Сropping is the process of removing unwanted information from an image. It is used when it is necessary to remove extraneous trash from an image, to improve its framing, or to isolate subject matter. But in the field of video coding and broadcasting, its main application is changing the aspect ratio.
Cropping is an alternative to image stretching and letterboxing.
Letterboxing is the process of filling the blank spaces. It has become a popular technique since the advent of widescreen television. Сropping allows you to remove these blank spaces, but it is worth the loss of large image space. For example, when fitting a widescreen 16:9 image to a 4:3 format, 25 percent of the original image is lost. Another limitation of cropping is the impossibility to revert it unless the original still exists or undo information exists. That problem can be partly solved by texture synthesis, but that technique can only work with natural patterns such as grass or sky.