Deep convolutional neural networks have been successfully applied to image
classification tasks. When these same networks have been applied to image
retrieval, the assumption has been made that the last layers would give the
best performance, as they do in classification. We show that for instance-level
image retrieval, lower layers often perform better than the last layers in
convolutional neural networks. We present an approach for extracting
convolutional features from different layers of the networks, and adopt VLAD
encoding to encode features into a single vector for each image. We investigate
the effect of different layers and scales of input images on the performance of
convolutional features using the recent deep networks OxfordNet and GoogLeNet.
Experiments demonstrate that intermediate layers or higher layers with finer
scales produce better results for image retrieval, compared to the last layer.
When using compressed 128-D VLAD descriptors, our method obtains
state-of-the-art results and outperforms other VLAD and CNN based approaches on
two out of three test datasets. Our work provides guidance for transferring
deep networks trained on image classification to image retrieval tasks.

本文研究采用卷积神经网络在图像检索中的应用，通过提取不同层的卷积特征，使用 VLAD 编码转化为一个单一向量，探讨不同层和规模在特征处理中的影响，发现较低的层次和更精细的尺度能够更好地提升图像检索的准确度，同时对比了其他方法，最终取得了业界领先的成果。