# S3Pool - Pooling with stochastic Spatial Sampling

# Idea

New kind of pooling aiming to learn from the information that is discarded by classical pooling. The image is spatially partioned and the rows and columns of a pooling grid are non-uniformly and stochastically chosen without replacement. It acts as a strong regularizer equivalent to implicit data augmentation because the network will see a different distorsion of the feature maps at each pass.

# Method

- Partition the image \((h,w)\) into strips of size \(g\) (both vertically and horizontally)
- Randomly select \(\frac{g}{s}\) rows and columns from each strip to obtain the downsampling grid -> The resulting feature map is of size \(\frac{h}{s}\), \(\frac{w}{s}\) (like a pooling with a window of size \(s\)) -> The pooling is likely to be non-uniform (different at each pass for the same image)
- At test time, it is replaced either by standard max pooling or average pooling

# Application

Tested on CIFAR10-100 and STL-10 with ResNet

This method performs better than dropout and classical stochastic pooling both with and without data augmentation.

The training time goes up by about 10-15%

Downsampling : regular and stochastic spatial with different grid sizes

Pooling strategies :