offset pages to facilitate book "gutter"

[tdlee]

Generating use-case to demonstrate the issue:

• Individual PDFs of short music scores (e.g. songs, church hymns).
• Each item usually one, two or three pages (occasionally a few more).
• Each page neatly centred, with good amount of (equal) margin to left and right. Good for individual printouts.
• Then using PDFsam's excellent "merge" capability to create a songbook or church hymnal of many items; could be a few hundred.

Emergent problem: The book format requires space for guttering in the centre. So left-hand pages need to be shifted left a few mm; likewise right-hand pages shifted right a few mm. (Alternate pages shifted left and right.)

Suggested solution:

Aside: I'm using PDFsam basic. I happen to see that PDFsam visual appears already to have a "crop" capability. Might this new capability be related?

Possible user-interface:

• Horizontal offset input box for amount (typically a few mm: -ve for left; +ve for right): all pages offset this amount.
• LH inverse tick box: for alternate pages, apply the inverse offset to alternate (LH) pages.

(My labels horizontal offset and LH inverse are merely provisional; better names are invited!)

Noting that the first page of content is conventionally a RH page, in the generating example the user would enter something like "3mm" (positive), and then also select the LH inverse box. So pp. 1, 3, 5... (RH) would be offset right by 3mm, and pp. 2, 4, 6... would be offset left by the corresponding amount, that is, by -3mm.

This would, of course, need to be preserved in the JSON workspace.

Note. A tempting first response might be "do this on the original PDFs". But this cannot work, because in book form the first page of some songs would be an LH page; of other songs they would be a RH page.

• At the point of individual song maintenance, the ultimate book destination (will its first page be LH or RH?) cannot be known;
• When any given item appears in two or more different books, it is likely to end up with its first page LH in one book and RH in another.

Another possible solution: An option to add extra space to the side of the page (in our case, the central gutter), again with the essential LH inverse concept. So the resulting merged PDF would have wider pages than the incoming PDFs. That route is probably fine for major print shops whose workflow is "print big then cut", but not good for ordinary users who require all workflows to be on standard sizes such as A4 (typical in Europe) or Letter (typical in US).