D-DL4CV-Lec15db-RoI_Align

RoI Align

How is it better than Rol pooling?

It resolve the problem of misalignment

Steps

1. Maps RoI to feature map (without rounding)
2. Divide the region into fixed grid of same size (don’t need to be integer)
3. Sample features at regular-spaced points in each subregion using “bilinear interpolation”
4. Conduct max pooling

Bilinear Interpolation

Bilinear interpolation is a method for estimating values at non-integer coordinates by using weighted average of the four nearest known values

$$f_{x,y}=\sum _{i,j}{f}_{i,j}\max (0,1-\left|x-i\right|)\cdot \max (0,1-\left|y-j\right|)$$

where

$$\begin{array}{rl}i& \in \left\{\lfloor x\rfloor -1,\dots ,\lceil x\rceil +1\right\}\\ j& \in \left\{\lfloor y\rfloor -1,\dots ,\lceil y\rceil +1\right\}\end{array}$$