23 research outputs found
Adaptive Superpixel for Active Learning in Semantic Segmentation
Learning semantic segmentation requires pixel-wise annotations, which can be
time-consuming and expensive. To reduce the annotation cost, we propose a
superpixel-based active learning (AL) framework, which collects a dominant
label per superpixel instead. To be specific, it consists of adaptive
superpixel and sieving mechanisms, fully dedicated to AL. At each round of AL,
we adaptively merge neighboring pixels of similar learned features into
superpixels. We then query a selected subset of these superpixels using an
acquisition function assuming no uniform superpixel size. This approach is more
efficient than existing methods, which rely only on innate features such as RGB
color and assume uniform superpixel sizes. Obtaining a dominant label per
superpixel drastically reduces annotators' burden as it requires fewer clicks.
However, it inevitably introduces noisy annotations due to mismatches between
superpixel and ground truth segmentation. To address this issue, we further
devise a sieving mechanism that identifies and excludes potentially noisy
annotations from learning. Our experiments on both Cityscapes and PASCAL VOC
datasets demonstrate the efficacy of adaptive superpixel and sieving
mechanisms
Learning Debiased Classifier with Biased Committee
Neural networks are prone to be biased towards spurious correlations between
classes and latent attributes exhibited in a major portion of training data,
which ruins their generalization capability. We propose a new method for
training debiased classifiers with no spurious attribute label. The key idea is
to employ a committee of classifiers as an auxiliary module that identifies
bias-conflicting data, i.e., data without spurious correlation, and assigns
large weights to them when training the main classifier. The committee is
learned as a bootstrapped ensemble so that a majority of its classifiers are
biased as well as being diverse, and intentionally fail to predict classes of
bias-conflicting data accordingly. The consensus within the committee on
prediction difficulty thus provides a reliable cue for identifying and
weighting bias-conflicting data. Moreover, the committee is also trained with
knowledge transferred from the main classifier so that it gradually becomes
debiased along with the main classifier and emphasizes more difficult data as
training progresses. On five real-world datasets, our method outperforms prior
arts using no spurious attribute label like ours and even surpasses those
relying on bias labels occasionally.Comment: Conference on Neural Information Processing Systems (NeurIPS), New
Orleans, 202
Active Learning for Semantic Segmentation with Multi-class Label Query
This paper proposes a new active learning method for semantic segmentation.
The core of our method lies in a new annotation query design. It samples
informative local image regions (e.g., superpixels), and for each of such
regions, asks an oracle for a multi-hot vector indicating all classes existing
in the region. This multi-class labeling strategy is substantially more
efficient than existing ones like segmentation, polygon, and even dominant
class labeling in terms of annotation time per click. However, it introduces
the class ambiguity issue in training since it assigns partial labels (i.e., a
set of candidate classes) to individual pixels. We thus propose a new algorithm
for learning semantic segmentation while disambiguating the partial labels in
two stages. In the first stage, it trains a segmentation model directly with
the partial labels through two new loss functions motivated by partial label
learning and multiple instance learning. In the second stage, it disambiguates
the partial labels by generating pixel-wise pseudo labels, which are used for
supervised learning of the model. Equipped with a new acquisition function
dedicated to the multi-class labeling, our method outperformed previous work on
Cityscapes and PASCAL VOC 2012 while spending less annotation cost
Quantification of systematic errors in the electron density and temperature measured with Thomson scattering at W7-X
The electron density and temperature profiles measured with Thomson
scattering at the stellarator Wendelstein 7-X show features which seem to be
unphysical, but so far could not be associated with any source of error
considered in the data processing. A detailed Bayesian analysis reveals that
errors in the spectral calibration cannot explain the features observed in the
profiles. Rather, it seems that small fluctuations in the laser position are
sufficient to affect the profile substantially. The impact of these
fluctuations depends on the laser position itself, which, in turn, provides a
method to find the optimum laser alignment in the future
Accelerated Bayesian inference of plasma profiles with self-consistent MHD equilibria at W7-X via neural networks
High- operations require a fast and robust inference
of plasma parameters with a self-consistent MHD equilibrium. Precalculated MHD
equilibria are usually employed at W7-X due to the high computational cost. To
address this, we couple a physics-regularized NN model that approximates the
ideal-MHD equilibrium with the Bayesian modeling framework Minerva. We show the
fast and robust inference of plasma profiles (electron temperature and density)
with a self-consistent MHD equilibrium approximated by the NN model. We
investigate the robustness of the inference across diverse synthetic W7-X
plasma scenarios. The inferred plasma parameters and their uncertainties are
compatible with the parameters inferred using the VMEC, and the inference time
is reduced by more than two orders of magnitude. This work suggests that MHD
self-consistent inferences of plasma parameters can be performed between shots.Comment: 18 pages, 6 figure
Soft x-ray tomograms are consistent with the magneto-hydrodynamic equilibrium in the Wendelstein 7-X stellarator
Ion temperature clamping in Wendelstein 7-X electron cyclotron heated plasmas
The neoclassical transport optimization of the Wendelstein 7-X stellarator has not resulted in the predicted high energy confinement of gas fueled electron-cyclotron-resonance-heated (ECRH) plasmas as modelled in (Turkin et al 2011 Phys. Plasmas 18 022505) due to high levels of turbulent heat transport observed in the experiments. The electron-turbulent-heat transport appears non-stiff and is of the electron temperature gradient (ETG)/ion temperature gradient (ITG) type (Weir et al 2021 Nucl. Fusion 61 056001). As a result, the electron temperature Te can be varied freely from 1 keV–10 keV within the range of PECRH = 1–7 MW, with electron density ne values from 0.1–1.5 × 1020 m−3. By contrast, in combination with the broad electron-to-ion energy-exchange heating profile in ECRH plasmas, ion-turbulent-heat transport leads to clamping of the central ion temperature at Ti ∼ 1.5 keV ± 0.2 keV. In a dedicated ECRH power scan at a constant density of 〈ne〉 = 7 × 1019 m−3, an apparent \u27negative ion temperature profile stiffness\u27 was found in the central plasma for (r/a < 0.5), in which the normalized gradient ∇Ti/Ti decreases with increasing ion heat flux. The experiment was conducted in helium, which has a higher radiative density limit compared to hydrogen, allowing a broader power scan. This \u27negative stiffness\u27 is due to a strong exacerbation of turbulent transport with an increasing ratio of Te/Ti in this electron-heated plasma. This finding is consistent with electrostatic microinstabilities, such as ITG-driven turbulence. Theoretical calculations made by both linear and nonlinear gyro-kinetic simulations performed by the GENE code in the W7-X three-dimensional geometry show a strong enhancement of turbulence with an increasing ratio of Te/Ti. The exacerbation of turbulence with increasing Te/Ti is also found in tokamaks and inherently enhances ion heat transport in electron-heated plasmas. This finding strongly affects the prospects of future high-performance gas-fueled ECRH scenarios in W7-X and imposes a requirement for turbulence-suppression techniques