Function render_changed 

pub fn render_changed(
    old: &GlyphBuffer,
    new: &GlyphBuffer,
    font: &Font<'_>,
    images: &impl Images,
    out: &mut Frame,
)

Makes out, which holds old as render drew it, hold new as render would draw it, redrawing only the 8 × 16 pixel tiles of the screen where they may differ: around a changed cell (one pixel more for a glyph’s edge), under a changed item (the old and the new one), the window of a changed backdrop, and a painted item whose picture changed.