Correcting Perspective and Keystoning

This article is about correcting the visual distortions that occur in photographs caused by cameras. The distortion occurs for a number of reasons, but primarily due to a poor quality CCD and lens. Digital Cameras usually display a grid to help align the camera with the object to minimize this distortion. In this photograph, the nearest corner edge is vertical and shown by the red line. However, the other two corner edges are not completely vertical, and consequently, the image appears incorrect and distorted. This distortion may be correctable on a computer using a graphics application such as GIMP, and this article focuses on the technique of using the software on a PC.

Freeware Software

I started looking on the Internet for freeware software that could correct this type of distortion, and found many trial programs that claimed to do the correction automatically. After testing many programs, I finally concluded that a freeware program for automated perspective correction does not exist yet, and I would have to learn how to make the corrections manually. I was under the impression that it might take a long time to learn, but in fact, it took only a few minutes of playing time.

Gimp was the only freeware graphics package I found that had the necessary tools to manually correct distortions. The only problem with it was that there was no guide to show how I could use it, so I spent a few minutes playing around experimenting and managed to find a way of making it work. After experimentation on a photograph, I managed to correct the perspective! I was surprised that nobody had written anything about this function, because I think it is one of its best and lesser-known features.

Gimp version 2.4

Gimp version 2.4 is fast and will easily run on an old Windows 2000 machine. I did not want to install a processor intensive heavy version that would take forever to load and slow the machine down. It does not show the grid lines by default, so you may have to learn how to enable the lines because you will need them as guides when making the corrections.

Show and Configure Grid

• In the "View" menu option, choose "Show Grid" to display the grid.
• In the "Image" menu option, choose "Configure Grid", which will display a dialog box showing the grid settings.

Distorted Perspective

Once you have a grid, you can see clearly that the vertical corner edges are sloped. In the picture above the outer edges, indicated by the thick lines, are clearly not vertical.

The Tool

Select the perspective correction tool shown by the circle.

Correct Left Side

The best way to use this tool is to drag the square markers at the corners only, and make small horizontal or vertical moves with the mouse. To correct the left corner simply move the top left hand cursor as shown by the arrow. It requires a small horizontal movement to straighten the corner edge making it vertical.

Correct Right Side

To correct the right hand side vertical corner simply move the square marker shown by the arrow 2.

Result

All the corner edges are now vertical. Once you are happy, press the "Transform", which makes the changes permanent. Finally, save the corrected photograph. It is not easy, and many people never get the hang of it, however, ultimately it is worth it if you want your images to appear correctly.