Seong Joon Oh

I am a research scientist at NAVER AI Lab, working on the challenges of deploying machine learning models in the real world. I am interested in training robust, probabilistic, and weakly-supervised models in pragmatic settings.

I received my PhD in computer vision and machine learning at Max-Planck Institute for Informatics in 2018, under the supervision of Bernt Schiele and Mario Fritz, with a focus on the privacy and security implications of CV and ML (Thesis). I received the Master of Mathematics with Distinction in 2014 and Bachelor of Arts in Mathematics as a Wrangler in 2013, both at University of Cambridge.

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I have tried to push certain fronts in ML research to make models truly useful and deployable in real life. They can be grouped into a few keywords.
Robustness. Changes in the input distribution shall not disrupt the model's predictive power. Ideally, a model should be robust against the shifts in input domain (e.g. natural and adversarial perturbations) and confounders (e.g. fairness).
Uncertainty. A model should know when it is going to get it wrong. This allows the users and downstream systems to make sensible and safe decisions based on the estimated confidence levels.
Weak Supervision. Human supervision is often a bottleneck for training a model on a new task. I have sought cost-effective surrogates for the annotations and corresponding training methodologies.
Privacy & Security. There are different privacy and security angles with which ML can be analyzed. One may question the "stealability" of a black-box model as an IP; one may also question the privacy guarantees for user data in the federated learning setup. Still others may wonder whether certain level privacy is achievable at all on internet, with the increasing volume of user data online and more widespread use of machine learning algorithms to process such data.
Evaluation. Correct evaluation is undoubtably important in research and industrial applications, yet it is surprisingly difficult. I have cleaned up benchmarks and evaluation protocols in a few domains.
Large-Scale ML. Some of the methodologies I have been involved in are designed for large-scale ML. They typically require minimal changes to the original ML system but bring consistent gains across the board.

It is difficult to find a CV researcher or practitioner who hasn't used (or at least heard of) the Class Activation Maps (CAM). It is a seminal feature attribution method that has left a deep mark on the vision research and applications. Notwithstanding its popularity, we found some practical and conceptual issues that makes CAM not as interpretable as it should be. We address the issues with a probabilistic treatment of the last layers of CNNs where the latent cue variable Z is trained via Marginal Likelihood (ML) or Expectation-Maximisation (EM) algorithms. The resulting Class Activation Latent Maps, or CALM, produces more precise and interpretable score maps.

The Tranformer architecture has successfully been adapted to visual models (e.g. ViT). However, Transformers, originally designed for language modelling, and ViT assign a constant ratio of computational loads between spatial and channel dimensions at different depths. We postulate this as a suboptimal design choice, as CNNs assign different ratios at different depths to maximise the utility of compute. We thus present Pooling-based Vision Transformer (PiT) that does this.

Recovering the dynamical systems, or the data generation process, behind time series data enables an effective and robust prediction, interpretation, and forecasting. There exist prior methods for recovering either continuous or discrete dynamics, but not the mixture. The underlying dynamics behind many real-world systems contain both continuous and discrete elements. For example, an aircraft essentially follows a continuous dynamics but goes through a discrete mode shift at touchdown. Such a system is referred to as a Stochastic Hybrid System (SHS). We present a framework that recovers SHS from time series data using ingredients like Neural ODEs and latent variable models.

ImageNet labels contain lots of noise (e.g. Shankar et al.). There have been efforts to fix them on the evaluation set, but not yet on the training set. We fix them on the training set (published at codebase), but with the help of a bigger image classifier, to make the task feasible at all. This is another trick that will improve the ImageNet & downstream task accuracies across the board.

Given an image, there are many ways to describe it in text. Given a text description, there are likewise many possible images that suits the description. Cross-model associations are of many-to-many nature. The usual deterministic embeddings cannot model this well. We introduce a probabilistic embedding scheme based on the Hedged Instance Embedding (ICLR'19) to handle the many-to-many mapping gracefully.

Journal extension of CVPR'20! It now contains more analyses, including the evaluation of input gradient variants as Weakly-Supervised Object Localization (WSOL) methods.

When you apply a momentum-based optimizer over scale-invariant parameters, their norms increase quite a bit. The norm increase doesn't contribute anything to the loss minimization while only slowing down the convergence. We fix this by appending a projection operation on SGD and Adam. This leads to performance improvements across the board.

Data augmentation is not as extensively studied in the video recognition tasks as in the static image recognition domain. We study the extension of popular static-image augmentation method, such as CutMix, on video recognition tasks.

Evaluating generative models is tricky. There are Inception Score and Fréchet Inception Distance measures indeed, and then (Improved) Precision and Recall metrics to separately examine the fidelity and diversity aspects. Yet, they are still not perfect. We address the issues with Improved Precision and Recall metrics and propose new metrics: Density and Coverage.

Models pick up correlations, rather than causal mechanisms, between inputs and outputs. De-biasing (and fairness) researches have guided models on "which cues to look at" through explicit bias labels or by re-weighting or re-generating training data to remove bias. We show that, for many application scenarios, it is possible to encode the "cues to look at" through model architecture and such expensive strategies are no longer needed.

