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PS: Straightening an image

Often when you scan things, or take pictures, you discover after you see the picture on your screen that you were a few degrees off the horizon. Usually this doesn't matter, and you can get away with cropping out the edges, and nobody will ever be able to tell.

Some times, however, you aren't so lucky, and will have to straighten the lines in an image considerably. You might be trying to use the Transform tool, or you might try to rotate the image in another way, but you will find that it can be incredibly hard to completely align an image. This is particularly frustrating if you have a line running along the length of an image; The edge of a building for example. If the line is only a fragment of a degree off, it becomes very obvious.

Luckily, Photoshop provides a useful way to solve the problem; Arbitrary rotation.

If you have a look in the Image – Rotate – Arbitrary, you see that you can fill in any value you want the image rotated. “Great”, I hear you think, “But how the hell do I know how many degrees I am off?”. Easy; Remember what I wrote about the Measurement tool (see the introduction to the tool palette)?

Use the ruler tool, and drag a line, using the ruler tool, along the line in the picture – this can be a vertical line or a horizontal line. Now, go back to the arbitrary rotation menu. Look! Magically, a number is filled in. This number is the exact inverse of the rotational error to the nearest 45 degrees. In other words, unless your image is spectacularly askew, you can just select the line, chose arbitrary rotation, and Photoshop sorts the rest out! When it is done, you can just crop off the edges (you will get a little blank space along the edges where the rotation happened), and you are done!

And now, in pictures:

	Such a gorgeous picture of Organic marmelade... But it's all crooked!
	Use the measurement tool, and make a line that runs exactly along whatever you want to straighten. If you want to use a vertical line instead, that's possible, Photoshop is clever enough to know what you mean.
	From the Image menu, select Rotate Canvas, then Arbitrary...
	... And the correct value will be filled in already! Just click OK. that gives you...
	The final picture! Use the marquee or crop tools to crop of the edges...
	... And you are left with a perfectly aligned photograph!