Improving the BoVW via discriminative visual n-grams and MKL strategies

The Bag-of-Visual-Words (BoVW) representation has been widely used to approach a number of different high-level computer vision tasks. The idea behind the BoVW representation is similar to the Bag-of-Words (BoW) used in Natural Language Processing (NLP) tasks: to extract features from the dataset, then build feature histograms that represent each instance. Although the approach is simple and effective facilitating its applicability to a wide range of problems, it inherits a well-known limitation from the traditional BoW; the disregarding of spatial information among extracted features (sequential information in text), which could be useful to capture discriminative visual-patterns. In this paper, we alleviate this limitation with the joint use of visual words and multi-directional sequences of visual words (visual n-grams). The contribution of this paper is twofold: (i) to build new simple-effective visual features inspired in the popular idea of n-gram representations in NLP and (ii) to propose the Multiple Kernel Learning (MKL) strategies to better exploit the joint use of visual words and visual n-grams in Image Classification (IC) tasks. For the former, we propose building a codebook of visual n-grams, and use them as attributes to represent images by means of the BoVW representation. For the second point, we consider the visual words and visual n-grams as different feature spaces, then we propose MKL strategies to better integrate the visual information. We evaluate our proposal in the image classification task using five different datasets: Histopathology, Birds, Butterflies, Scenes and a subset of 6 classes of CalTech-101. Experimental results show that the proposed strategies exploiting our visual n-grams, outperforms or is competitive with (i) the traditional BoVW, (ii) the BoVW using visual n-grams under traditional fusion schemes (e.g., ensemble based classifiers) and (iii) other approaches in the literature for IC that consider the spatial context.