How does google books highlight text in a scanned image

Question

If I do a keyword search here:

http://books.google.com/ebooks/reader?id=s1gVAAAAYAAJ&printsec=frontcover&output=reader&pg=GBS.PA7

The highlighted text in the "flowing text" seems easy enough but if you switch to show "scanned pages" it still shows the highlighted text? Anybody know how they accomplish this.

Answer 1

Through an OCR process, either when they scanned the image or dynamically when you request the page.