Function write_pdf_3d 

pub fn write_pdf_3d(
    title: &str,
    pages: &[&SheetDrawing],
    three_d: Option<&Pdf3d>,
) -> Vec<u8> 

write_pdf, carrying a live 3D model (super::pdf3d) when three_d is given: a 3D annotation over each placement it names, and — when it asks for one — a last page given over to the model with a button per saved view.

With 3D the file is PDF 1.7 (per-view node visibility is a 1.7 feature) and pages carry /Annots; the rest of this module’s contract holds — the U3D stream is ASCII85-encoded, so every byte is still ASCII. Without it the output is byte for byte write_pdf’s.

5 … 4+2n      the sheet pages, (page, contents) pairs as above
5+2n, 6+2n    the 3D page and its contents, when asked for
next          the 3D stream (the U3D, and every saved view in /VA)
then          per placement: its 3D annotation, its (empty) appearance
then          the 3D page's annotation, its appearance, a link per view