How 9patchimage works?

I am working with android's 9 patch image 开发者_StackOverflow中文版emulator to create 9 patch image.

Can anyone tell me how it actually works? On what basis it Scale the image? What are the parameters it considers to stretch image?

Here are great paragraphs about 9 patch drawables https://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch

It's basically a PNG image with 1px border around it. With the black pixels on the border you define an area witch is stretchable and area in which the content is allowed.

More information under the link provided by Konstatin.