Coding selection and pixmap tools?

[Juhana Sadeharju <kouhia nic funet fi>]

I would like to code the following scrapbook-copy operation:

(1) User defines a rectangular selection.
(2) The selection area is re-rendered to an offscreen pixmap.
(3) The offscreen pixmap is saved to file.
(3b) The offscreen pixmap is copied to GNOME/KDE clipboard, from
which it can be pasted to a word editor.

Evince already has a code for rectangular selection. I may check that.

How to code the rest? To where?
For example: check dpi, ask selection size in pixels, allocate pixmap,
render to pixmap.

Is there a way to find out what is a suitable resolution for the
offline pixmap? Think about a math text with subsubscripts. If not,
then I could render the selection with increasing resolution, and
decide when the pixmap does not get any new details.

Juhana