I am not sure what middle size I should use for the stretchable part of a retina image. The document says:
During scaling or resizing of the image, areas covered by a cap are not scaled or resized. Instead, the pixel area not covered by the cap in each direction is tiled, left-to-right and top-to-bottom, to resize the image. This technique is often used to create variable-width buttons, which retain the same rounded corners but whose center region grows or shrinks as needed. For best performance, use a tiled area that is a 1x1 pixel area in size.
But aren't retina images supposed to be 2x the size of non-retina images? If so, shouldn't the middle be 1x1 point == 2x2 pixel?
Both 1x1 pixel and 2x2 pixel areas will work. 1x1 may get you more performance as per docs, but 2x2 is much more convenient for scaling down retina size resources.