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<article xmlns:xlink="http://www.w3.org/1999/xlink">
  <front>
    <journal-meta />
    <article-meta>
      <title-group>
        <article-title>Combination of image and location information for snake species identi cation using ob ject detection and E cientNets</article-title>
      </title-group>
      <contrib-group>
        <contrib contrib-type="author">
          <string-name>Alex Michailutschenko</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Obioma Pelka</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
          <xref ref-type="aff" rid="aff1">1</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Johannes Ruckert</string-name>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Leon Willemeit</string-name>
          <email>leon.willemeit002g@stud.fh-dortmund.de</email>
          <xref ref-type="aff" rid="aff0">0</xref>
        </contrib>
        <contrib contrib-type="author">
          <string-name>Christoph M. Fri</string-name>
          <email>christoph.friedrichg@fh-dortmund.de</email>
        </contrib>
        <aff id="aff0">
          <label>0</label>
          <institution>Department of Computer Science, University of Applied Sciences and Arts Dortmund (FHDO)</institution>
          ,
          <addr-line>Emil-Figge-Str. 42, 44227 Dortmund</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
        <aff id="aff1">
          <label>1</label>
          <institution>Department of Diagnostic and Interventional Radiology and Neuroradiology, University Hospital Essen</institution>
          ,
          <addr-line>Essen</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
        <aff id="aff2">
          <label>2</label>
          <institution>Institute for Medical Informatics, Biometry and Epidemiology (IMIBE), University Hospital Essen</institution>
          ,
          <addr-line>Essen</addr-line>
          ,
          <country country="DE">Germany</country>
        </aff>
      </contrib-group>
      <abstract>
        <p>Snake species identi cation based on images is important to quickly treat patients su ering from snake bites using the correct antivenom. The SnakeCLEF 2020 challenge, which is part of the LifeCLEF research platform, is focused on this task and provides snake images and associated location information. This paper describes the participation of the FHDO Biomedical Computer Science Group (BCSG) in this challenge. The implemented machine learning work ow uses Mask Region-based Convolutional Neural Network (Mask R-CNN) for object detection, various image pre-processing steps, E cientNets for classi cation as well as di erent methods to fuse image and location information. The best model submitted before the challenge deadline achieved a macro-averaging F1-score of 0.404. After the expiration of this deadline, the results could be improved up to a macro-averaging F1-score of 0.594.</p>
      </abstract>
      <kwd-group>
        <kwd>snake species identi cation</kwd>
        <kwd>object detection</kwd>
        <kwd>E</kwd>
        <kwd>cientNets image classi cation</kwd>
        <kwd>metadata inclusion</kwd>
      </kwd-group>
    </article-meta>
  </front>
  <body>
    <sec id="sec-1">
      <title>-</title>
      <p>
        This paper explains the participation of University of Applied Sciences and Arts
Dortmund (FHDO) Biomedical Computer Science Group (BCSG) at the
Conference and Labs of the Evaluation Forum (CLEF) 20204 SnakeCLEF challenge5
for snake species identi cation [
        <xref ref-type="bibr" rid="ref20">20</xref>
        ]. This challenge is part of the LifeCLEF 2020
research platform which focuses on the automated identi cation of species [
        <xref ref-type="bibr" rid="ref14">14</xref>
        ]
and consists of four challenges. The implemented approach in this paper is
inspired by an article [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] about the winning entry of round 2 of the AICrowd Snake
Species Identi cation Challenge6.
      </p>
      <p>
        The identi cation of snake species is important as there are approximately
between 81,410 and 137,880 victims of snakebites dying every year [29]. These
deaths result from inaccurate knowledge about the species and consequently
about the antivenom needed [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ].
      </p>
      <p>
        The high diversity of snake species [27] and their partially similar appearances
lead to confusion [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ] and make this choice more complicated. It is also mentioned,
that an increasing amount of people who were bitten by a snake bring pictures
of the snake, for example, taken with a smartphone, or the killed snake itself to
the physician [
        <xref ref-type="bibr" rid="ref5">5</xref>
        ].
      </p>
      <p>
        Therefore, the target of the SnakeCLEF challenge is the improved and robust
identi cation of snake species based on photographs [
        <xref ref-type="bibr" rid="ref20">20</xref>
        ].
      </p>
      <p>In this article, the experiments and results of FHDO BCSG are presented.
For this reason, Section 2 describes previous work in this eld of research.
Afterwards, the general machine learning work ow is illustrated in Section 4, followed
by a description of the achieved results in Section 5. Finally, the results are
summarized in Section 6.
2</p>
    </sec>
    <sec id="sec-2">
      <title>Related Work</title>
      <p>Automated identi cation of snake species using machine learning is rarely
studied, resulting from small datasets of annotated images.</p>
      <p>
        James et al. [
        <xref ref-type="bibr" rid="ref13">13</xref>
        ] described a semiautomatic approach, where taxonomical
features have been extracted from images to discriminate six di erent species.
