Traffic Sign Detection under Challenging Conditions: A Deeper Look Into Performance Variations and Spectral Characteristics

08/29/2019 ∙ by Dogancan Temel, et al. ∙ Georgia Institute of Technology 14

Traffic signs are critical for maintaining the safety and efficiency of our roads. Therefore, we need to carefully assess the capabilities and limitations of automated traffic sign detection systems. Existing traffic sign datasets are limited in terms of type and severity of challenging conditions. Metadata corresponding to these conditions are unavailable and it is not possible to investigate the effect of a single factor because of simultaneous changes in numerous conditions. To overcome the shortcomings in existing datasets, we introduced the CURE-TSD-Real dataset, which is based on simulated challenging conditions that correspond to adversaries that can occur in real-world environments and systems. We test the performance of two benchmark algorithms and show that severe conditions can result in an average performance degradation of 29 challenging conditions through spectral analysis and show that challenging conditions can lead to distinct magnitude spectrum characteristics. Moreover, we show that mean magnitude spectrum of changes in video sequences under challenging conditions can be an indicator of detection performance. CURE-TSD-Real dataset is available online at https://github.com/olivesgatech/CURE-TSD.

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CURE-TSD

CURE-TSD: Challenging Unreal and Real Environments for Traffic Sign Detection


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I Introduction

Transportation systems are transformed by disruptive technologies that are based on autonomous systems. In order for autonomous systems to seamlessly operate in real-world conditions, they need to be robust under challenging conditions. In this study, we focus on automated traffic sign detection systems and investigate the effect of challenging conditions in algorithmic performance. Currently, the performance of traffic sign detection algorithms are tested with existing traffic sign datasets in the literature [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12], which have been very useful to develop and evaluate state-of-the-art traffic sign recognition and detection algorithms. However, these datasets are usually very limited in terms of type and severity of challenging conditions. Moreover, there is no metadata corresponding to the types and the levels of these conditions. Traffic sign size can be considered as the only metadata corresponding to the challenge level, which can degrade the detection performance significantly [13]. Besides the limitations of challenging conditions, it is not feasible to assess the effect of most of the challenging conditions in existing datasets because of limited control over the acquisition process. A number of conditions change simultaneously, which makes it impossible to investigate the effect of a specific condition. To overcome the shortcomings of existing datasets and enable assessing the effect of challenging conditions one at a time, we introduced traffic sign recognition [14] and detection datasets [15, 16]. We hosted the first IEEE Video and Image Processing (VIP) Cup [15] within the IEEE Signal Processing Society and obtained an algorithmic benchmark for the CURE-TSD dataset, which is based on video sequences corresponding to real-world as well as simulator environments. In this study, we focus on the real-world sequences denoted as CURE-TSD-Real

dataset. Specifically, we investigate the effect of challenging conditions over the average performance of top two benchmark algorithms that are based on deep neural networks.

Recently, adversarial examples have been commonly used in the literature to test the vulnerability of recognition and detection systems [17, 18]. Even though adversarial examples have been successful in deceiving these generic object recognition and detection systems, they have not been effective in traffic sign recognition and detection systems [19, 20]. Lu et al. [21] showed that adversarial examples deceive traffic sign detection systems only in limited scenarios and Das et al. [20]

showed that a simple compression stage can minimize the effect of adversarial attacks in traffic sign recognition. Adversarial examples are useful to assess the limits of existing systems with special inputs that are optimized for deception. However, such adversarial examples do not necessarily correspond to real-world challenging conditions. Moreover, previous studies mainly focused on feeding adversarial data directly into the classification model. However, in real world, challenging conditions can directly affect the data acquired by sensors rather than classifiers. In this study, we differentiate from previous studies by focusing on challenging conditions corresponding to adversaries that can naturally occur in real-world environments and systems as shown in Fig. 

1. On contrary to adversarial studies in the literature that require model information to design input images, we designed challenging conditions independent of the detection algorithms, which enables a black-box performance assessment. Previously, we performed black-box assessment of object detection APIs with realistic challenging conditions in [22, 23], and investigated the robustness and out-of-distribution classification performance of traffic sign classifiers in [24, 25].

