A page that intersects a Viewport, and which of its pixels show.
public VisiblePage(int PageIndex, PixelRectangle Region, double X, double Y)
The zero-based page.
The visible pixels, in the coordinates of Region: a render of the whole page at the viewport's zoom and the layout's rotation, origin at its top-left corner.
The viewport x of the page's pixel origin: a tile's pixel (u, v) shows at (X + u, Y + v).
The viewport y of the page's pixel origin.
ISO 32000-2 §14.11.2 (the crop box is the region displayed). Spec #33 story 54 (viewer capabilities 1 and 14).