The dataset contained 1,299 images and the least frequent class included 88
images. Using di erent feature selection approaches, it has been concluded that the
bottom-view taxonomical features are less important for the species identi
cation than the front- and side-view features.
      </p>
      <p>
        As manual extraction of features describing the appearance of a snake is
tedious, recent articles used automated feature extraction, for example, texture
features [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ] or deep learning [
        <xref ref-type="bibr" rid="ref18 ref2 ref3 ref9">2,3,9,18</xref>
        ].
4 https://clef2020.clef-initiative.eu/, [last accessed: 2020-07-17]
5 https://www.imageclef.org/SnakeCLEF2020, [last accessed: 2020-07-17]
6
https://www.aicrowd.com/challenges/snake-species-identificationchallenge, [last accessed: 2020-07-17]
      </p>
      <p>
        Texture features were used in Amir et al. [
        <xref ref-type="bibr" rid="ref4">4</xref>
        ] to distinguish between 22
different species. Their dataset contained 349 images and the least frequent snake
species included three images. Using classical machine learning methods, the
approach achieved a classi cation accuracy of 87 %.
      </p>
      <p>
        Patel et al. [
        <xref ref-type="bibr" rid="ref18">18</xref>
        ] used deep learning methods to develop an application for
smartphones which distinguishes images of nine di erent snake species,
occurring on the Galapagos Islands in Ecuador. To this end, object detection, as well
as classi cation algorithms, have been used. The training dataset for their
implementation has been a bundle of three data sources, two internet searches of the
platforms Google and Flickr were combined with an image dataset provided by
the Ecuadorian institution Tropical Herping7. In total, 250 images were collected
and the least frequent class contained seven images. Di erent model
architectures have been tested for object detection and image classi cation. The model
which was based on Faster Region-based Convolutional Neural Network (Faster
R-CNN) [23] ResNet [
        <xref ref-type="bibr" rid="ref11">11</xref>
        ] achieved the best classi cation accuracy of 75 %. The
authors state that a larger amount of training samples would be important for
further investigations in this eld.
      </p>
      <p>
        Abdurrazaq et al. [
        <xref ref-type="bibr" rid="ref2">2</xref>
        ] used three di erent Convolutional Neural Network
(CNN) architectures to distinguish ve di erent snake species. They used a
dataset containing 415 images. For the least frequent snake species, 72 images
were available. The best results were achieved using a medium-sized classi cation
network.
      </p>
      <p>
        Abeysinghe et al. [
        <xref ref-type="bibr" rid="ref3">3</xref>
        ] used a deep Siamese network [
        <xref ref-type="bibr" rid="ref6">6</xref>
        ] to classify a relatively
small dataset containing 200 images of 84 species based on World Health
Organization (WHO) venomous snake database8. The approach described in their
article concentrated on single-shot learning as the dataset included 3 to 16
images per species. The achieved results of the automated classi cation model
performed worse than human classi cation accuracy. Pairwise classi cation results
exceed class prediction accuracy.
      </p>
      <p>
        As already mentioned, Gokula Krishnan [
        <xref ref-type="bibr" rid="ref9">9</xref>
        ] described the results of round 2
of the AICrowd Snake Species Identi cation Challenge. The solution which
achieved the best results has used object detection as a pre-processing step
to focus on the image parts containing the snake. On this basis, E cientNets
were applied afterwards for image classi cation. In round 2 the dataset included
187,720 images assigned to 85 classes.
3
      </p>
    </sec>
    <sec id="sec-3">
      <title>Dataset</title>
      <p>The training dataset used in the actual SnakeCLEF and AICrowd Snake Species
Identi cation Challenge round 4 consists of 245,185 red-green-blue- (RGB-)
color-space-images (models trained on the training dataset were referred to as
T1) assigned to 783 di erent snake species. Additionally, a validation dataset is
7 https://www.tropicalherping.com/, [last accessed: 2020-07-17]
8 https://apps.who.int/bloodproducts/snakeantivenoms/database/, [last
cessed: 2020-07-17]
ac14000
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/su t/aa /su t/su i/nn /aem i/sne ii/r / / / /su lsau )81=
iilitttrccsknanodoga− iitrsab−m liitrrcscahphoaphooh−mm iiiirrcscanabnuoh− iffsahophgeo−m ijiiysauheb−m lffrycaenodo− irrzscvceehaepoa−wm llijdaapaan− iltttscsaunupnuahohp−m lliiirscsysaohnopb− ilililrscssspohdophooepa− itrcsycspheohhnopha−m (n
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b
available, which includes another 14,029 images (models trained on the
training and validation dataset were referred to as T2). The class distribution of the
snake species is highly unbalanced as can be seen in the bar plot of the absolute
class frequencies depicted in Figure 1.</p>
      <sec id="sec-3-1">
        <title>3.1 Image Dataset</title>
        <p>
          An analysis of the dataset with AntiDupl9 revealed 1,713 duplicate images in
the training set. Some of these duplicates are associated with di erent species
like \Image not found" images, that are the result of download problems. Other
duplicates are correctly associated with several species as they depict distinct
9 https://github.com/ermig1979/AntiDupl, [last accessed: 2020-07-15]
snakes. When the mean squared di erence between images in AntiDupl is relaxed
to 2 %, another 2,114 duplicates can be found. These are the result of di erent
jpeg compression rates for the same image, resize operations or deletion of
copyright information. Another problem that has been found are out-of-class images,
that have been injected by the organizers. These images contain no snakes but
for example, ice-hockey players, churches, other animals, persons, and mangas.
