  The performance of face photo-sketch translation has improved a lot thanks to
deep neural networks. GAN based methods trained on paired images can produce
high-quality results under laboratory settings. Such paired datasets are,
however, often very small and lack diversity. Meanwhile, Cycle-GANs trained
with unpaired photo-sketch datasets suffer from the \emph{steganography}
phenomenon, which makes them not effective to face photos in the wild. In this
paper, we introduce a semi-supervised approach with a noise-injection strategy,
named Semi-Cycle-GAN (SCG), to tackle these problems. For the first problem, we
propose a {\em pseudo sketch feature} representation for each input photo
composed from a small reference set of photo-sketch pairs, and use the
resulting {\em pseudo pairs} to supervise a photo-to-sketch generator
$G_{p2s}$. The outputs of $G_{p2s}$ can in turn help to train a sketch-to-photo
generator $G_{s2p}$ in a self-supervised manner. This allows us to train
$G_{p2s}$ and $G_{s2p}$ using a small reference set of photo-sketch pairs
together with a large face photo dataset (without ground-truth sketches). For
the second problem, we show that the simple noise-injection strategy works well
to alleviate the \emph{steganography} effect in SCG and helps to produce more
reasonable sketch-to-photo results with less overfitting than fully supervised
approaches. Experiments show that SCG achieves competitive performance on
public benchmarks and superior results on photos in the wild.
