or a lamp post, when there is no horizon. Choose a line near the middle of the frame, where lens distortion bends things least.
When you let go, the picture is rotated to match your line. Check the result before you accept the crop.
Letting Paint do it
Auto straighten in the Crop tool and Image > Auto Straighten both look for strong lines in the picture, using a method called the Hough transform, and level the picture from them. They work well when there's an obvious horizon or plenty of straight architecture. On a portrait or a forest they have less to go on, so check the result and fall back to drawing a line if it looks off.
Keeping the corners
Rotating a photo leaves wedge shaped gaps at the corners, and normally you crop them away. That costs part of the picture. The Crop tool can fill the empty corners with Smart Fill instead, building them from the surrounding picture. It works best when the corners are sky, grass, sand or water, and less well when there is detail such as a person or text near the edge. You can also choose whether cropped pixels are deleted or simply hidden, so the extra is still there if you change your mind.
Getting a good result
Straighten first, then crop for composition, since the two affect each other. The Crop tool's overlays help with the second step: rule of thirds, a grid, diagonal, triangle, golden ratio and golden spiral. Press O to cycle through them and Shift+O to turn the spiral. A grid overlay is also handy just for checking that a horizon now really is level.
Remember that a horizon can be level while verticals still lean. That is a perspective problem rather than a tilt, and rotation won't fix it.
When the problem is perspective
If tall buildings lean inwards or a document photographed at an angle looks like a trapezium, straightening isn't enough. Paint by Zaweba has Perspective Crop for flattening four corners into a rectangle, and Perspective Warp and Wide Angle Correction for converging lines. For a simple tilted snapshot, though, one line drawn with the Crop tool is all it takes.
Scans of several prints
If you are straightening scanned prints, File > Automate > Split and Straighten Photos opens each print on a flatbed scan upright as its own document, which saves rotating them one by one.
