Words, phrases and sentences bear either positive, negative or neutral polarities (or sentiment). "A good idea" is positive, "a bad idea" is negative, and "a spontaneous idea" is neutral. Negation and other words can function as polarity shifters, i.e. the polarity of phrases and sentences is compositional. "This is not a bad idea" is positive, while "They failed to realise their good idea" is negative. 
Moreover, positive and negative evaluations on the document level do not alternate all of a sudden. Rather they are also indicated by polarity shifters, be it within sentences ("This is a good idea but I have no means to realise it") or between sentences (... "There are a few flaws to mention, though" ...). 

The project aims at the conceptualisation and implementation of a system carrying out bi-directional sentiment composition. The term "bi-directional" is meant to denote the insight that polarity sometimes is projected in a top-down fashion on a word, phrase or even sentence that already has got a polarity value bottom-up. We propose a novel architecture for sentiment composition and especially pay attention to  cases of polarity conflict in bi-directional polarity assignments.