In addition to investigating the effect of challenging conditions in traffic sign detection performance, we also analyze the effect of challenging conditions in terms of spectral characteristics. In [26], Van der Schaaf and Van Hateren analyzed the power spectrum of natural images and showed that there is a common characteristic followed by natural images. In [27], Torralba and Olivia extracted more information based on spectrum related to the openness of images, the semantic category of scenes, the recognition of objects, and the depth of scenes. Based on these observations and findings, a direct comparison between spectrum of challenge-free sequences and challenging sequences can be affected by the context of the scene. However, if we first obtain the difference between challenge-free and challenging sequences and then obtain the power spectrum, we can limit the effect of context and concentrate on the change with respect to reference conditions, which is the methodology pursued in this study.

The rest of this paper is organized as follows: In Section II, we analyze existing traffic sign datasets. In Section III, we provide a general description of the CURE-TSD-Real dataset, describe challenging conditions, and briefly introduce benchmark algorithms. We discuss traffic sign detection performance under challenging conditions and analyze spectral characteristics of these conditions in Section IV. Specifically, we explain the performance metrics in Section IV-A, describe the training and test datsets in Section IV-B, report performance variation under challenging conditions in Section IV-C, perform a spectral analysis of challenging conditions in Section IV-D, and analyze the relationship between detection performance and spectral characteristics in Section IV-E. Finally, we conclude our work in Section V.





Number of
videos
Number of
annotated
images
Number of
annotated
signs
Annotation
information
Resolution
Number of
sign types
Annotated
sign size
Acquisition
location
Challenging
conditions
(*=annotated)



RUG [1]
N/A N/A N/A N/A 360X270 3 N/A Netherlands
illumination
overcast
blur

BelgiumTS [2]
4 9,006 13,444
sign type
bounding box
3D location
1,628x1,236 62
9x10 to
206x277
Belgium
illumination
occlusion
overcast

BelgiumTSC [3]
N/A 7,125 7,125
sign type
bounding box
22x21 to
674x527
62
11x10 to
562x438
Belgium
illumination
occlusion
deformation

Stereopolis [4]
N/A 273 273
sign type
bounding box
1,920x1,080 10
25x25 to
204x159
France
illumination
occlusion
overcast

STS [5]
N/A 3,488 3,488
sign type
bounding box
visibility status
road status
1,280x960 7
3x5 to
263x248
Sweden
illumination
occlusion*
shadow, blur*
overcast, rain

GTSRB [6, 7]
N/A 51,840 51,840 sign type
15x15 to
250x250
43
15x15 to
250x250
Germany
illumination
occlusion, blur
deformation
perspective

LISA [9]
17 tracks 6,610 7,855
sign type
bounding box
occlusion status
road status
640x480 to
1,024x522
47
6x6 to
167x168
USA
illumination
occlusion*
shadow, blur
reflection
codec error
dirty lens
decolorization*
overcast

GTSDB [8]
N/A 900 1,206
sign type
bounding box
1,360X800 43
16-128
longer
edge
Germany
illumination
occlusion
blur, shadow
haze, rain
overcast

TT-100K [10]
N/A 100,000 30,000
sign type
bounding box
pixel map
2,048x2,048 45
2x7 to
397x394
China
illumination
occlusion
overcast
haze, shadow

CTSD [11]
N/A 1,100 1,574
sign type
bounding box
1,024x768 &
1,270x800
48
26x26 to
573x557
China
illumination
occlusion
shadow, rain,
overcast
dirty lens
reflection
haze, blur

CCTSDB [12]
N/A 10,000 13,361
sign category
bounding box
1,024x768 &
1,270x800 &
1,000x350
3 classes
10x11 to
380x378
China
illumination
occlusion
shadow, rain
overcast
dirty lens
reflection
haze, blur

CURE-TSD-Real
2,989 896,700 648,186
sign type
bounding box
challenge type
challenge level
1,628x1,236 14
10x11 to
206x277
Belgium
rain*, snow*
shadow*, haze*
illumination*
decolorization*
blur*, noise*
codec error*
dirty lens*
occlusion
overcast





  • Online sources of the datasets are hyperlinked to the dataset names in the first column of this table.




