Howto cmake opencv in Raspian

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Instructions not detailed enough for me - have created temp working directory holding unzipped opencv files. Have cmake 2.8.9 built ok. pwd = directory holding the unzipped files

Tried cmake -D CMAKE_BUILD_TYPE=RELEASE -D CMAKE_INSTALL_PREFIX=/usr/local/opencv where /usr/local/opencv was created to be target, but get errors indicating the _PREFIX should be the source Tried again with reference to the 'modules' directory that is in pwd Error says does not exist

What directory should I be in (pwd) ?? Will cmake walk directory structure to make all components ?? Are the args used correct for this function ??