How to Square All of Your Corners in GIMP

Thursday, June 16, 2011  at 6:58 AM
Learn to adjust for photos that are not lined up properly.  Welcome to the eighth article in my Photography Editing in GIMP series!

Previous articles:

Article 1: Easy Photography Editing in GIMP
Article 2: How to Crop in GIMP
Article 3:  How to Control Saturation in GIMP
Article 4: How to Control Contrast and Lightness
Article 5: How to Change Color Photos into Black and White
Article 6: How to Level a Horizon
Article 7: How to Correct a Skewed Perspective in GIMP

These tutorials use the free photo-editing program GIMP.  I am not being paid to endorse GIMP--I just really like it!

How to Square All of Your Corners

By using the three tools, shear, rotate and perspective, you can turn a photo from this:

French Carousel by Danalynn C

into this:

French Carousel by Danalynn C

Step 1: Open the picture with GIMP, set up your guide lines, then rotate the photo to the correct vertical orientation using the Rotate tool.  I usually compare my vertical guide lines to a vertical line that is in the middle of the photo and is straight on for this step, as that line will not be distorted by perspective. 

French Carousel by Danalynn C

Step 2: Following your guide lines, adjust the perspective using the Perspective tool.  Focus on making all of the other vertical and horizontal lines straight.

French Carousel by Danalynn C

Step 3: Some people like to use Shear when squaring edges.  Shear allows you to push the one set of sides of the image so that, even though they are still parallel, they do not form right angles with the other sides.  I am not such a fan of shear because I I feel that Rotate and Perspective keep the original angles more intact and the picture ends up looking more natural.  Here is an example of what shear does:

French Carousel by Danalynn C

Step 4: Once all of your edges are squared, save!

French Carousel by Danalynn C
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