Table I: Main characteristics of publicly available datasets and CURE-TSD-Real dataset*

Ii Existing Datasets

We summarize the main characteristics of existing traffic sign datasets in Table I, which includes number of video sequences, number of annotated frames/images, number of annotated signs, annotation information, resolution, number of sign types, annotated sign size, acquisition location, and challenging conditions. When a category does not apply to a specific dataset, there is a ‘not applicable’ abbreviation (N/A). We report the characteristics of publicly available datasets based on the reference papers as well as available dataset files. The majority of listed datasets are based on images whereas LISA provides short tracks up to 30 frames per track and BelgiumTS provides 4 videos whose number of frames varies between 2,001 and 6,001. Total number of annotated images varies from 273 to 100,000 in which number of traffic signs is in between 273 and 51,840. Annotations are mainly based on sign types and bounding box coordinates but 3D location, pixel map, visibility status, occlusion condition, and road status are also provided in certain datasets. The resolution of the traffic sign detection datasets is in between 360x270 and 2,048x2,048 and the sign size in all listed datasets vary from 2x7 to 573x557. The number of traffic sign types vary from 3 to 62. Challenging conditions are not annotated or explicitly described in majority of the datasets. Therefore, we visually inspected these datasets to list apparent challenging conditions, which include illumination, occlusion, shadow, blur, reflection, codec error, dirty lens, overcast, haze, and deformation of the traffic signs. The majority of the listed datasets were captured in Europe but recent studies also include China and USA. All of the datasets include images captured with color cameras but LISA dataset also includes grayscale images directly obtained from car cameras.

(a) speed limit (b) good vehicles (c) no overtaking (d) no stopping (e) no parking (f) stop (g) bicycle (h) hump (i) no left (j) no right (k) priority to (l) no entry (m) yield (n) parking
Figure 2: Traffic signs types in the CURE-TSD-Real dataset.

Iii CURE-TSD-Real Dataset

Iii-a General Information

Among the datasets analyzed in Section II, BelgiumTS [2] and LISA [9] are the only datasets that provide partial video sequences. When this study was conducted, tracks in the LISA dataset were available but not the video sequences. Therefore, we utilized the BelgiumTS [2] dataset to obtain video sequences. We selected a subset of the traffic signs in the BelgiumTS dataset as shown in Fig. 2 and labeled consecutive frames only for these sign types. Sign types were selected according to the compatibility with the synthesized part of the CURE-TSD dataset, which is not considered in this paper. In total, there were four main sequences in BelgiumTS, which included 3,001, 6,201, 2,001, and 4,001 frames. We grouped 300 consecutive frames as individual videos and obtained a total of videos. In the BelgiumTS dataset, annotations were provided for specific frames and annotating the missing frames could have resulted in an inconsistency among the labels. Therefore, we annotated all the frames including the ones with existing labels and extended the number of annotated frames from 9,006 to 14,700 using the Video Annotation Tool from Irvine, California (VATIC) [28]. Specifically, we utilized the JavaScript version on the browser and labeled a traffic sign if half of the sign was visible. We considered the original video sequences as baseline and added challenging conditions at different levels to test the performance limits of traffic sign detection algorithms.

Figure 3: All challenge types and levels corresponding to a sample frame in the CURE-TSD-Real video sequences.

Iii-B Challenging Conditions

We processed original video sequences with 12 challenge types to obtain challenging video sequences as illustrated in Fig. 3. Postproduction of challenging conditions scaled up the dataset size from 14,700 images to 896,700 images. We adjusted the level of challenging conditions through visual inspection rather than numerical progression. Challenge-free sequences were considered as level 0 and we added five different levels for each challenge type. Levels were adjusted according to the following rules: level 1 does not affect the visibility of traffic signs perceptually, level 2 affects the visibility of small and distant traffic signs, level 3 makes the visibility of small and distant traffic signs difficult, level 4 makes the visibility of small and distant traffic signs challenging, and level 5 makes the visibility of small and distant traffic signs nearly impossible. Simulated conditions and implementation details are listed as follows:

  • [label=,leftmargin=*]

  • tests the effect of color acquisition error, which was implemented using Black & White color correction filter version 1.0. The filter settings were: Reds, Yellows, Greens, Cyans, Blues, and Magentas. We utilized multiple adjustment layers to compound the effect of the color correction filter and created multiple distinct levels.

  • tests the effect of dynamic scene acquisition. was implemented with the Camera Lens Blur filter version whose radius was set to , , , , and along with Hexagan Iris Shape. For the challenge, Gaussian Blur filter version was used whose Bluriness levels were set to and . On contrary to lens blur, Gaussian blur is distributed in all directions, which leads to less structured blurred objects.

  • tests the effect of encoder/decoder error, which was implemented using Time Displacement filter version . Max Displacement Time was set to and .

  • tests the effect of underexposure, which was implemented using Exposure filter version . The master channel Exposure parameter was set to and .

