Interlacing is an option when you create GIF and PNG bitmap files. When an interlaced file is loaded into an application which supports interlacing, the picture first appears as a low resolution image. As more of the file is loaded, the resolution increases. This technique is used commonly on the Internet to improve the appearance of bitmaps as they are downloaded. It will almost look like the picture is coming into focus. If you do not use interlacing, the picture will only be displayed when the whole image has been downloaded.