Click anywhere on the page to see that spot life-size. It is the only way to tell whether the dots and accents of the type came through.
Paper
Grey paper and the shadow of the gutter go white. Strokes are left alone: this cannot lighten anything darker than the paper around it.
Specks
Only marks smaller than the type on that page, and only where there is clear paper all around them. That is what a dot on an i and a caron on a č never have.
Bands
The strip down the side where the lid did not reach, the shadow the gutter throws, and the smear where that shadow burns out. Thin, starting at the edge of the sheet, and fading at its fringe: a photograph is none of those.
Tilt
Measured once the page is clean and applied last, so the bands and the specks are looked for on the pixels the scanner made rather than on pixels an angle has already moved.
Curve
Drag the handles on the page so the two curves follow the top and the bottom of the block of type as it bends. Each page keeps its own pair.
Margins
The block of type is slid to the middle of the sheet, so it stops jumping about as you turn the pages. Nothing is cropped and nothing is resized: the page only slides, and what goes off the edge is the white the block was pushed against.
Output
Cleaning draws every page again, so the pages that come out are pictures made at this resolution rather than the bytes that went in.