Paper:
We propose a method for semi-supervised semantic segmentation using the adversarial network. While most existing discriminators are trained to classify input images as real or fake on the image level, we design a discriminator in a fully convolutional manner to differentiate the predicted probability maps from the ground truth segmentation distribution with the consideration of the spatial resolution. We show that the proposed discriminator can be used to improve the performance on semantic segmentation by coupling the adversarial loss with the standard cross entropy loss on the segmentation network. In addition, the fully convolutional discriminator enables the semi-supervised learning through discovering the trustworthy regions in prediction results of unlabeled images, providing additional supervisory signals. In contrast to existing methods that utilize weakly-labeled images, our method leverages unlabeled images without any annotation to enhance the segmentation model. Experimental results on both the PASCAL VOC 2012 dataset and the Cityscapes dataset demonstrate the effectiveness of our algorithm.

Review:
This paper describes techniques for training semantic segmentation networks. There are two key ideas:
- Attach a pixel-level GAN loss to the output semantic segmentation map. That is, add a discriminator network that decides whether each pixel in the label map belongs to a real label map or not. Of course, this loss alone is unaware of the input image and would drive the network to produce plausible label maps that have no relation to the input image. An additional cross-entropy loss (the standard semantic segmentation loss) is used to tie the network to the input and the ground-truth label map, when available.
- Additional unlabeled data is utilized by using a trained semantic segmentation network to produce a label map with associated confidences; high-confidence pixels are used as ground-truth labels and are fed back to the network as training data.
The paper is fine and the work is competently done, but the experimental results never quite come together. The technical development isn’t surprising and doesn’t have much to teach researchers working in the area. Given that the technical novelty is rather light and the experimental benefits are not quite there, I cannot recommend the paper for publication in a first-tier conference.
Some more detailed comments:
1. The GAN and the semi-supervised training scheme appear to be largely independent. The GAN can be applied without any unlabeled data, for example. The paper generally appears to present two largely independent ideas. This is fine, except they don’t convincingly pan out in experiments.
2. The biggest issue is that the experimental results do not convincingly indicate that the presented ideas are useful.
2a. In the “Full” condition, the presented approach does not come close to the performance of the DeepLab baseline, even though the DeepLab network is used in the presented approach. Perhaps the authors have taken out some components of the DeepLab scheme for these experiments, such as multi-scale processing, but the question then is “Why?”. These components are not illegal, they are not cheating, they are not overly complex and are widely used. If the authors cannot demonstrate an improvement with these components, their ideas are unlikely to be adopted in state-of-the-art semantic systems, which do use these components and are doing fine.
2b. In the 1/8, 1/4, and 1/2 conditions, the performance of the baselines is not quoted. This is wrong. Since the authors are evaluating on the validation sets, there is no reason not to train the baselines on the same amount of labeled data (1/8, 1/4, 1/2) and report the results. The training scripts are widely available and such training of baselines for controlled experiments is commonly done in the literature. The reviewer is left to suspect, with no evidence given to the contrary, that the presented approach does not outperform the DeepLab baseline even in the reduced-data conditions.
A somewhat unflattering view of the work would be that this is another example of throwing a GAN at everything to see if it sticks. In this case, the experiments do not indicate that it did.