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Anytime a computer application renders an image, it
reserves a chunk of memory from the graphics card and fills it with
information which is then drawn to the screen. The memory is released
once this operation is complete, but its contents are not erased. This
is because each application assumes that whoever will next reserve this
memory will overwrite it with its own data.

Scrape works by reserving some memory and
reading its contents without writing anything to it first, thus
collecting traces left behind by recently used programs.