Discovering image semantics in codebook derivative space

The sparse coding based approaches for image recognition have recently shown improved performance than traditional bag-of-features technique. Due to high dimensionality of the image descriptor space, existing systems usually require very large codebook size to minimize coding error in order to get satisfactory accuracy. While most research efforts try to address the problem by constructing a relatively smaller codebook with stronger discriminative power, in this paper, we introduce an alternative solution by enhancing the quality of coding. Particularly, we apply the idea similar to Fisher kernel to the coding framework, where we use the image-dependent codebook derivative to represent the image. The proposed idea is generic across multiple coding criteria, and in this paper, it is applied to enhance the locality-constraint linear coding (LLC). Experiments show that, the extracted new feature, called "LLC+," achieved significantly improved accuracy on several challenging datasets even with a small codebook of 1/20 the reported size used by LLC. This obviously adds to LLC+ the modeling accuracy, processing speed and codebook training advantages.