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Title: Efficacy of Pixel-Level OOD Detection for Semantic Segmentation

Paper content:
The detection of out of distribution samples for image classification has been widely researched.
Safety critical applications, such as autonomous driving, would benefit from the ability to localise the unusual objects causing the image to be out of distribution.
This paper adapts state-of-the-art methods for detecting out of distribution images for image classification to the new task of detecting out of distribution pixels, which can localise the unusual objects.
It further experimentally compares the adapted methods on two new datasets derived from existing semantic segmentation datasets using PSPNet and DeeplabV3+ architectures, as well as proposing a new metric for the task.
The evaluation shows that the performance ranking of the compared methods does not transfer to the new task and every method performs significantly worse than their image-level counterparts.
Figure 1: Image from the LostAndFound dataset (Pinggera et al., 2016) , where two unlikely objects (storage crates) are almost entirely incorrectly predicted to be road.
The Max Softmax method clearly highlights these crates as OOD.
(best viewed in colour)
Many applications using machine learning (ML) may benefit from out of distribution (OOD) detection to improve safety.
When inputs are determined to be out of distribution, the output of an ML algorithm should not be trusted.
A large body of research exists for detecting entire images as OOD for the task of image classification.
Image-level OOD detection outputs a classification for the entire image; this coarse level of detection may be inadequate for many safety critical applications, including autonomous driving.
Most of the pixels in an image taken from an onboard camera will be in distribution (ID), i.e. an image of a road scene with cars, people, and roadway-but an unusual object that was not part of the training set may cause only a small number of OOD pixels.
Extending the framework to semantic segmentation networks will allow each pixel to have an "in" or "out of" distribution classification.
Applied to autonomous driving, groups of pixels classified as OOD would be considered as unknown objects.
Depending on the location of the unknown objects, a planner would then proceed with caution or hand over control to a safety driver.
Another application is automatic tagging of images with OOD objects, which would then be sent for human labelling.
Figure 1 shows a failure case where OOD detection is beneficial.
The two crates are predicted as road.
The right image of this figure shows the result of pixel-level OOD detection using one of the proposed methods, which clearly identifies the unusual objects.
This paper adapts existing state-of-the-art image-level OOD detection methods to the new task of pixel-level OOD classification and compares their performance on a new dataset designed for this task.
In addition to adapting the methods, we address the question of whether the best-performing image-level methods maintain their performance when adapted to the new task.
In order to answer this question, we also propose pixel-level OOD detection performance metrics, drawing both on existing image-level OOD detection and semantic segmentation performance metrics.
Further, we design two new datasets for pixel-level OOD detection with test images that contain both pixels that are in distribution and pixels that are out of distribution, evaluated with two different network architectures-PSPNet (Zhao et al., 2016) and DeeplabV3+ (Chen et al., 2018) .
Somewhat surprisingly, our evaluation shows that the best performing pixel-level OOD detection methods were derived from image-level OOD detection methods that were not necessarily the best performing on the image-level OOD detection task.
In summary, the contributions of this paper are the following:
• adaptation of image-level OOD detection methods to pixel-level OOD detection and their evaluation; • training and evaluation datasets for pixel-level OOD detection evaluation derived from existing segmentation datasets; and • a new metric for pixel-level OOD detection, called MaxIoU.
The drop in performance for pixel-level OOD detection is likely due to features that cause large disruptions at the pixel-level, but would not affect an entire image; for example, shadows, occlusion, and far away objects.
Figure 7 shows an example of shadows and far away objects in the bottom row.
At the end of the road, most pixels are high OOD values as well as the right side of the scene, which is in the shade of a building.
The top row of Figure 7 shows an interesting failure case of a flooded road being predicted as road with a low OOD value.
As can be seen in all example outputs, class boundaries are highlighted.
A classical computer vision algorithm was developed, using a series of erosion, dilation and other filters to remove these boundaries.
In general performance was increased; however, the increase was on the order of 10 −3 .
Several methods for detecting OOD pixels were adapted from image-level OOD detection, as well as a pixel uncertainty estimation.
These methods were compared using metrics previously established by OOD detection works, as well as a new metric that has roots in the semantic segmentation task.
This paper also contributed two new datasets for pixel-level OOD classification derived from semantic segmentation datasets that have common classes but also unique ones.
There is great room for improvement for pixel-level OOD detection.
One shortcoming for all the methods compared in this paper is the ability to distinguish between class boundary pixels and OOD pixels.
We tested classical computer vision techniques that could be used to visually fix this problem, but the performance increase was negligible.
The ODIN and Mahalanobis methods have the best performance with PSPNet and SUN dataset, beating the VarSum, Mutual Information, and Confidence methods by a significant margin.
However, Mutual Information has the best performance with DeeplabV3+ and the IDD dataset, with the other methods following closely.
Therefore the ODIN, Mahalanobis, and Mutual Information methods should be considered the baseline for further research in pixel-level OOD detection.
Understanding the faults of pixel-level OOD detectors is crucial for progress.
This would include categorising the failure cases of a detector.
For example, understanding why a flooded road is not highlighted, and what makes that different to shadows falsely being highlighted.

assistant: text: Evaluating pixel-level out-of-distribution detection methods on two new real world datasets using PSPNet and DeeplabV3+.