Positive and negative relief
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Positive and negative relief is an art technique that are usually done over graph paper to show a pattern will "pop" up in the page. Sometimes it is considered as an optical trick. 
The technique is done by alternating two contrasting colors, such as black and white, on the grids. A pattern is then drawn over the grids, and the parts of the pattern in the grid will be coloured by the opposite colour that the grid inherits.