  • tests the effect of occlusion because of dirt over camera lens, which was implemented by overlaying a set of dusty lens images.

  • tests the effect of overexpsoure in acquisition, which was implemented with the Exposure filter version . The master channel Exposure parameter was set to , , , , and .

  • tests the effect of acquisition noise, which was implemented using the Noise filter version . The Amount of Noise parameter was set to , , , , and .

  • tests the effect of occlusion due to rain, which was implemented using the Gradient Ramp generator version with colors # 0F1E2D and # 5A7492 to create a blueish hue over the video, and CC Rainfall generator from Cycore Effects HD 1.8.2 version . The Opacity level was set to with adjustment layers and the Drops option was set to , , , and .

  • tests the effect of non-uniform lighting due to shadow. Based on the description of Merriam Webster [29], shadow refers to partial darkness or obscurity within a part of space from which rays from a source of light are cut off by an interposed opaque body. In this study, darkness/obscurity refers to the vertical patterns and space refers to the traffic sign. In Fig. 3, we can observe that shadow partially occludes the traffic sign and the levels corresponds to the darkness of the occluded region. The condition was implemented using Venetian Blinds filter version . Transition Completeness was , Direction was , and Width was . Finally, Opacity was set to , and .

  • tests the effect of occlusion due to snow, which was implemented using the Glow filter version with color # FFFFF to create a white hue over the video, and CC Snowfall generator from Cycore Effects HD 1.8.2 version . Glow Threshold was , Glow Intensity was , Glow Operation was Screen, and Glow Dimension was Horizontal. Drops option in the CC Snowfall generator was set to , and using adjustment layers.

  • tests the effect of occlusion due to haze, which was implemented using the Ellipse Shape Layer filter version with radial gradient fill using color # D6D6D6 in the center with opacity and color # 000000 at the edges with opacity. The shape and focal point location of the ellipse was manually controlled to closely follow the furthest point in the video, which created a sense of depth to the scene and emulated the behaviour of haze. In addition to Ellipse filter, Smart Blur version , Exposure version , and Brightness & Contrast version were utilized. In the Smart Blur filter, Radius was set to , and Threshold was set to . In the Exposure filter, Radius was and Gamma Correction was . In the Brightness & Contrast filter, Brightness was set to and Contrast was set to . Solid Layer was used with a color code of # CECECE and opacity was set to , , , , and .

Challenge types were selected and synthesized based on the discussion with the Multimedia Signal Processing Technical Committee and IEEE Signal Processing Society during the VIP Cup 2017 competition process. In the original competition proposal, we proposed challenging conditions including , , , and . Based on the recommendations of the committee members and follow-up discussions, we added the remaining challenging conditions including , , , , , and . All of the challenging conditions other than were observed in the prior real-world traffic datasets [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12] as summarized in Table I, which indicates the relevance and significance of these conditions. To simulate challenging conditions, we utilized the state-of-the-art visual effects and motion graphics software Adobe(c) After Effects, which has been commonly used for realistic image and video processing and synthesis in the literature [30, 31, 32]. We provided the details of challenge generation process including the operators and parameters so that challenging condition synthesis can be reproducible, and researchers can build on top of the initial configurations. Challenging conditions do not have to be used all together and researchers can select the challenging conditions that are relevant and sufficient for their target application. This dataset can be considered as an initial step to assess the robustness of traffic sign detection algorithms under controlled challenging conditions, which can be further enhanced by the research community.

Iii-C Benchmark Algorithms

In this study, we analyze the average performance of two top performing algorithms in the VIP Cup traffic sign detection challenge [15, 16]

. Both of the algorithms are based on state-of-the-art convolutional neural networks (CNNs) including U-Net

[33], ResNet [34], VGG[35], and GoogLeNet [36]. In both algorithms, localization and recognition of the traffic signs are performed by separate CNNs. Details of the finalist algorithms are summarized as follows: First algorithm includes a VGG-based challenge type detection stage followed by a histogram equalization and a ResNet-based denoising depending on the challenge type. Traffic signs are localized by a U-Net architecture and recognized by a custom CNN architecture. Second algorithm extracts features with a pretrained GoogLeNet architecture whose features at the end of Inception 5B layer are fed into the regression layer to obtain sign locations, which are further classified by a custom CNN architecture.

Term Description/Formulation
Positive () Total number of traffic signs
True positive () Total number of correct traffic sign detections
False positive () Total number of false traffic sign detections
False negative() Number of undetected traffic signs
Precision (prec)
Recall (rec)
score
Table II: Detection performance metrics. ( or ) is used to adjust the relative importance of and .

