It is easy, when taking a picture, to hold the camera not quite
perfectly vertical, resulting in a picture where things are
tilted at an angle. In GIMP, the way to fix this is to use the
Activate this by clicking its icon in the Toolbox, or by
pressing the “R” key capitalized) while inside the image.
Make sure the Tool Options are visible, and at the top, make sure for
“Affect:” that the left button (“Transform
Layer”) is selected. If you then click the mouse inside the
image and drag it, you will see a grid appear that rotates as you
drag. When the grid looks right, click
or press the enter key, and the image will be rotated.
Now as a matter of fact, it isn't so easy to get things right by this
method: you often find that things are better but not quite perfect.
One solution is to rotate a bit more, but there is a disadvantage to
that approach. Each time you rotate an image, because the rotated
pixels don't line up precisely with the original pixels, the image
inevitably gets blurred a little bit. For a single rotation, the
amount of blurring is quite small, but two rotations cause twice as
much blurring as one, and there is no reason to blur things more than
you have to. A better alternative is to undo the rotation and then do
another, adjusting the angle.
Fortunately, GIMP provides another way of doing it that is
considerably easier to use: in the Rotate Tool Options, for the
Transform Direction you can select "Backward (Corrective)". When you
do this, instead of rotating the grid to compensate for the error, you
can rotate it to line up
with the error. If this seems confusing, try it and you will see that
it is quite straightforward.
Note: New in GIMP 2.2 is the option to preview the results of
transformations, instead of just seeing a grid. This makes it easier
to get things right on the first try.
After you have rotated an image, there will be unpleasant triangular
"holes" at the corners. One way to fix them is to create a background
that fills the holes with some unobtrusive or neutral color, but
usually a better solution is to crop the image. The greater the
rotation, the more cropping is required, so it is best to get the
camera aligned as well as possible when you take the picture in the
When you take a picture with a digital camera, you have some control
over what gets included in the image but often not as much as you
would like: the result is images that could benefit from trimming.
Beyond this, it is often possible to enhance the impact of an image by
trimming it so that the most important elements are placed at key
points. A rule of thumb, not always to be followed but good to keep in
mind, is the “rule of thirds”,
which says that maximum impact is obtained by placing the center of
interest one-third of the way across the image, both widthwise and
To crop an image, activate the
tool in the Toolbox, or by pressing the “C”
key (capitalized) while inside the image. With the tool active,
clicking and dragging in the image will sweep out a crop rectangle. It
will also pop up a dialog that allows you to adjust the dimensions of
the crop region if they aren't quite right. When everything is
perfect, hit the button in the dialog.