|John's introductory article about GIMP
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photos need at least some straightening, even those shot using a
tripod, unless the photographer has taken the trouble to incorporate
some kind of leveling tool, like a bubble level. You may want to
do a bit of preparation before you dig into this. To start, click
the first link at the left and do some reading about GIMP, and if you
haven't already, you should download, install and start GIMP.
The trick with GIMP is to just use the Rotate tool from the GIMP Toolbox, but first,
1. Open the image to be straightened: Use the File → Open
command. You may get a message from GIMP about the image's
embedded color profile. You can Keep or Convert the profile-- it
doesn't matter for the purposes of this tutorial.
2. Next, identify either the horizon or any line in the image that
should be parallel with the horizon like the top of a wall or a
sidewalk, or any line or object that should be vertical, like the
center of a nice, straight tree. Drag a horizontal guideline from
the ruler above the image, or a vertical guideline from the ruler at
the left of it by moving the cursor into the ruler and clicking and
dragging. Stop dragging when your guideline intersects or is
close to the element in the image to which you want to straighten it.
3. Select the Rotate tool by clicking or double-clicking it. You
can find it in the toolbox by hovering your mouse above each tool until
a little box appears with the name of the tool. These little
boxes are called “tooltips.” Also, each tool button also features a
graphic showing what the tool does. The Rotate tool has two
rectangles, one straight, and one rotated, and two tiny, white arrows
that show the rotation of the second rectangle.
4. Each tool features a set of options that change the way the tool
works. These options appear in the area below the toolbox when you
select the tool. Set the Rotate tool Options for straightening as
Transform: Layer...the first choice, and a Rotate tool default, meaning that this option is pre-chosen for you
Direction: Normal...the default
Interpolation: Sinc (Lanczos3)...the default scaling option
Clipping: Crop with aspect...NOT a default, so you have to change this one
Preview: Image...the default
and ignore the rest of the Options.
5. Next, drag the Rotate tool in the image. You will notice that
the image rotates as you drag, and that the Rotate dialog
appears. If the dialog window is in the way, move it out of the
way by dragging its titlebar (the fat part of its border at the top
where you see the word Rotate next to the little fox.) Now, drag
in the image until the element of the image that you chose in Step 2,
above, is lined up with your guideline. After you finish
rotating, finalize the operation by clicking the Rotate button in the
6. The image has now shrunk a bit as a result of the rotating and its
attendant cropping. The next thing to do is to use the Autocrop
Image command in the Image menu to get rid of the empty part of the
canvas that now appears around the image: Click Image → Autocrop Image.
7. Next, get rid of your guideline: Click Image → Guides → Remove all Guides.
8. Finally, don't forget to save your changes: Click File → Save.
GIMP is a powerful and free program. In the next article in this
series, John will explore the why's and wherefore's of cropping images
January 17, 2012