Iv Traffic Sign Detection under Challenging Conditions

Iv-a Performance Metrics

We calculate precision, recall, , and metrics to measure the traffic sign detection performance as described in Table II

. A detection is considered correct if intersection over union (IoU), also known as Jaccard index, is at least 0.5. IoU is obtained by calculating the overlapping area between the ground truth bounding box and the estimated bounding box, and diving the overlaping area by the area of the union between these boxes.

Iv-B Training and Test Sets

There are 49 reference video sequences with 300 frames per video as described in Section III-A. Video sequences were split into training and test sequences by following 7:3 splitting ratio, which led to 34 training video sequences and 15 test video sequences. For each reference video sequence, we generated 60 challenging sequences based on 12 challenge types and 5 challenge levels. For each challenge type, there are 170 (34x5) video sequences (51,000 frames) in the training set and 75 (15x5) video sequences (22,500 frames) in the test set. We set the number of video sequences same for each challenge type and level to eliminate any bias towards a specific challenge type or level. Overall, there are 2,074 video sequences (622,200 frames) in the training set and 915 video sequences (274,500 frames) in the test set.

(a) Precision versus challenge levels (b) Recall versus challenge levels (c) versus challenge levels (d) versus challenge levels
Figure 4: Average traffic sign detection performance of top two algorithms with respect to challenge levels over all categories. Performance variations between challenge-free and severe conditions are reported for each metric in percentage.
Algorithms Top-I Top-II Average Top-I-II
Challenge Types Precision Recall Precision Recall Precision Recall
- 0.35 0.29 0.34 0.30 0.65 0.07 0.25 0.09 0.50 0.18 0.30 0.20
Decolorization 0.00 0.00 0.00 0.00 0.67 0.07 0.24 0.08 0.34 0.03 0.12 0.04
100 100 100 100 4 10 6 10 32 82 60 79
Lens blur 0.19 0.12 0.17 0.13 0.54 0.08 0.25 0.10 0.44 0.10 0.22 0.12
45 59 49 57 17 11 1 10 12 45 24 41
Codec error 0.04 0.02 0.03 0.02 0.17 0.01 0.04 0.01 0.11 0.01 0.03 0.01
89 95 91 94 73 89 86 88 79 93 89 93
Darkening 0.19 0.14 0.18 0.15 0.62 0.07 0.25 0.09 0.41 0.11 0.22 0.12
47 52 48 51 4 0 0 0 19 41 27 39
Dirty lens 0.10 0.04 0.07 0.04 0.60 0.07 0.24 0.08 0.36 0.05 0.16 0.06
72 87 78 86 7 8 7 8 28 71 48 68
Exposure 0.04 0.01 0.02 0.01 0.25 0.02 0.09 0.03 0.14 0.02 0.05 0.02
90 98 95 98 61 66 65 66 71 92 82 91
Gaussian blur 0.24 0.03 0.11 0.04 0.54 0.08 0.25 0.09 0.41 0.06 0.18 0.07
33 89 68 87 17 6 2 5 18 70 40 66
Noise 0.33 0.10 0.24 0.11 0.65 0.04 0.18 0.06 0.50 0.07 0.21 0.08
6 68 30 63 1 39 27 38 1 62 29 58
Rain 0.00 0.00 0.00 0.00 0.45 0.05 0.17 0.06 0.22 0.02 0.09 0.03
100 100 100 100 31 35 31 35 55 87 71 85
Shadow 0.27 0.23 0.26 0.24 0.64 0.06 0.22 0.07 0.46 0.15 0.25 0.16
24 21 22 21 1 17 13 17 8 21 16 20
Snow 0.28 0.04 0.13 0.05 0.60 0.06 0.22 0.08 0.44 0.05 0.17 0.06
20 87 63 85 6 14 12 14 11 72 41 68
Haze 0.22 0.00 0.00 0.00 0.64 0.07 0.25 0.09 0.44 0.04 0.13 0.05
37 100 99 100 1 2 0 1 11 79 56 77
Average 0.16 0.06 0.10 0.07 0.53 0.06 0.20 0.07 0.36 0.06 0.15 0.07
55 80 70 78 18 22 21 22 29 68 48 65
Table III: Detection performance of top-I, top-II, and average top-I-II algorithms under challenging conditions for each challenge type and performance metric. First row reports the detection performance over reference video sequences without simulated challenging conditions. For each challenge type, we report the detection performance in the corresponding first row and percentage performance degradation () with respect to the challenge-free conditions in the corresponding second row. At the last two rows, we report the average detection performances and performance degradations () with bold font over all challenge types.

