EdgeFiles

Edge files are used for rendering frame borders, and are specified with the edgeFile attribute of a &lt;Backdrop&gt; tag, or the edgeFile element of a backdrop specification table.

An edge file is just an image file (obeying the normal WoW file size rules), which will then be split horizontally by the UI engine into 8 equally sized slices (The actual file dimensions shouldn't matter as long as the width is a multiple of 8).

The slices are placed as follows (going from left to right)


 * Left edge (Left side is outside edge)
 * Right edge (Right side is outside edge)
 * Top edge (Left side is outside edge. Rotated 90 degrees Counterclockwise)
 * Bottom edge (Right side is outside edge. Rotated 90 degrees Counterclockwise)
 * Top Left corner (Left and Top sides are outside edges)
 * Top Right corner (Right and Top sides are outside edges)
 * Bottom Left corner (Left and Bottom sides are outside edges)
 * Bottom Right corner (Right and Bottom sides are outside edges)

The frame border is then made by repeating the edge images in between the four corner images.

The EdgeSize parameter on the frame's backdrop indicates how wide the image will be stretched on to the frame.

Example
This is an example of an edge file used in the game for a wooden border in the Achievement UI

If we split it into 8 section it is easyer to see how this works using the above key working from left to right: