Nine-patch Image in Android

Nine-patch

A NinePatchDrawable graphic is a stretchable bitmap image, which Android will automatically resize to accommodate the contents of the View in which you have placed it as the background. An example use of a NinePatch is the backgrounds used by standard Android buttons — buttons must stretch to accommodate strings of various lengths. A NinePatch drawable is a standard PNG image that includes an extra 1-pixel-wide border. It must be saved with the extension .9.png, and saved into the res/drawable/ directory of your project.

The border is used to define the stretchable and static areas of the image. You indicate a stretchable section by drawing one (or more) 1-pixel-wide black line(s) in the left and top part of the border (the other border pixels should be fully transparent or white). You can have as many stretchable sections as you want: their relative size stays the same, so the largest sections always remain the largest.

You can also define an optional drawable section of the image (effectively, the padding lines) by drawing a line on the right and bottom lines. If a View object sets the NinePatch as its background and then specifies the View's text, it will stretch itself so that all the text fits inside only the area designated by the right and bottom lines (if included). If the padding lines are not included, Android uses the left and top lines to define this drawable area.

The difference between the different lines, the left and top lines define which pixels of the image are allowed to be replicated in order to stretch the image. The bottom and right lines define the relative area within the image that the contents of the View are allowed to lie within.