To identify them for exclusion from the training set, a standard ImageNet [
          <xref ref-type="bibr" rid="ref8">8</xref>
          ]
classi er with 1,000 classes and based on a ResNet50 [
          <xref ref-type="bibr" rid="ref11">11</xref>
          ] architecture has been
used and a positive list of snake and reptile classes, that are part of the ImageNet
Large Scale Visual Recognition Challenge 2012 (ILSRVC2012) [25] dataset has
been used. With this classi er, about 4,000 out-of-class images have been
identied and the e ects of the reduced dataset (abbreviated as D1 hereafter) has been
tested and compared to the un ltered dataset. The results of this comparison
are summarized in Table 7.
3.2
        </p>
      </sec>
      <sec id="sec-3-2">
        <title>Metadata</title>
        <p>The images are associated with metadata that provides information about the
continent and country of the place where the image has been taken. For some
snake depictions, the information is not given and only \UNKNOWN" is
provided in the metadata. This information could be used for better classi cation.
It should be noted, that the number of snake species in the dataset does not
match the natural occurrence of a snake in a location. For example, the most
frequent species with German country information in the dataset is pantherophis
guttatus, the corn snake which is not endemic in Germany but is the pet snake
number one in Germany. Accordingly, the data set takes into account that pet
snakes can also attack humans.
4</p>
      </sec>
    </sec>
    <sec id="sec-4">
      <title>Methods</title>
      <p>
        This section describes the work ow used to learn a discrimination between the
di erent snake species. The generalized work ow is depicted in Figure 2. The
work ow is modular and in the course of the challenge, it was examined how
di erent implementations of the individual modules a ect the classi cation
performance on the test dataset. In this section, the components are described more
precisely and di erent implementations of them are demonstrated. The work ow
has been implemented using the programming language Python 3.6.9 [28] and
was based on Keras 2.2.4-tf [
        <xref ref-type="bibr" rid="ref7">7</xref>
        ] with a Tensor ow 2.1.0 [
        <xref ref-type="bibr" rid="ref1">1</xref>
        ] backend. For the
inference on the AICrowd submission platform, Tensor ow 2.0.0 was used for reasons
of compatibility.
      </p>
      <p>Image pre-processing included an optional object detection stage and a
mandatory stage, where rectangular images were transferred to a square shape
afterwards. Additionally, the images were augmented, optionally branded using
locational information and fed into the deep learning training network. Finally, an
optional multiplication of the prediction probabilities and the a priori
probability distribution of the snake species occuring at the corresponding location has
been implemented.</p>
      <p>Input image</p>
      <p>Object detection</p>
      <sec id="sec-4-1">
        <title>Preprocessing</title>
        <p>Image preprocessing
Augmentation
Image branding
Model training
Metadata
multiplication</p>
      </sec>
      <sec id="sec-4-2">
        <title>Classification</title>
        <p>
          The idea of using an object detection stage before executing an image classi
cation was inspired by the winning team [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ] of round 2 of the AICrowd Snake
Species Identi cation Challenge. Object detection has been implemented using
the Mask R-CNN procedure, rst described by He et al. [
          <xref ref-type="bibr" rid="ref10">10</xref>
          ]. Mask R-CNN
performs instance segmentation as it extracts a bounding box, a class label and a
pixel-wise segmentation mask for each object detected in an image. The Mask
R-CNN algorithm is organized using two di erent stages. In the rst stage, a
backbone CNN extracts a feature map from the original image. In this paper,
Resnet-50 has been used as a backbone. Afterwards but also in the rst stage,
a Region Proposal Network (RPN) is used to identify candidate object regions.
So-called anchor boxes are used in this step to prede ne bounding boxes. The
second stage consists of a Region of Interest (ROI) align network which extracts
multiple possible ROI sections. Based on these sections, a fully connected layer
network is trained to perform a parallel softmax classi cation for class identi
cation (snake vs. background in this case) and a regression task to specify bounding
boxes. Additionally, a CNN-based mask classi er is employed for pixel-wise
segmentation. In this article, the backbone model weights were initialized by the
model weights trained on the ImageNet [
          <xref ref-type="bibr" rid="ref8">8</xref>
          ] dataset. The training on the snake
dataset has been implemented in two di erent phases. First all layers except the
layers which are included in the backbone were trained using 20 epochs to warm
up the model and afterwards 30 epochs were performed to train the entire model.
