Instance Normalization

This repo implements the Instance Normalization with different programming languages or libraries: Tensorflow (Keras), Numpy, C++, Eigen, CUDA. Each sample includes both the forward and backward passes and results are checked compared with Keras's InstanceNormalization Instance.

Instance Normalization Forward Pass

Suppose $N, C, D$ are batch, channel and feature dimensions respectively.

$\mu, \sigma ^2: (N, C, 1)$

$\gamma, \beta: (1, C, 1)$

$\mu = \frac{1}{D}\sum_{i=1}^{D}x_i$

$\sigma^2 = \frac{1}{D}\sum_{i=1}^{D}(x_i - \mu)^2$

$\hat{x} = \frac{x - \mu}{\sqrt{\sigma^2 + \epsilon}}$

$y=\gamma\hat{x} + \beta$

Instance Normalization Backward Pass

$dy: (N, C, D)$

$d\mu, d\sigma ^2: (N, C, 1)$

$d\gamma, d\beta: (1, C, 1)$

$d\gamma=dy\cdot \frac{\partial y}{\partial \gamma}=\sum_{i=1}^{ND}dy\cdot\frac{x-\mu}{\sqrt{\sigma^2 + \epsilon}}$

$d\beta=dy\cdot \frac{\partial y}{\partial \beta}=\sum_{i=1}^{ND}dy$

Here we can view $y=\gamma\hat{x} + \beta$ as a composition function $y(i(x), \sigma^2(x), \mu(x))$ , where $i(x)=x$ and $\sigma^2(x)$ , $\mu(x)$ are defined as above. Therefore, we get:

$dx = \frac{\partial L}{\partial i}\cdot\frac{\partial i}{\partial x} + \frac{\partial L}{\partial \sigma^2}\cdot\frac{\partial \sigma^2}{\partial x} + \frac{\partial L}{\partial \mu}\cdot\frac{\partial \mu}{\partial x}$

Compute $\frac{\partial L}{\partial i}\cdot\frac{\partial i}{\partial x}$

$\frac{\partial L}{\partial i} = \frac{\partial L}{\partial y}\cdot\frac{\partial y}{\partial i} = dy \cdot \gamma \cdot \frac{1}{\sqrt{\sigma^2 + \epsilon}}$

$\frac{\partial i}{\partial x}=1$

Compute $\frac{\partial L}{\partial \sigma^2}\cdot\frac{\partial \sigma^2}{\partial x}$

$\frac{\partial L}{\partial \sigma^2} = \frac{\partial L}{\partial y}\cdot\frac{\partial y}{\partial \sigma^2} = \sum_{i=1}^{D}dy \cdot \gamma (x-\mu)(-\frac{1}{2})(\sigma^2 + \epsilon)^{-\frac{3}{2}}$

$\frac{\partial \sigma^2}{\partial x}=\frac{2}{D}(x-\mu)$

Compute $\frac{\partial L}{\partial \mu}\cdot\frac{\partial \mu}{\partial x}$

$\frac{\partial L}{\partial \mu}=\frac{\partial L}{\partial \mu}\cdot \frac{\partial \mu}{\partial \mu} + \frac{\partial L}{\partial \sigma^2}\cdot \frac{\partial \sigma^2}{\partial \mu}\\=\frac{\partial L}{\partial y}\cdot \frac{\partial y}{\partial \mu}\cdot 1 + \frac{\partial L}{\partial \sigma^2}\cdot \frac{\partial \sigma^2}{\partial \mu}\\=\sum_{i=1}^{D}dy\cdot \gamma \cdot\frac{-1}{\sqrt{\sigma^2+\epsilon}}+\sum_{i=1}^{D}\frac{\partial L}{\partial \sigma^2}\cdot \frac{1}{D}(-2)(x-\mu)$ , where the last term is 0

$\frac{\partial \mu}{\partial x}=\frac{1}{D}$