On 2013–07–14 Wolfgang Schuster wrote:

> Overlays are a framed feature (the page background is a frame too)
> and textbackgrounds are images which are drawn on the page like
> overlays.

MetaPost graphics can be hooked into the textbackground using the mp
and method keys. So apparently there is a way to use arbitrary
background graphics.

It would be a natural consistent interface if overlays worked with
the background key. But I assume it's due to the page break ability
of textbackground that makes it more complicated to implement.

BTW: Is there any documentation, except the source, about the mp and
method keys?

Marco