In visual recognition, the images are frequently modeled as sets of local features (bags). We show that bag of words, a common method to handle such cases, can be viewed as a special match kernel, which counts 1 if two local features fall into the same regions partitioned by visual words and 0 otherwise. Despite its simplicity, this quantization is too coarse. It is, therefore, appealing to design match kernels that more accurately measure the similarity between local features. However, it is impractical to use such kernels on large datasets due to their significant computational cost. To address this problem, we propose an efficient match kernel (EMK), which maps local features to a low dimensional feature space, average the resulting feature vectors to form a set-level feature, then apply a linear classifier.

In visual recognition, the images are frequently modeled as sets of local features (bags). We show that bag of words, a common method to handle such cases, can be viewed as a special match kernel, which counts 1 if two local features fall into the same regions partitioned by visual words and 0 otherwise. Despite its simplicity, this quantization is too coarse. It is, therefore, appealing to design match kernels that more accurately measure the similarity between local features. However, it is impractical to use such kernels on large datasets due to their significant computational cost.