I have an image that I've scanned that I would like to straighten. The image is in a bordered box but the scan wasn't straight, so the image appears rotated. How can I straighten the image so the sides of the box are vertical and parallel as they should be?

So it would just be by eye?

There is a deskew plugin: http://registry.gimp.org/node/2958 Webpage gone but the tinyurl link seems to work. Might / might not work in windows.

edit: looks like some clever person posted a linux binary there.
If using Windows these are samj's deskew plugins.
https://dl.dropboxusercontent.com/u/105 ... -32-64.zip
Unzip, use the appropriate set of .exe and dll's for 32bit or 64 bit.

If you use linux and can compile this works for me
https://github.com/prokoudine/gimp-deskew-plugin

Otherwise there will have to be some human input

Using the rotate tool - set it to corrective mode and rotate the grid using the mouse. Line it up with an edge or text



applied looks like this.



Then there are scripts/plugin aids. Set a path to an edge or text, apply the plugin.
One is ofnuts ofn-layer-straightener.py http://sourceforge.net/projects/gimp-to ... s/scripts/ about 4 down. Un-zip pop into your plugins folder.

Set the path tool to polygonal mode to make it easier.

like this



Result is the same as rotate tool.

All this assumes that there is no distortion on the scan. Might have to apply a perspective transform to make absolutely rectangular.

edit: A quick video on the plugins and setting up some keyboard shortcuts to speed up the process if you have been using a sheet-feed scanner

https://youtu.be/IG9SAUBLKc0 2 1/2 minutes

I do a lot of scanning too. The beds on the affordable scanners are so small, my images are always slanted. :-)

Steps:
1. Click the Measure Tool (so you don't have to eyeball it alone)
2. At the top of your bordered box, draw a line from one side to the other. You may want to zoom in. A quick way to zoom in is to hold the Ctrl key and roll the mouse wheel up and down. In the second video below, he wanted the bridge to be straight. The drawn line will be on an angle.
3. Look at the bottom of Gimp, in the status bar it will show the angle in degrees that you will want to correct.
4. Click the Rotate Tool.
5. Change Direction to Corrective because this ONLY moves the grid which helps you visually see what is going on. I'm not quite sure how Normal is used. It moves both. Show image preview should be checked.
6. Optional Change: Change Clippings to Crop with Aspect, and Number of lines
7. Click the image to display the Rotate Tool's dialog box.
8. Enter the angle value from #3. I entered it in with a negative sign (i.e., -2.18). I then clicked in the text field for "Center X:" so I could see the preview. The grid should have turned and the lines are parallel to your bordered box top (the one you measured). Make any adjustments if you don't like what you see. Click Rotate.
9. To verify, you can drag down a guideline from the top or measure again.
10. Depending on your Rotation options in #6, you may need to either crop the image or Image > Fit Canvas to Layers

A combo of these two videos helped me see how the Tools work and how they could be applied:
Straighten a horizon using rotate tool - GIMP Beginners' Guide ep45
https://www.youtube.com/watch?v=Kc79tmnw7_I
Gimp- Rotating Photos perfectly with Measure Tool
https://www.youtube.com/watch?v=aS70VvqxxcU

With my ofn-layer-straightener script (see above) it would be:
1. Click the Measure Tool Path tool (so you don't have to eyeball it alone)
2. At the top of your bordered box, draw a line from one side to the other. You may want to zoom in. A quick way to zoom in is to hold the Ctrl key and roll the mouse wheel up and down. In the second video below, he wanted the bridge to be straight. The drawn line will be on an angle.
3. Look at the bottom of Gimp, in the status bar it will show the angle in degrees that you will want to correct.You don't need to read any values
4. Click the Rotate Tool. Not needed
5. Change Direction to Corrective because this ONLY moves the grid which helps you visually see what is going on. I'm not quite sure how Normal is used. It moves both. Show image preview should be checked. Not needed
6. Optional Change: Change Clippings to Crop with Aspect, and Number of lines. Not needed
7. Click the image to display the Rotate Tool's dialog box. Not needed
8. Enter the angle value from #3. I entered it in with a negative sign (i.e., -2.18). I then clicked in the text field for "Center X:" so I could see the preview. The grid should have turned and the lines are parallel to your bordered box top (the one you measured). Make any adjustments. Click Rotate. Just call the script

Additionally, if you "make any adjustments" in step 8 then the initial measurement has no purpose. If you have enough grid lines in the Rotate tool you can set one sufficiently parallel to what you want to straighten.

PS: Your 2nd video is a very bad example. The bridge isn't horizontal due to perspective. Unless you have a real horizon (sea, or very flat country side) there is no horizontal reference in a photo. However, verticals should be vertical. In the video above, he should have made the smokestack vertical....