There may be documents that should never be shredded.
I’m thinking here particularly about the antics of the East German Stasi when they tried to destroy evidence of their activities.
Subsequent efforts to restore the paperwork are heroic but fraught with difficulties.
Today’s invention is a simple modification to existing printers intended to make shredding itself less effective.
If we are talking about printing with black ink on a white surface, we could arrange that one of the components of the black (say cyan) would be decreased, perhaps linearly as a function of position across the page. Another colour component could be varied similarly down the page.
These gradients would be so small as to be invisible to the naked eye.
If such a document found itself shredded, it would be possible to use a sensitive colorimeter to arrange the fragments by position before reassembling the original.