Function full_size_rect 

pub fn full_size_rect(view: Rect, width: u32, height: u32) -> Rect

Where a picture of width×height sits in view at full size.

Contained — the whole picture is always visible — and never enlarged past its own pixels: a 96 px thumbnail blown up to a window is not “full size”, it is an invention. A picture smaller than its viewport therefore sits centred at its own size.