Iv-C Performance Variation under Challenging Conditions

As reference performance, we calculate the average detection performance of benchmarked algorithms for challenge-free sequences. Then, we calculate the detection performance under varying challenging conditions and levels. Average detection performance under different challenge levels are reported in Fig. 4. Y-axis corresponds to detection performance and x-axis corresponds to challenge levels. In addition to reporting average detection performance for each challenge level, we also report the percentage performance degradation under severe challenging conditions (level 5). Based on the results in Fig. 4, detection performance significantly decreases with respect to reference challenge-free conditions. Specifically, detection performance degrades by in precision, in recall, in score and in score.

To understand the effect of challenging condition types over traffic sign detection, we report the performance of top-I team, top-II team, and their average (top-I-II) in Table III. The detection performance of top-I team degrades by in precision, in recall, in score, and in score. For top-II team, the performance degradation is in precision, in recall, in score, and in score. We can observe that overall performance degradation for each team vary between and in which the variation is higher for top-I team. When team results are averaged (Top-I-II), overall performance degrades by in precision, in recall, in score, and in score. Precision under challenge is the only category in which performance remains almost same as reported in top-I-II results. In other challenge categories, precision degradation varies between and . Performance degradation in recall is more significant than precision in all challenge categories, which varies between and . Performance variation in score is in between and .

Even though almost all the simulated challenging conditions degrade detection performance, there are few exceptions for top-II algorithm in ( precision), ( precision) ( recall, , and ), and ( recall and ). The effect of significantly depends on the benchmark algorithm and the increase in precision under for top-II algorithm does not exceed . In the category, detection performance of top-I algorithm degrades in term of all metrics whereas performance variation of top-II algorithm does not exceed . When traffic signs under severe challenging conditions are compared in Fig. 3, we can observe that structural information including high frequency components mostly remain intact in category whereas chroma information is distorted. Therefore, we can conclude that top-I algorithm significantly relies on color information whereas top-II does not rely as much. Challenging conditions based on blur filter out high frequency components that can be used to recognize and localize traffic signs. But, at the same time, filtering out high frequency components can eliminate certain false detections, which can explain the minor performance enhancement in the aforementioned exceptional categories.

Figure 5: Average detection performance degradation under challenging conditions for each challenge type.

We report the average performance degradation over all the performance metrics and algorithms to understand the overall effect of challenging conditions types in Fig. 5. and result in the highest performance degradation in the challenge categories. As observed in sixth row from top in Fig. 3, condition can significantly saturate descriptive regions of traffic signs that are critical for recognition, which results in a high performance degradation. In case of , we observe visual artifacts that corrupt the structural characteristics of the sign. In addition to the structural artifacts, challenge can relocate a significant portion of the traffic sign to a new location as shown in the third row from top in Fig. 3, which would not satisfy the required overlap between the ground truth location and the detected location even though the sign can be recognized accurately. Benchmark algorithms can be vulnerable to challenging weather conditions including , , and , which lead to a performance degradation between and . In real-world scenarios, outer surface of the camera lens or the window surface in front of the camera can get dirty because of the weather and road conditions, which can affect the visibility of traffic signs due to occlusion. Based on the experiments, occlusions can reduce the overall traffic sign detection performance by half. , , and challenges degrade the detection performance between and whereas results in the least performance degradation by . Because of the difficulty of realistic shadow generation, we used a simple periodic pattern to simulate local effect, which can be considered as a partial effect. We can observe that the performance degradation in the category is almost double the degradation in the category, which is proportional to the ratio of the darkened regions when degraded images are compared in both categories as observed in fourth row and tenth row from top in Fig. 3. When traffic sign images with high level conditions are observed in Fig. 3, images may appear almost entirely dark and can be considered as the most challenging condition perceptually. However, the perceptual level of darkness depends on the display settings and if images are observed under different display configurations, it can be possible to observe descriptive parts of the traffic sign even under high level conditions.