The implementation of the Mask R-CNN used in this article is an adaption10
of the implementation of Abdulla11 transferred to use Tensor ow 2.1.0. No data
augmentation has been used for object detection. The threshold of minimum
detection con dence has been set to 0.3. Stochastic gradient descent (SDG) was
used as an optimizer to train the model, momentum was set to 0.9. Further
parameters include a weight decay, which was set to 0.0001 and a batch size of
8 was used.
        </p>
        <p>
          In order to train the object detection model, the annotated snake images
available from the winning solution of round 2 of the AICrowd Snake Species
Identi cation Challenge [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ] (O1 in Section 5) were used initially. Later, 400
additionally annotations were added to this dataset (O2 in Section 5) to investigate
whether the object detection and thus the classi cation performance can be
improved. The object detection results can be found in Table 3. Since Mask R-CNN
is used in this approach only for object detection and not for instance
segmentation, it may be an adequate solution to use Faster R-CNN instead of the Mask
R-CNN. However, the results of the TensorFlow Object Detection application
programming interface (API) [
          <xref ref-type="bibr" rid="ref12">12</xref>
          ], which represents a guide to choose an
adequate object detection architecture shows an increased mean average precision
(mAP) of 39.0 for the Microsoft Common Objects in Context (COCO) dataset
[
          <xref ref-type="bibr" rid="ref17">17</xref>
          ] for Mask R-CNN object detection in comparison to Faster RCNN, which
achieves a mAP of 38.712. The use of the Mask R-CNN object detection makes
it easier to supplement segmentation data prospectively, which was not used
during this challenge due to a lack of time.
4.2
        </p>
        <sec id="sec-4-2-1">
          <title>Image Pre-processing</title>
          <p>As most of the deep learning classi cation models expect input images of square
shape and prede ned dimensions, it has been important to transform the mostly
rectangular images, or image parts detected by the object detection into square
shape and adjust the image for the expected image dimension of the classi
cation model. There are di erent possibilities for the extraction of quadratic from
rectangular images. The methods used in this paper are described in this section
and the implemented combinations of this methods are summarized in Table
1. The results of the experiments achieved using di erent image pre-processing
methods are summarized in Table 4.</p>
          <p>Resize The least complex possibility has been to rescale images without
consideration of aspect ratio. This resulted in highly distorted images so the
texture and the shape of the snake have been disturbed especially for images with
10 https://github.com/DiffPro-ML/Mask_RCNN, [last accessed: 2020-06-30]
11 https://github.com/matterport/MaskRCNN, [last accessed: 2020-06-30]
12 https://github.com/tensorflow/models/blob/master/research/object_
detection/g3doc/tf2_detection_zoo.md [last accessed: 2020-08-03]
strongly di erent image dimensions. In this paper, two rescaling procedures, one
considering and one retaining the aspect ratio were compared to each other. In
the latter case, images had to be padded with further information to transfer
them to a square shape.</p>
          <p>Scaling Another problem which occurs during pre-processing is the problem
of upscaling. Upscaling small images lead to poor image quality. It has been
suspected that this could bring di culties in texture recognition. In this paper,
approaches which did and did not use upscaling for image pre-processing were
compared to each other. If upscaling has been avoided, approaches were needed
to pad pixel information for the remaining image sections.</p>
          <p>
            Fill boundaries As previously mentioned, there were some di erent cases,
where padding was required to get input images with preset image dimensions.
One strategy to solve this issue has been to pad the image by a monochrome
color. Koitka and Friedrich [
            <xref ref-type="bibr" rid="ref16">16</xref>
            ] recommended padding with a color matching to
the image instead of using a prede ned color (usually black or white). Since black
is usually the most frequently occurring color in shady images, this approach
used the average color of the original image or rather of the cropped areas as an
alternative to pad the image.
          </p>
          <p>In combination with object detection, it has been possible to increase the ROI
and thus pad the image using background information instead of monochrome
color. In this case, the image section prede ned by the object detection
workow has been expanded as long as a quadratic section is found or one of the
dimensions of the original image were smaller than the expected dimension of
the square. If this happened, the average color of the image has been used to
pad protruding boundaries. It has been attempted to include background evenly
on all sides to center the snake. Sometimes this was not possible, for example,
if the snake was located in a corner of the original image. In this case, the ROI
has been moved to include background information of the remaining directions,
thus the snake has not been centered in the image.</p>
          <p>No consideration of the aspect ratio Up-scaling No padding
Consideration of the aspect ratio Up-scaling Monochrome padding
Consideration of the aspect ratio No up-scaling Monochrome padding
Consideration of the aspect ratio No up-scaling Background padding</p>
        </sec>
        <sec id="sec-4-2-2">
          <title>Data Augmentation</title>
          <p>Data augmentation has been used to expand the training images and avoid
over tting. In each epoch of the training process, the images were randomly
transformed. These transformations included random cropping of approximately
10 % of the image pixels per dimension, a rotation in the range of 40 , a
widthshift, height-shift, random shearing, zooming each with a factor of 0.2, as well as
the possibility of horizontal ipping. If pixel positions were generated during this
procedure, for which no image information has been available, those were lled
using the value of the nearest available image position. During the challenge, the
work ow has been adapted to speed up the image classi cation procedure. In
the later version of the work ow those pixels used black as a monochrome color.
