Put two piles of scans together, and put a document on one sheet

[tannevaled]

Two verbs a page-manipulation library should have had.

Interleave takes pages from each document in turn. It is what a single-sided feeder leaves behind: a stack run through twice comes out as two files — the fronts and the backs — which is one document taken apart. Merging them end to end gives every front then every back; this gives the document. A shorter pile simply runs out, which is what a scanner leaves when the last sheet is single-sided.

OnePage stacks every page onto one sheet, one below the other. A page is a unit of paper, not a unit of reading: a receipt, a chat log and a web page cut into A4 are one thing a printer divided, and this puts them back so what is read scrolls rather than turns. Narrower pages are centred, because a column that jumps from side to side is harder to read than one that does not.

The order is asserted, not looked at

A PDF's origin is at the bottom, so the first page takes the largest offset and the last sits at zero. Reversed, every page is still there and the document is upside down — which no count of pages would catch.

And poppler was asked rather than ourselves

Three pages carrying a mark at a different height each, stacked and rendered:

	measured	computed
page 1's mark	28%	26.7%
page 2's mark	48%	46.7%
page 3's mark	70%	68.3%

Parity: BentoPDF's Alternate & Mix Pages and Combine to Single Page.

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