Iv-D Spectral Analysis of Challenging Conditions

In this section, we investigate the effect of challenging conditions over the spectral characteristics of video sequences. In the CURE-TSD-Real dataset, there are challenge-free sequences. Corresponding to each challenge-free reference video, there are video sequences with distinct challenge conditions ( types levels). For each challenge-free reference video, we obtain the pixel-wise and frame-wise difference between the reference video and challenging video, which results in the residual video. In total, we obtain ( videos challenge configurations) residual videos, which correspond to residual videos per challenge type. For each residual video, we calculate the power spectrum per frame to obtain a power spectrum sequence. We calculate the power spectrum of a residual frame as

(1)

where is the residual frame, is the absolute value, is the -

discrete Fourier transform, and

is the logarithm. We calculate the pixel-wise average of power spectrum sequences for each challenge type and quantize them to obtain the average magnitude spectrum maps in Fig. 6.

(a) Decolorization (b) Lens blur (c) Codec error (d) Darkening (e) Dirty lens (f) Exposure (g) Gaussian blur (h) Noise (i) Rain (j) Shadow (k) Snow (l) Haze
Figure 6: Average magnitude spectrum maps of video sequences corresponding to different challenge types.

In the average magnitude spectrum maps, central region corresponds to low frequency components and corners represent high frequency components. Color coding is based on the magnitude of the frequency components. Color of the spectrum elements varies from dark blue to yellow as their magnitude increases. We can observe that challenging conditions lead to characteristic spectral shapes that can be used to analyze the effect of these conditions. Even though and correspond to perceptually very distinct images as observed in Fig. 3, their spectral representations correspond to an almost identical pattern in Fig. 6(d) and Fig. 6(f). Spectral representation of challenge indicates that high-frequency components remain similar to the challenge-free sequences whereas low-frequency components get significantly distorted. and result in non-uniform deformations that affect the visibility of certain regions in an image, which lead to similar spectral representations. In the challenge, lens blur and Gaussian blur lead to a similar pattern in which boundaries of horizontal and vertical regions correspond to cutoff frequencies as observed in Fig. 6(b) and Fig. 6(g). Challenging conditions result in dominant vertical patterns in the and the challenges as observed in Fig. 3, which correspond to a more predominant horizontal pattern in spectral representations as shown in Fig. 6(i) and Fig. 6(j). Moreover, we observe discrete peeks in the spectral representations of the challenge in Fig. 6(j) because of the periodic shadow patterns. In the challenge, falling particles are the main occluding factor whereas in the challenge, piled up snow significantly occludes certain regions, which limits the highest spectral components to a more central region as observed in Fig. 6(k). and challenge lead to a peak at DC along with minor low frequency degradations as shown in Fig. 6(a) and Fig. 6(h). In the challenge, we observe local shifts of certain regions in the images as shown in Fig. 3, which leads to an almost symmetric spectral representation in Fig.6(c) without the sharp horizontal and vertical lines. On contrary to all other challenges, challenge results in deformation over the images that varies in terms of shape and size, which leads to a granular structure in the spectral representation as shown in Fig. 6(e).

Figure 7: Average magnitude spectrum maps of video sequences for different challenge levels and types.

Previously, we investigated the effect of challenge types in spectral representations of residual videos. To understand the effect of challenge levels in spectral representations, we also need to analyze the changes in spectrum with respect to the severity of challenging conditions. In Fig. 7, we show the average magnitude spectrum maps of video sequences for different challenge levels and types. Minor condition corresponds to level one, medium condition corresponds to level three, and major condition corresponds to level five challenges. Each spectrum corresponds to the average of video sequences with a specific challenge type and level. The most significant spectral change with respect to challenge levels occur in case of and whereas the least change occur in case of , , and .

In the aforementioned experiments and analysis, we focused on the effect of individual challenging conditions and levels because video sequences in the CURE-TSD-Real dataset include one challenging condition at a time. Therefore, it is not possible to directly assess the effect of concurrent challenging conditions. In order to test the capability of spectral representations under concurrent challenging conditions with an example, we combined and conditions and obtained their magnitude spectrums as shown in Fig. 8. We obtained each magnitude spectrum map by averaging frame-level magnitude spectrums of 49 video sequences (14,700 frames). Spectral maps corresponding to concurrent and conditions are shown in Fig. 8(c) and Fig. 8(f). In addition to spectral maps of concurrent and conditions, we included the spectral maps of isolated and conditions in Fig. 8(a), Fig. 8(b), Fig. 8(d), and Fig. 8(e) to visually compare them next to each other.

