An effective learning strategy for cascaded object detection

• Object detection is frequently a complex, severely unbalanced classification problem.
• A cascade of node classifiers allows us to efficiently handle the complexity.
• In our proposal, each node classifier is trained with a ranking-based algorithm.
• Ranking effectively faces the imbalance between object and non-object patches.
• Our method is effective if compared to other learning strategies for skewed classes.

To distinguish objects from non-objects in images under computational constraints, a suitable solution is to employ a cascade detector that consists of a sequence of node classifiers with increasing discriminative power. However, among the millions of image patches generated from an input image, only very few contain the searched object. When trained on these highly unbalanced data sets, the node classifiers tend to have poor performance on the minority class. Thus, we propose a learning strategy aimed at maximizing the node classifiers ranking capability rather than their accuracy. We also provide an efficient implementation yielding the same time complexity of the original Viola–Jones cascade training. Experimental results on highly unbalanced real problems show that our approach is both efficient and effective when compared to other node training strategies for skewed classes.