Robust Scene Classification with Cross-Level LLC Coding on CNN Features

Convolutional Neural Network (CNN) features have demonstrated outstanding performance as global representations for image classification, but they lack invariance to scale transformation, which makes it difficult to adapt to various complex tasks such as scene classification. To strengthen the scale invariance of CNN features and meanwhile retain their powerful discrimination in scene classification, we propose a framework where cross-level Locality-constrained Linear Coding and cascaded fine-tuned CNN features are combined, which is shorted as cross-level LLC-CNN. Specifically, this framework first fine-tunes multi-level CNNs in a cascaded way, then extracts multi-level CNN features to learn a cross-level universal codebook, and finally performs locality-constrained linear coding (LLC) and max-pooling on the patches of all levels to form the final representation. It is experimentally verified that the LLC responses on the universal codebook outperform the CNN features and achieve the state-of-the-art performance on the two currently largest scene classification benchmarks, MIT Indoor Scenes and SUN 397.