4.4</p>
        </sec>
        <sec id="sec-4-2-3">
          <title>Image Classi cation</title>
          <p>
            E cientNets As also used by Gokula Krishnan [
            <xref ref-type="bibr" rid="ref9">9</xref>
            ], E cientNets, rst
described in Tan and Le [26], were used for classi cation in this approach. The
baseline E cientNet-B0 architecture is generated using an architecture search
that parallely optimizes accuracy on a prede ned classi cation task and Floating
Point Operations Per Second (FLOPS) [26]. Based on this baseline model, larger
models of the same family are created by scaling the depth, height and
resolution of the baseline model uniformly. The di erent models of this family achieve
state-of-the-art classi cation accuracy on ImageNet [
            <xref ref-type="bibr" rid="ref8">8</xref>
            ]. Additionally, the
architecture is smaller and faster on inference compared to other existing CNNs [26].
E cientNets were successfully adapted to di erent machine learning problems
using transfer learning [26].
          </p>
          <p>
            Various models of the E cientNets family were used in this competition
from E cientNet-B0 up to E cientNet-B4 networks (B0 - B4 in Section 5).
The results of using di erent models of the E cientNets family can be found
in Table 6. The model weights were initialized by a model pre-trained using
noisy student [30]. The E cientNets were extended by a atten layer, a dense
layer with 1000 neurons and Swish [22] as an activation function and a dense
layer with 783 neurons, which corresponds to the number of snake species and
softmax activation were added to the previous architecture. The described model
was trained for a few epochs on the snake classi cation task to warm-up the
network. In this phase only the newly added layers and the batch normalization
layers have been trained. Afterwards all layers were trained for a larger number
of epochs (N10+50 denotes a warm-up phase including ten epochs and 50 epochs
are used to train the entire model). Di erent batch sizes were used as further
parameters to train the model (32 is encoded as BS32, 64 as BS64 etc., BS64/32
means that a batch size of 64 has been used during the warm-up phase and a
batch size of 32 has been used afterwards). The chosen batch size depended on
the image size (e.g., an image size of 128 128 is encoded as S128 in Section 5) the
classi cation model and the available graphics processing unit (GPU) memory.
The results of models using di erent image sizes can be found in Table 5. The
learning rate ( ) was likewise adjusted depending on the batch size (LR1 encodes
a learning rate of 10 4 during the warm-up phase and 10 5 during ne-tuning
and LR2 encodes a learning rate of 10 5 during the warm-up phase and 10 6
during ne-tuning in Section 5). All submissions described in this paper used the
Adam optimizer ( 1 = 0:9, 2 = 0:999, = 10 7) [
            <xref ref-type="bibr" rid="ref15">15</xref>
            ] to minimize categorical
cross entropy. The implementation of the classi cation model work ow used an
E cientNets 1.1.0 implementation of Tensor ow Keras 2.2.4 [
            <xref ref-type="bibr" rid="ref7">7</xref>
            ].
          </p>
          <p>Since the dataset of the challenge had very unbalanced class frequencies,
different class weight functions were used in order to implement an oversampling.
Equation 1 describes a linear class weight function (W1 in Section 5) and
Equation 2 describes a function where very low frequencies were less oversampled
(W2 in Section 5). For both equations, F (c) denotes the frequency of class c.
For comparability reasons, one model has been trained without class weights.
w1(c) =
max F (c)</p>
          <p>F (c)
w2(c) = 1</p>
          <p>1
q max F (c) + 0:5</p>
          <p>
            F (c)
Polyak Averaging Polyak averaging, based on the approach of Polyak [
            <xref ref-type="bibr" rid="ref21">21</xref>
            ] and
Ruppert [24], is a method to combine the learned weights of di erent epochs
during the model training in order to obtain a nal model with more robust
weights. In this paper, it has been tested if Polyak averaging leads to improved
classi cation results (P1 denotes the described Polyak averaging in Section 5).
Therefore the learned weights of the last ve epochs were averaged using an
exponential function described in Equation 3, where i has a value of 1 for the
last epoch, 2 for the penultimate epoch and 5 for the fth last epoch.
          </p>
          <p>Wpolyak(i) = exp
i
2
(1)
(2)
(3)
4.5</p>
        </sec>
        <sec id="sec-4-2-4">
          <title>Addition Of Location Information</title>
          <p>Optionally, location information was added to some models by multiplying the
prediction probabilities of the classi cation model by the a priori probability of
the snake class for the speci ed location (M1 denotes the multiplication of the
locational distribution). The a priori probabilities were estimated by the relative
frequency distribution of the snake species at the location in the training and
validation dataset. Usually, the country information was used in this step, only
if this information was missing, the distribution of the continent has been used
instead. For some images, both country and continent information were missing.
In those cases, the frequency distribution of the entire dataset has been used.
