Removing barrel or pincushion distortion

Removing barrel or pincushion distortion is a special case of 2-D warping. The image is automatically divided into a 4$\times$4 region marked by a blue grid. The corners and the points where the blue lines intersect are control points. To remove barrel distortion, move the corner control points away from the center of the image. To remove pincushion distortion, move the corner control points closer the center of the image.

There are two ways to move the control points:

1. Click near the control point with the mouse and drag the control point in the desired direction.
2. Click near the control point to select it and release the mouse button. Then use the arrow keys on your keyboard to move the control point. This has the advantage of moving the point by a fixed number of pixels each time.

Next, move the intermediate control points along the edge of the image so that the image is warped in a smooth manner. It is okay if the control points are off the edge of the image.

When finished, press the Esc key. The blue guide lines will disappear, indicating that warping is finished.