What is: CycleGAN?

CycleGAN, or Cycle-Consistent GAN, is a type of generative adversarial network for unpaired image-to-image translation. For two domains $X$ and $Y$, CycleGAN learns a mapping $G : X \rightarrow Y$ and $F: Y \rightarrow X$. The novelty lies in trying to enforce the intuition that these mappings should be reverses of each other and that both mappings should be bijections. This is achieved through a cycle consistency loss that encourages $F\left(G\left(x\right)\right) \approx x$ and $G\left(Y\left(y\right)\right) \approx y$. Combining this loss with the adversarial losses on $X$ and $Y$ yields the full objective for unpaired image-to-image translation.

For the mapping $G : X \rightarrow Y$ and its discriminator $D\_{Y}$ we have the objective:

$\mathcal{L}\_{GAN}\left(G, D\_{Y}, X, Y\right) =\mathbb{E}\_{y \sim p\_{data}\left(y\right)}\left[\log D\_{Y}\left(y\right)\right] + \mathbb{E}\_{x \sim p\_{data}\left(x\right)}\left[log(1 − D\_{Y}\left(G\left(x\right)\right)\right]$

where $G$ tries to generate images $G\left(x\right)$ that look similar to images from domain $Y$, while $D\_{Y}$ tries to discriminate between translated samples $G\left(x\right)$ and real samples $y$. A similar loss is postulated for the mapping $F: Y \rightarrow X$ and its discriminator $D\_{X}$.

The Cycle Consistency Loss reduces the space of possible mapping functions by enforcing forward and backwards consistency:

$\mathcal{L}\_{cyc}\left(G, F\right) = \mathbb{E}\_{x \sim p\_{data}\left(x\right)}\left[||F\left(G\left(x\right)\right) - x||\_{1}\right] + \mathbb{E}\_{y \sim p\_{data}\left(y\right)}\left[||G\left(F\left(y\right)\right) - y||\_{1}\right]$

The full objective is:

$\mathcal{L}\_{GAN}\left(G, F, D\_{X}, D\_{Y}\right) = \mathcal{L}\_{GAN}\left(G, D\_{Y}, X, Y\right) + \mathcal{L}\_{GAN}\left(F, D\_{X}, X, Y\right) + \lambda\mathcal{L}\_{cyc}\left(G, F\right)$

Where we aim to solve:

$G^{\*}, F^{\*} = \arg \min\_{G, F} \max\_{D\_{X}, D\_{Y}} \mathcal{L}\_{GAN}\left(G, F, D\_{X}, D\_{Y}\right)$

For the original architecture the authors use:

• two stride-2 convolutions, several residual blocks, and two fractionally strided convolutions with stride $\frac{1}{2}$.
• instance normalization
• PatchGANs for the discriminator
• Least Square Loss for the GAN objectives.