The softmax function was applied after this multiplication, to normalize the
results.</p>
          <p>Another variant has been implemented based on the previously described
procedure. The sole exception has been that the raw prediction probabilities
of images with missing country and continent information were not multiplied
(abbreviated as M2 in Section 5). As a second variation of this method, all
prediction probabilities were multiplied by a binary variant of the frequency
distribution, which thus denotes if a snake was or was not present at a location
(M3 in Section 5). The results achieved using the di erent metadata integration
strategies are summarizes in Table 8.</p>
          <p>During the experiments of the FHDO BCSG a few alternatives have been
investigated. These methods were only tested in small experiments and are not
described in this paper for reasons of clarity.</p>
          <p>
            Image Branding As an alternative to the simple multiplication of the
location distributions, an approach has been implemented, which directly adds the
location information into the classi cation network. This has been done using a
binary image branding technique introduced in Pelka et al. [
            <xref ref-type="bibr" rid="ref19">19</xref>
            ], which adds grey
(RGB = [102,102,102]) boxes encoding the location information directly to the
images. The height of the boxes was set to 8 pixels while the width (bw) depends
on the image dimensions d and is described in Equation 4.
          </p>
          <p>bw =
d
8
4
(4)
The rst box starts directly at the left border of the image and after every box,
space was left for 4 pixels.</p>
          <p>The continent information has been added as binary boxes at the top border
of the image, while the country information has been added at the bottom border
of the image. Since a distinction has been made between seven continents as well
as the \unknown"-class, every box at the top of the image represents a continent
(abbreviated as M4). A similar approach to encode the country information
would result in small boxes because 189 countries had to be distinguished. The
used image branding approach is illustrated in Figure 3. In this case, a binary
encoding of the country index has been chosen, so that eight boxes could be used
to represent 28 = 256 di erent countries. Hereafter, the combined branding of
continent and country information is abbreviated as M5.
5</p>
        </sec>
      </sec>
    </sec>
    <sec id="sec-5">
      <title>Results</title>
      <p>In this section, the classi cation results for the test dataset of the challenge are
described. Table 2 summarizes the most relevant successful submissions of the
FHDO BCSG for the SnakeCLEF challenge.</p>
      <p>This table is mainly used to give an overview about the submitted models.
In order to get a better insight into the partial results and the e ects of the
di erent methods used, partial aspects are considered in individual tables in the
further course of this section. It was possible to submit models to the AICrowd
Snake Species Identi cation Challenge after the submission deadline of
SnakeCLEF expired. Therefore Table 2 presents a few models which achieve better
Original image</p>
      <p>Branding lookup tables</p>
      <p>Branded image
Continent</p>
      <p>Africa
Continent encoding: …</p>
      <p>Index Encoding
0 10000000
… …
South America 6 00000010</p>
      <p>Unknown 7 00000001</p>
      <sec id="sec-5-1">
        <title>Country encoding</title>
        <p>Country
Afghanistan
…
Brazil
…
Zimbabwe
results than the best submission in the SnakeCLEF challenge. In order to avoid
miscommunication, the submissions in Table 2 are listed in chronological order
and the deadline of the challenge is highlighted.</p>
        <p>The results of the di erent object detection datasets are summarized in Table
3. This table only presents the parameters, which are necessary for this
comparison. It should be noted, that all the other parameters of the compared models
are identical, as can be veri ed in Table 2. This type of presentation is also used
in subsequent tables.</p>
        <p>The comparison of submissions 68418 and 68450, as re ected in Table 3
shows, that the macro-averaging F1-score (abbreviated as F1 hereafter) increased
by 0.010 when additional images were annotated, whereas log loss remains stable.
Moreover, the number of images where no snakes were identi ed decreases from
141 to 123 in the joined training and validation dataset.</p>
        <p>Additionally, submission 68678 is a model, which was trained using no object
detection. This model is not completely comparable to any other models, but
submission 68632 di ers only in the image pre-processing step. Comparing those
two models, shows a slightly better performance of the model which used the
object detection. As previously mentioned, this comparison is not entirely fair.</p>
        <p>Table 4 summarizes the results of models trained based on di erent
preprocessing methods. As can be seen, pre-processing in uenced the classi cation
results achieved for the test dataset. Remarkable was the good performance of
the submission 68506 which used image resizing without consideration of the
aspect ratio. This model achieved the best F1 of 0.452. It has been expected,
that this image pre-processing would achieve bad results as major distortions
were possible, so in some cases humans were not able to recognize snakes in
those images.</p>
        <p>
          The second-best result was achieved for submission 67962. In this submission,
the ROIs detected during object detection have been expanded and thus were
padded using background information. The submission reached an F1 of 0.403
Abbreviations: O1: Object detection dataset from [
          <xref ref-type="bibr" rid="ref9">9</xref>
          ], O2: Expanded dataset, I1:
No aspect ratio, up-scaling, no padding, I2: Aspect ratio, up-scaling, monochrome
padding, I3: Aspect ratio, no up-scaling, monochrome padding, I4: Aspect ratio, no
up-scaling, background padding, Sx: Image size: x x pixels, Bx: E cientNet-Bx, BSx:
Batch size of x for image classi cation, BSx/y: Batch size: warm-up-phase: x,
netuning: y, W1: Linear weights, W2: Nonlinear weights, LR1: Learning rate warm-up
phase: 10 4, ne-tuning: 10 5, LR2: Warm-up phase: 10 5, ne-tuning: 10 6, Nx+y:
Training epochs warm-up phase: x, ne-tuning: y, P1: Polyak averaging, D1: Reduced
dataset, T1: Training dataset, T2: Training + test dataset, M1: Multiplication of
metadata, M2: Multiplication without unknown cases, M3: Binary multiplication,
M4: Continent branding, M5: Continent and country branding
and thus outperformed the F1 of submission 68432, which used a monochrome
color padding strategy, by 0.034. The comparison between submission 68432
and submission 67727 shows a slightly positive e ect of using upscaling, as the
F1 of submission 67727 is 0.020 higher than the F1 of submission 68432. The
previously described comparison is based on small images containing 128 128
pixels, for future investigations, it would be interesting how the pre-processing
methods a ect larger images.
