By Ian Pullen
One of these improvements is the Cage Transform Tool that introduces a new powerful and versatile way to transform photos and areas within photos. This won't be immediately useful to all GIMP users, though it could be a useful way for photographers to reduce the effects of perspective distortion. In this tutorial, I'm going to use an image that exhibits perspective distortion as the basis of showing you how to use the new tool.
Perspective distortion occurs when the lens of a camera has to be inclined in order to get the whole of a subject in frame, such as a when photographing a tall building. For the purpose of this tutorial, I deliberately induced perspective distortion by getting down low and taking a photo of a door into an old barn. If you look at the image, you'll see that the top of the door appears to be narrower than the bottom and that is the distortion that we're going to correct. While it is a bit of a rickety barn, I can assure you that the door is, by and large, rectangular in reality.
If you've got a photo of a tall building or something similar that suffers from perspective distortion, you can use that image to follow along. If not, you can download a copy of the photo that I've used and work on that.