I have long waited for this moment since CVPR'17. Weakly-Supervised Object Localization, or WSOL, has in fact been not weakly supervised in a strict sense. Design choices and hyperparameters are validated with the localization annotations! This paper explains why researchers had to rely on localization validation -- without localization supervision, there is no way to force a model to not extract cues from background regions. We propose a new fair benchmark acknowledging the need for localization annotations and show that WSOL methods since CAM in 2016 have not introduced much gain.

Scene text recognition works well, but there are remaining corner cases. An example is texts with unusual orientations and arrangements (e.g. BMW logo). We focus on this corner case and propose a model based on self-attention.

Journal version of my first paper ICCV'15, after five years! We have developed the version two of the ICCV'15 system that outperforms the methods that have appeared in the meantime.

Book chapter version of ICLR'18! We build connections between our black-box inspection methodology and the explainable AI.

Federated learning allows sensitive user data to never leave the device and still be used for training. It is considered a safer option than sending the user data directly to the server. But is it? We show that users may be identified and linked based on the model updates communicated between the device and server.

There has been a line of research on simple regularization techniques like CutMix (ICCV'19) and other lines of research on robustness and uncertainty. We make a happy marriage of the two and measure how well the regularization techniques improve robustness and uncertainty of a model.

A simple solution that works surprisingly well! Cut and paste patches from other images during training. Quite likely, you will see a performance boost.

Scene text recognition field has long suffered from the lack of a unified agreement on the evaluation protocol. We provide a standard protocol. We also provide a unified view on the previous methods and discover a novel combination of existing modules that turns out to be the state of the art.

There has been quite some work on representing uncertainty for classification or regression tasks. Is there a way to represent uncertainty for instance embedding models too? We show that it is possible to train probabilistic representatitons for instances based on their inherent ambiguity.

Can a bad guy hijack an RL agent? We show that it is possible to let an agent pursue an alternative reward by introducing small adversarial perturbations in the input stream.

Recipes for training a high-performance model are not cheap. Think about the GPU-and-research-scientist-and-engineer hours to find the right architectural components and optimizer hyperparameters. What if they can be stolen by examining the model responses to certain inputs?

Adversarial perturbation solutions (ICCV'17) produce visually pleasant protections with high protection rates, but their effects may be confined to a handful of recognition systems. We propose another solution based on face inpainting that changes the face to a fictitious yet natural-looking identity. It is effective against a broader set of recognition systems.

We stop and look back on the visual privacy papers (ICCV'15, ECCV'16, ICCV'17).

If face blurring doesn't work (ECCV'16), how should we shield our personal photos online against recognition systems? We propose a solution based on adversarial perturbations and the game theoretic considerations for the evaluation therein.

There has been quite some work around training models for localization tasks (e.g. semantic segmentation) from the image tag supervision only. But is this fundamentally possible without relying on extensive validation with full localization annotations? We argue that certain priors are necessary at the very least to encode the extent of objects. Saliency, we argue, is a handy prior.

We casually use pronouns to refer to others. For machines, however, referring to people with pronouns necessitates new types of data and training strategies to explicitly localize and link people across frames. We do that.

But can you still be recognized even with a blur on your face? Quite likely.

You are a janitor at Taj Mahal. Against you will, sightseers take photos with your face in the background. How can you opt out of being present in someone else's photo? We present a mobile-system based solution.

How well does a CNN model recognize people in personal photos? Even when people don't look at cameras, CNN finds out who they are, based on the context (e.g. location and social connections).

Deeply-learned computer vision models are data-hungry and manual annotations are expensive. Can we train models with “weaker” annotations? This tutorial provides an overview of the vast literature on weakly supervised learning methods in computer vision. We also discuss the limitations of current state-of-the-art methods and evaluation metrics. We propose future research directions that hopefully will spur disruptive progress in weakly supervised learning.

• Serving as a reviewer for CVPR, NeurIPS, ICML, ICCV, ECCV, ICLR, etc.
• 5 x Best Reviewer Awards.

• Samsung PhD Scholarship 2014-2018.
• Vensi Thawani Prize 2014: For a distinctive achievement in mathematics.
• William Pochin Scholarship 2014: For a distinctive achievement in mathematics.
• William Pochin Scholarship 2013: For the First Class honour in mathematics.
• Meritorious Winner 2012 at the Mathematical Contest in Modelling.
• William Pochin Scholarship 2011: For the First Class honour in mathematics.

• Towards reliable machine learning. KAIST, Daejun, April 2020.
• Towards reliable machine learning. IPIU, Jeju, Feb 2020.
• Towards reliable machine learning. Azothbio, Seoul, January 2020.
• Topics in ML research - security and uncertainty. Postech, Pohang, June 2019.
• Topics in ML research - security and uncertainty. UNIST, Ulsan, May 2019.
• OCR in the wild. NeurIPS, Vancouver, December 2019.
• Towards reverse-engineering deep neural networks. Google, Mountain View, July 2018.
• Understanding and controlling user privacy in social media photos. KIST IMRC, Seoul, August 2017.
• Understanding and controlling user privacy in social media photos. MPI Intelligent Systems, Tübingen, June 2017.

Template based on Jon Barron's website.