Digital Photo Processing - Straighten the Horizon Line

In your digital photos, is the horizon line or the main subject not straight? Most higher-end digital imaging software come with free-form rotation tools where you can adjust rotation not only by 90 degree increments but by a degree or tenth of a degree. With this feature you should be able to straighten up virtually any image.

Unfortunately, one byproduct of rotation is the addition of blank space around your photo that must be cropped out. This can be done with a rectangular mask and cut/paste tools, or a cropping function if provided.

After cropping, your aspect ratio (number of horizontal pixels versus vertical pixels or vice-versa depending on your framing) will change, so before you print or otherwise use the photo you may also have to resample it slightly or crop out more information.

Now is where your math skills may come in.

For example, let's say you took a photo with a 5 megapixel digital camera. The original width was 2,560 and the height 1,920 for an aspect ratio of 4:3.

1,920 divided by 3 = 640
2,560 divided by 640 = 4.

After a slight rotation and cropping the image turns out to be 2,523 by 1,875 for an aspect ratio of approximately 4.096 to 3.

1,875 divided by 3 = 625
2,523 divided by 625 = 4.0368.

In this case, there are too many horizontal pixels. How many?

625 times 4 = 2,500.
2,523 - 2,500 = 23.

Thus, you need to crop 23 horizontal pixels from this photo to maintain the 4:3 aspect ratio. Afterwards, your photo will be 2,500 x 1,875 and have the correct aspect ratio:

1,875 divided by 3 = 625
625 times 4 = 2,500.