(a) Minor exposure (b) Major exposure (c) Major exposure and minor rain (d) Minor rain (e) Major rain (f) Major rain and minor exposure
Figure 8: Average magnitude spectrum maps of video sequences corresponding to rain, exposure, and a combination of rain and exposure at different challenge levels.

In case of concurrent conditions, we can observe that major condition dominates the spectral representation. Spectral map of concurrent major and minor (Fig. 8(f)) is similar to the spectral maps of (Fig. 8(d-e)) in terms of asymmetry between horizontal and vertical components. Moreover, spectral map of concurrent major and minor (Fig. 8(c)) is similar to the spectral map of major exposure (Fig. 8(b)). Thus, we can mention that dominant conditions mostly determine the shape of the spectral maps. However, we can still observe differences in the spectral maps when we compare concurrent major and minor condition with solely major condition. For example, spectral map of major (Fig. 8(e)) and spectral map of concurrent major and minor (Fig. 8(f)) are still separable from each other in terms of shape and color. Meanwhile, spectral map of major and minor (Fig. 8(c)) and spectral map of major (Fig. 8(b)) are separable from each other in terms of color, which reflects differences in terms of spectral magnitude. Based on this example, we can express that spectral maps can reflect the impact of two concurrent conditions, but identification of the concurrent conditions may not be as straightforward as the identification of individual conditions.

Iv-E Detection Performance versus Spectral Characteristics

Even though challenge levels affect the spectral representations, high level spectral shapes remain similar in majority of the challenging conditions. The intensity of the magnitude spectrums can be used to quantify the changes in spectral representations, which can be an indicator of the detection performance degradations. In Fig. 9, we show the relationship between detection performance and mean magnitude spectrum. Specifically, we computed the detection performance under varying challenge levels and calculated the mean magnitude spectrum corresponding to the varying challenge levels. We can observe that an increase in mean magnitude spectrum generally corresponds to a decrease in detection performance. To measure the correlation between traffic sign detection performance and mean magnitude spectrum, we calculated the Spearman rank order correlation coefficient, which is reported for each detection performance metric in Table IV. Specifically, we measured the correlation between mean magnitude spectrum and detection performance for each challenge category and obtained the average of these correlation coefficients. Based on the experiments, correlation between detection performance and mean magnitude spectrum is for precision, for recall, for score, and for score.

Figure 9: Detection performance versus mean magnitude spectrum of residual video sequences.

Spectral representations can be used to analyze the changes in images and videos and these changes can be quantified by measuring the changes in spectral representations. A direct mean pooling operation is a straightforward approach to quantify spectrums of residual sequences. However, detection algorithms do not necessarily react identically to changes at different frequencies. Therefore, instead of a direct mean pooling operation, a weighted pooling can be performed by considering the relative importance of frequency bands for traffic sign detection. For example, in JPEG compression [37], the objective is to compress the image as much as possible without visual artifacts. To achieve this objective, quantization tables were designed based on psychovisual experiments to compress signal components according to the their perceptual significance. Similarly, a significance map can be designed for traffic sign detection application to quantify the changes in spectral components according to their algorithmic significance. Spectral analysis approach investigated in this study requires a reference video. Therefore, to estimate the traffic sign detection performance, we need to obtain the images of the same scene at different conditions. Such a system is feasible for a fixed camera setup in which we can capture the same region at different times. To deploy such systems to mobile platforms, we need to focus on no-reference spectral representations in which there is no need for a reference video.

Estimated Metric Precision Recall
Estimation Performance
(Spearman Correlation)
0.643 0.848 0.657 0.810
Table IV: Detection performance degradation estimation with mean magnitude spectrum under challenging conditions.

V Conclusion

We analyzed the average performance of benchmark algorithms in the CURE-TSD-Real dataset and showed that detection performance can significantly degrade under challenging conditions. and resulted in the most significant performance degradation with more than whereas resulted in the least degradation with around . Challenging weather conditions , , and resulted in at least performance degradation. Around performance degradation in highlighted the importance of color information for certain algorithms in sign detection. Detection performance degradation based on , and is in between and whereas exceeds . Our frequency domain analysis showed that simulated challenging conditions can correspond to distinct spectral patterns and magnitude of these spectral patterns can be used to estimate the detection performance under challenging conditions. Degradation estimation perfomance based on spectral representations was in between and in terms of Spearmen correlation. As future work, adaptive pooling and no-reference spectral analysis are promising research directions that can be further investigated to estimate detection performance of algorithms by solely considering the environmental conditions.

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