16. A computer system for determining category classifiers applicable to videos of a digital video repository, the system comprising: a computer processor; and a computer program executable by the computer processor and performing actions comprising: creating a category-instance repository comprising relationships between categories and instances of categories, the creating comprising: applying textual patterns over a corpus of documents comprising textual portions, and computing strengths for category-instance relationships based at least in part on frequencies of co-occurrence of the categories and instances; training a set of video concept classifiers on the videos and on textual metadata associated with the videos, each of a plurality of the classifiers corresponding to a concept derived from the textual metadata; removing instances of categories, responsive to the instances not corresponding to any of the concepts; filtering the categories of the category-instance repository based on the concepts associated with the video concept classifiers to remove categories that are not likely to be accurately recognized in videos; removing inconsistent categories by: identifying, as video concept classifiers associated with a category, video concept classifiers of concepts with labels corresponding to labels of the instances of the category; applying the video concept classifiers associated with the category to a video, thereby obtaining concept scores; computing a consistency score for the category based at least in part on the concept scores; and removing the category responsive to the computed consistency score indicating some threshold level of inconsistency; and determining, for each of a plurality of the categories, a category classifier based at least in part on the video concept classifiers of concepts associated with the category.
17. The system of claim 16 , the actions further comprising: applying one of the determined category classifiers to one of the videos, thereby obtaining a score; and responsive to the obtained score indicating that the video represents the category, adding a textual representation of the category to the textual metadata of the video.