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Title: Generative Adversarial Networks for Extreme Learned Image Compression

Paper content:
We propose a framework for extreme learned image compression based on Generative Adversarial Networks (GANs), obtaining visually pleasing images at significantly lower bitrates than previous methods.
This is made possible through our GAN formulation of learned compression combined with a generator/decoder which operates on the full-resolution image and is trained in combination with a multi-scale discriminator.
Additionally, if a semantic label map of the original image is available, our method can fully synthesize unimportant regions in the decoded image such as streets and trees from the label map, therefore only requiring the storage of the preserved region and the semantic label map.
A user study confirms that for low bitrates, our approach is preferred to state-of-the-art methods, even when they use more than double the bits.
Image compression systems based on deep neural networks (DNNs), or deep compression systems for short, have become an active area of research recently.
These systems (e.g. BID6 BID33 ) are often competitive with modern engineered codecs such as WebP (WebP), JPEG2000 BID37 ) and even BPG (Bellard) (the state-of-the-art engineered codec).
Besides achieving competitive compression rates on natural images, they can be easily adapted to specific target domains such as stereo or medical images, and promise efficient processing and indexing directly from compressed representations BID41 .
However, deep compression systems are typically optimized for traditional distortion metrics such as peak signal-to-noise ratio (PSNR) or multi-scale structural similarity (MS-SSIM) BID44 .
For very low bitrates (below 0.1 bits per pixel (bpp)), where preserving the full image content becomes impossible, these distortion metrics lose significance as they favor pixel-wise preservation of local (high-entropy) structure over preserving texture and global structure.
To further advance deep image compression it is therefore of great importance to develop new training objectives beyond PSNR and MS-SSIM.
A promising candidate towards this goal are adversarial losses BID13 which were shown recently to capture global semantic information and local texture, yielding powerful generators that produce visually appealing high-resolution images from semantic label maps BID43 .In
this paper, we propose and study a generative adversarial network (GAN)-based framework for extreme image compression, targeting bitrates below 0.1 bpp. We
rely on a principled GAN formulation for deep image compression that allows for different degrees of content generation. In
contrast to prior works on deep image compression which applied adversarial losses to image patches for artifact suppression BID33 BID12 , generation of texture details BID25 , or representation learning for thumbnail images BID35 , our generator/decoder operates on the full-resolution image and is trained with a multi-scale discriminator BID43 .We
consider two modes of operation (corresponding to unconditional and conditional GANs BID13 BID31 ), namely• generative compression (GC), preserving the overall image content while generating structure of different scales such as leaves of trees or windows in the facade of buildings, and • selective generative compression (SC), completely generating parts of the image from a semantic label map while preserving user-defined regions with a high degree of detail.We emphasize that GC does not require semantic label maps (neither for training, nor for deployment). A
typical use case for GC are bandwidth constrained scenarios, where one wants to preserve the full image as well as possible, while falling back to synthesized content instead of blocky/blurry blobs for regions for which not sufficient bits are available to store the original pixels. SC
could be applied in a video call scenario where one wants to fully preserve people in the video stream, but a visually pleasing synthesized background serves the purpose as well as the true background.In the GC operation mode the image is transformed into a bitstream and encoded using arithmetic coding. SC
requires a semantic/instance label map of the original image which can be obtained using off-the-shelf semantic/instance segmentation networks, e.g., PSPNet and Mask R-CNN BID18 , and which is stored as a vector graphic. This
amounts to a small, image dimension-independent overhead in terms of coding cost. On the
other hand, the size of the compressed image is reduced proportionally to the area which is generated from the semantic label map, typically leading to a significant overall reduction in storage cost.For GC, a comprehensive user study shows that our compression system yields visually considerably more appealing results than BPG (Bellard) (the current state-of-the-art engineered compression algorithm) and the recently proposed autoencoder-based deep compression (AEDC) system . In particular
, our GC models trained for compression of general natural images are preferred to BPG when BPG uses up to 95% and 124% more bits than those produced by our models on the Kodak (Kodak) and RAISE1K BID11 data set, respectively. When constraining
the target domain to the street scene images of the Cityscapes data set BID9 , the reconstructions of our GC models are preferred to BPG even when the latter uses up to 181% more bits. To the best of our
knowledge, these are the first results showing that a deep compression method outperforms BPG on the Kodak data set in a user study-and by large margins. In the SC operation
mode, our system seamlessly combines preserved image content with synthesized content, even for regions that cross multiple object boundaries, while faithfully preserving the image semantics. By partially generating
image content we achieve bitrate reductions of over 50% without notably degrading image quality.
The GC models produce images with much finer detail than BPG, which suffers from smoothed patches and blocking artifacts.
In particular, the GC models convincingly reconstruct texture in natural objects such as trees, water, and sky, and is most challenged with scenes involving humans.
AEDC and the MSE baseline both produce blurry images.We see that the gains of our models are maximal at extreme bitrates, with BPG needing 95-181% more bits for the C = 2, 4 models on the three datasets.
For C = 8 gains are smaller but still very large (BPG needing 21-49% more bits).
This is expected, since as the bitrate increases the classical compression measures (PSNR/MS-SSIM) become more meaningful-and our system does not employ the full complexity of current state-of-the-art systems, as discussed next.State-of-the-art on Kodak: We give an overview of relevant recent learned compression methods and their differences to our GC method and BPG in Table 1 in the Appendix.
BID33 also used GANs (albeit a different formulation) and were state-of-the-art in MS-SSIM in 2017, while the concurrent work of is the current state-of-the-art in image compression in terms of classical metrics (PSNR and MS-SSIM) when measured on the Kodak dataset (Kodak).
