Understanding the Effective Receptive Field in Deep Convolutional Neural Networks
Introduction
What is the Receptive Field (RF) of a unit?
Why is RF important?
The concept of receptive field is important for understanding and diagnosing how deep Convolutional neural networks (CNNs) work. Since anywhere in an input image outside the receptive field of a unit does not affect the value of that unit, it is necessary to carefully control the receptive field, to ensure that it covers the entire relevant image region. In many tasks, especially dense prediction tasks like semantic image segmentation, stereo and optical flow estimation, where we make a prediction for each single pixel in the input image, it is critical for each output pixel to have a big receptive field, such that no important information is left out when making the prediction.
How to Increase RF size?
Make the network deeper by stacking more layers, which increases the receptive field size linearly by theory, as each extra layer increases the receptive field size by the kernel size.
Add sub-sampling layers to increase the receptive field size multiplicatively.
Modern deep CNN architectures like the VGG networks and Residual Networks use a combination of these techniques.