        </p>
        <p>ID Pre-processing pipeline F1 Log loss
68506 No aspect ratio, up-scaling, without padding (I1) 0.452 6.648
67727 Aspect ratio, up-scaling, monochrome padding (I2) 0.389 6.650
68432 Aspect ratio, no up-scaling, monochrome padding (I3) 0.369 6.650
67962 Aspect ratio, no up-scaling, background padding (I4) 0.403 6.650
Table 5 summarizes the o cial classi cation results achieved using di erent
image sizes as model input. The results of the comparison corresponds to other
experiments executed during the challenge and shows that models trained on
larger image input sizes achieved better classi cation results. Increasing the
image size from 128 128 to 196 196 boosted the F1 by approximately 0.080. The
used image sizes may look striking, because E cientNet-B0 models are usually
trained using images including 224 224 pixels and E cientNet-B4 models are
optimized for an image size of 380 380 pixels. The use of small images in this
approach resulted from the fact that some early submissions failed because of
memory issues. The problem has been xed after the deadline of the SnakeCLEF
challenge expired. Some of the later submissions used larger image sizes
consistent to the original E cientNets input sizes and thus achieved better results.</p>
        <p>Next, the in uence of di erent model architectures on the classi cation
results were investigated. In Table 6, a comparison is presented concerning di erent
model architectures. The comparison shows, concurrently to some experiments
not listed here for reasons of clarity, increased F1 for upscaled models.
Submission 67727, which was based on an E cientNet-B2 architecture outperformed
submission 67700 by an increase of the macro averaging F1-score of 0.037.
Submission 68541, which represents an E cientNet-B4 architecture, achieved an F1
of 0.426 and thus outperformed the results of submissions 67727 and 67700 by
0.037 and 0.074. It should be noted that all of the submissions summarized in
Table 6 were trained using an image size of 128 128 pixels which is due to some
memory issues already mentioned before.</p>
        <p>Some additional experiments were performed comparing di erent top layer
architectures, for lack of time those were not completely comparable to each
other, especially because the number of epochs used for training di ered for
most of the models. For this reason these results are not elaborated in this
paper.</p>
        <p>ID Model architecture F1 Log loss
67700 E cientNet-B0 (B0) 0.352 6.651
67727 E cientNet-B2 (B2) 0.389 6.650
68541 E cientNet-B4 (B4) 0.426 6.648</p>
        <p>It has been mentioned in Section 4 that di erent weight functions can be used
to overcome unbalanced class distributions. The results of submissions 67727,
67882 and 67901 show, that the function introduced in Equation 2, which has
been used in submission 67727 achieved a macro averaging F1-score of 0.389
and thus outperformed submission 67882, which used a linear class weight
function and achieved an F1 of 0.365 and submission 67901, which used no class
weights and achieved a macro averaging F1-score of 0.377. One possible reason
for the poor results of the linear weighting could be the high di erences in class
frequencies, which lead to larger weights for rare classes.</p>
        <p>The results of the dataset ltering strategies, which are presented in
Table 7, have been inconclusive. For the work ow used in submissions 67675 and
67700, the model trained on the reduced dataset performed worse than the model
trained on the complete dataset. The opposite behaviour has been observed for
the work ow used in submissions 67696 and 67727, which achieved F1 of 0.392
and 0.389. As the ltering removed images from the training dataset, where no
snakes were present and no clear bene t is reached using this ltering, one could
assume, that there might be some images in the test dataset where no snakes
are present.</p>
        <p>ID Strategy for dataset ltering F1 Log loss
67675 Duplicates and plausibility ltering (D1) 0.338 6.652
67700 No ltering (-) 0.352 6.651
67696 Duplicates and plausibility ltering (D1) 0.392
67727 No ltering (-) 0.389</p>
        <p>As can be noted in Table 2, most of the earlier submissions, achieved high
log losses of about 6.6, while others had log losses of about 1 with a confusing
dependency to the achieved macro averaging F1-scores. This problem appeared
because of softened prediction results if softmax normalization is used after the
multiplication of the location frequencies and has been xed using
maximumnormalization instead (e.g., submissions 68520, 68574, 68575 and 68655).</p>
        <p>The results of adding metadata to improve image classi cation are described
in Table 8. It can be seen, that adding metadata to a model by multiplying
the model results using the a priori probability of the snake class for the given
location lead to an increased F1. Submission 68574, which was trained using the
same work ow as submission 68520, except adding metadata, outperforms this
model by an increase of F1 of 0.109. As can be seen looking at submission 68575
those results can be further improved by a value of 0.016, if the multiplication
is only used for available country and continent information. Submission 68655
exhibited a similar result of 0.445 using binary information about the availability
of a species in a country or continent.</p>
        <p>The results of the submissions 69365, 69750, 69768 and 69849 show that
models which used the image branding presented in Section 4, achieved no bene t
in comparison to multiplying the raw predictions by the location information.
