Return a blank canvas for a document with nothing on it, instead of
undefined. A thumbnail wants the absence (no picture is better than a
white square); an export wants the blank page.
OptionalbackgroundIndex
backgroundIndex?:number
Which background page to draw beneath the objects. Defaults to the
document's own active page — the page the author was last looking at,
and the only one the objects' coordinates are guaranteed to suit.
Return a blank canvas for a document with nothing on it, instead of
undefined. A thumbnail wants the absence (no picture is better than a white square); an export wants the blank page.