[webkit-help] The purpose of NinePieceImage

[hap 497]

Hi,

In the RenderBoxModelObject class, it has method to paint NinePieceImage:

bool RenderBoxModelObject::paintNinePieceImage(GraphicsContext*
graphicsContext, int tx, int ty, int w, int h, const RenderStyle*
style,
                                               const NinePieceImage&
ninePieceImage, CompositeOperator op)
{
//...

}

Can you please tell me what is a NinePieceImage? Under what situation
will Webkit create a NinePieceImage for RenderBoxModelObject?

Thank you.