Notably, all methods except ours (BPG, Rippel et al., and Minnen et al.) employ adaptive arithmetic coding using context models for improved compression performance.
Such models could also be implemented for our system, and have led to additional savings of 10% in .
Since Rippel et al. and Minnen et al. have only released a selection of their decoded images (for 3 and 4, respectively, out of the 24 Kodak images), and at significantly higher bitrates, a comparison with a user study is not meaningful.
Instead, we try to qualitatively put our results into context with theirs.In Figs. 12-14 in the Appendix, we compare qualitatively to BID33 .
We can observe that even though BID33 use 29-179% more bits, our models produce images of comparable or better quality.
In FIG1 , we show a qualitative comparison of our results to the images provided by the concurrent work of , as well as to BPG (Bellard) on those images.
First, we see that BPG is still visually competitive with the current state-of-the-art, which is consistent with moderate 8.41% bitrate savings being reported by in terms of PSNR.
Second, even though we use much fewer bits compared to the example images available from , for some of them (Figs. 15 and 16) our method can still produce images of comparable visual quality.Given the dramatic bitrate savings we achieve according to the user study (BPG needing 21-181% more bits), and the competitiveness of BPG to the most recent state-of-the-art , we conclude that our proposed system presents a significant step forward for visually pleasing compression at extreme bitrates.Sampling the compressed representations: In FIG1 we explore the representation learned by our GC models (with C = 4), by sampling the (discrete) latent space ofŵ.
When we sample uniformly, and decode with our GC model into images, we obtain a "soup of image patches" which reflects the domain the models were trained on (e.g. street sign and building patches on Cityscapes).
Note that we should not expect these outputs to look like normal images, since nothing forces the encoder outputŵ to be uniformly distributed over the discrete latent space.However, given the low dimensionality ofŵ (32 × 64 × 4 for 512 × 1024px Cityscape images), it would be interesting to try to learn the true distribution.
To this end, we perform a simple experiment and train an improved Wasserstein GAN (WGAN-GP) BID14 onŵ extracted from Cityscapes, using default parameters and a ResNet architecture.
4 By feeding our GC model with samples from the WGAN-GP generator, we easily obtain a powerful generative model, which generates sharp 1024 × 512px images from scratch.
We think this could be a promising direction for building high-resolution generative models.
In FIG5 in the Appendix, we show more samples, and samples obtained by feeding the MSE baseline with uniform and learned code samples.
The latter yields noisier "patch soups" and much blurrier image samples than our GC network.
Figure 6 : Mean IoU as a function of bpp on the Cityscapes validation set for our GC and SC networks, and for the MSE baseline.
We show both SC modes: RI (inst.), RB (box).
D + annotates models where instance semantic label maps are fed to the discriminator (only during training); EDG + indicates that semantic label maps are used both for training and deployment.
The pix2pixHD baseline BID43 was trained from scratch for 50 epochs, using the same downsampled 1024 × 512px training images as for our method.
The heatmaps in the lower left corners show the synthesized parts in gray.
We show the bpp of each image as well as the relative savings due to the selective generation.In FIG2 we present example Cityscapes validation images produced by the SC network trained in the RI mode with C = 8, where different semantic classes are preserved.
More visual results for the SC networks trained on Cityscapes can be found in Appendix F.7, including results obtained for the RB operation mode and by using semantic label maps estimated from the input image via PSPNet .Discussion
: The quantitative evaluation of the semantic preservation capacity (Fig. 6 ) reveals that the SC networks preserve the semantics somewhat better than pix2pixHD, indicating that the SC networks faithfully generate texture from the label maps and plausibly combine generated with preserved image content. The mIoU
of BPG, AEDC, and the MSE baseline is considerably lower than that obtained by our SC and GC models, which can arguably be attributed to blurring and blocking artifacts. However,
it is not surprising as these baseline methods do not use label maps during training and prediction.In the SC operation mode, our networks manage to seamlessly merge preserved and generated image content both when preserving object instances and boxes crossing object boundaries (see Appendix F.7). Further,
our networks lead to reductions in bpp of 50% and more compared to the same networks without synthesis, while leaving the visual quality essentially unimpaired, when objects with repetitive structure are synthesized (such as trees, streets, and sky). In some
cases, the visual quality is even better than that of BPG at the same bitrate. The visual
quality of more complex synthesized objects (e.g. buildings, people) is worse. However, this
is a limitation of current GAN technology rather than our approach. As the visual
quality of GANs improves further, SC networks will as well. Notably, the
SC networks can generate entire images from the semantic label map only.Finally, the semantic label map, which requires 0.036 bpp on average for the downscaled 1024 × 512px Cityscapes images, represents a relatively large overhead compared to the storage cost of the preserved image parts. This cost vanishes
as the image size increases, since the semantic mask can be stored as an image dimension-independent vector graphic.
We proposed and evaluated a GAN-based framework for learned compression that significantly outperforms prior works for low bitrates in terms of visual quality, for compression of natural images.
Furthermore, we demonstrated that constraining the application domain to street scene images leads to additional storage savings, and we explored combining synthesized with preserved image content with the potential to achieve even larger savings.
Interesting directions for future work are to develop a mechanism for controlling spatial allocation of bits for GC (e.g. to achieve better preservation of faces; possibly using semantic label maps), and to combine SC with saliency information to determine what regions to preserve.
In addition, the sampling experiments presented in Sec. 6.1 indicate that combining our GC compression approach with GANs to (unconditionally) generate compressed representations is a promising avenue to learn high-resolution generative models.

assistant: text: GAN-based extreme image compression method using less than half the bits of the SOTA engineered codec while preserving visual quality