If you want to peek at the implementation of some of the core filters, I'd suggest starting with levels.cpp/h. All the colorspaces are demonstrated* and the algorithms are simple (lookup tables).
*Don't depend on these filters to learn everything, though. There are nuances like RGB being "upside-down" and YV12's alignment issues.
@neuron2 - I think the sample you're referring to is actually still on Ben's page.
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