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Basic Photo Editing with GIMP- part 1: Straightening a Photo
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Most 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 follows:

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 dialog.

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 with GIMP.
January 17, 2012

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