How to Correct Image Perspective with the Gimp
Converging verticals occur in an image if the camera is tilted upwards. Large format film cameras offer a limited opportunity to avoid this problem by distorting the camera bellows to move the lens upward while keeping the plane of the lens vertical. This option is unavailable for digital cameras but can be simulated in software.
The Gimp, an open source image editor offers a feature set equivalent to that of Adobe Photoshop but can be confusing and require more experimentation than Photoshop.
The Gimp Perspective correction tool allows correction of converging verticals on the whole image or on a selection.
Finding the tool
Having opened a copy of the image on which you want to work there are two options: The Gimp Menu bar and the toolbox. To open the tool from the menu select Tools--> Transform Tools--> perspective or press shift-P. This will open a dialogue box and display a default grid. Ignore the numbers in the dialogue box that define the transformation matrix.
Alternatively open the toolbox and select the perspective tool. This gives more control over the process: you can choose whether or not to show a grid (default is no grid) , the number of grid lines, the spacing between the lines and the transformation algorithm used. When the verticals are corrected a pixel may be moved so that its red green and blue values have to be distributed over one or more pixels taking account of the values in the target pixel. Each algorithm produces slightly different results but generally the default is good enough.
Using the tool
Having opened the tool and selected grid options, select a gridline near one of the verticals you want to correct and click the mouse on it. You can then drag the gridline and you will get a preview of the transformed picture. This will not be a clean preview because it has to be done fast.
If you are correcting converging verticals, which look line this
You need to overcompensate because the whole image is transformed, so if you drag the left hand vertical you get something like this.
You then need to drag the right hand vertical to get the desired result. It may be necessary to drag left and right hand verticals several times in order to get the image right.
Once you are happy click Transform on the dialogue boy and after a few seconds the image will be transformed and can be saved.
The Gimp Perspective Correction tool offers options for correcting converging verticals in an image and is useful when these cannot be avoided at the time the picture is taken. There are several options available and the ability to transform only a selection raises creative possibilities, not explored here.