It can be noted, that submission 69750, which combined country and continent
branding achieved a poor F1 of 0.361, whereas the model, which used only
continent branding (submission 69768), achieved a better F1 of 0.437. One possible
reason for this might be the use of the complex positional encoding of the country
information which is hard to learn for a CNN, which focuses more on local di
erences. Because of the limited time, it was not possible to investigate this problem
more thoroughly, thus additional investigations should be under examination in
future work. The combination of continent branding and multiplication of the a
priori probability distribution in submission 69849 achieved similar results than
the model, which used no branding, but the multiplication.
69365 Multiplication (M1) 0.460
69750 Branding continent and country (M5) 0.361
69768 Branding continent (M4) 0.437
69849 Branding continent and multiplication (M4+M1) 0.459</p>
        <p>The best model submitted before the SnakeCLEF deadline expired was
submission 68023 which was based on an E cientNet-B4 model architecture and
achieved an F1 of 0.404. Due to the previously mentioned memory issues, this
model used an unusual small image size of 128 128 pixels. The newly added
layers of the described model were trained with a warm-up phase of ten epochs and
another 50 epochs were used to ne-tune the entire model. During the training
process a batch size of 64, a learning rate of 0.0001 and Adam optimizer have
been used. Location information was added using the described multiplication
procedure. Polyak averaging with exponential weights has been used to combine
the results of the last ve training epochs. The Polyak averaging achieved an
improvement of F1 of 0.001 in comparison to submission 67734 which used no
Polyak averaging.</p>
        <p>The best submission after the SnakeCLEF deadline expired was submission
69888 which achieved a macro-averaging F1-score of 0.594 and a log loss of 1.064.
The main di erences in comparison to the best model before the deadline
expired were, that the model was trained using the prede ned image dimensions
of an E cientNet-B4, which are 380 380 pixels. Due to the increased image
size, a smaller batch size of 13 and a decreased learning rate has been used.
Furthermore, 109 instead of 50 training epochs have been applied, and the location
distribution was multiplied only for known countries and continents. The model
included no Polyak averaging.
6</p>
      </sec>
    </sec>
    <sec id="sec-6">
      <title>Conclusions</title>
      <p>In conclusion, it can be stated that snake species identi cation is a challenging
task, primarily because of the high diversity of snake species, high intra-class
variance, and low inter-class variance.</p>
      <p>The main improvements in snake species classi cation presented in this paper
are based on increasing image size, combining location and image information as
well as upscaled model architecture. The results presented in this article show
improved classi cation results using an object detection strategy previously to
the image classi cation. However, a plausibility ltering of the training dataset
showed no clear improvement. Some di erences were detected in dependence
of the pre-processing steps. Nevertheless, no clear insights could be achieved
about which steps are particularly promising for good classi cation results. The
implementation and application of the di erent pre-processing steps turned out
to be relatively time-consuming. Besides, it has been previously mentioned, that
there were some memory issues which lead to a focus on small image sizes as
well as less upscaled model architectures in the early course of the challenge.
Thus the time needed to optimize the classi cation parameters more precisely
and to try out di erent optimizers was reduced. It is expected, that the results
may be further improved by adjusting those parameters.</p>
      <p>Due to the use of AICrowd as a submission platform, it has been possible
to test a large number of di erent models. This enables to get direct feedback
about the performance on the test dataset, and thus gives a good estimate about
which methods gets the most promising results. In addition it facilitates the
comparison between teams before the deadline expires. In this article, it has been
mentioned before, that there were some memory issues which were related to the
architecture of the test environment. In some cases debugging has been
complicated because the logs were not accessible. These concerns were compensated by
the very prompt and useful help from the organizers.</p>
    </sec>
    <sec id="sec-7">
      <title>Acknowledgment</title>
      <p>The work of Louise Bloch and Obioma Pelka was partially funded by a PhD
grant from University of Applied Sciences and Arts Dortmund, Germany.</p>
      <p>The authors want to thank Raphael Brungel for the constructive proofreading
of the manuscript.
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