% Copyright (C) 2014-2020 by Thomas Auzinger <thomas@auzinger.name>

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\newacronym{xai}{XAI}{Explainable Artificial Intelligence}
\newacronym{grad-cam}{Grad-CAM}{Gradient-weighted Class Activation Mapping}
\newacronym{cam}{CAM}{Class Activation Mapping}
\newacronym{oid}{OID}{Open Images Dataset}
\newacronym{ap}{AP}{Average Precision}
\newacronym{iou}{IOU}{Intersection over Union}
\newacronym{map}{mAP}{mean average precision}
\newacronym{resnet}{ResNet}{Residual Neural Network}
\newacronym{cnn}{CNN}{Convolutional Neural Network}

\begin{document}

\frontmatter % Switches to roman numbering.
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\begin{danksagung*}
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\begin{acknowledgements*}
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\begin{kurzfassung}
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\begin{abstract}
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% \chapter{Introduction}
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\chapter{Evaluation}

The following sections contain a detailed evaluation of the model in
various scenarios. First, we present metrics from the training phases
of the constituent models. Second, we employ methods from the field of
\gls{xai} such as \gls{grad-cam} to get a better understanding of the
models' abstractions. Finally, we turn to the models' aggregate
performance on the test set and discuss whether the initial goals set
by the problem description have been met or not.

\section{Object Detection}
\label{sec:yolo-eval}

The object detection model was pre-trained on the COCO~\cite{lin2015}
dataset and fine-tuned with data from the \gls{oid}
\cite{kuznetsova2020} in its sixth version. Since the full \gls{oid}
dataset contains considerably more classes and samples than would be
feasibly trainable on a small cluster of GPUs, only images from the
two classes \emph{Plant} and \emph{Houseplant} have been
downloaded. The samples from the Houseplant class are merged into the
Plant class because the distinction between the two is not necessary
for our model. Furthermore, the \gls{oid} contains not only bounding
box annotations for object detection tasks, but also instance
segmentations, classification labels and more. These are not needed
for our purposes and are omitted as well. In total, the dataset
consists of 91479 images with a roughly 85/5/10 split for training,
validation and testing, respectively.

\subsection{Training Phase}
\label{ssec:yolo-training-phase}

The object detection model was trained for 300 epochs on 79204 images
with 284130 ground truth labels. The weights from the best-performing
epoch were saved. The model's fitness for each epoch is calculated as
the weighted average of \textsf{mAP}@0.5 and \textsf{mAP}@0.5:0.95:

\begin{equation}
  \label{eq:fitness}
  f_{epoch} = 0.1 \cdot \mathsf{mAP}@0.5 + 0.9 \cdot \mathsf{mAP}@0.5\mathrm{:}0.95
\end{equation}

Figure~\ref{fig:fitness} shows the model's fitness over the training
period of 300 epochs. The gray vertical line indicates the maximum
fitness of 0.61 at epoch 133. The weights of that epoch were frozen to
be the final model parameters. Since the fitness metric assigns the
\textsf{mAP} at the higher range the overwhelming weight, the
\textsf{mAP}@0.5 starts to decrease after epoch 30, but the
\textsf{mAP}@0.5:0.95 picks up the slack until the maximum fitness at
epoch 133. This is an indication that the model achieves good
performance early on and continues to gain higher confidence values
until performance deteriorates due to overfitting.

\begin{figure}
  \centering
  \includegraphics{graphics/model_fitness.pdf}
  \caption[Object detection fitness per epoch.]{Object detection model
    fitness for each epoch calculated as in
    equation~\ref{eq:fitness}. The vertical gray line at 133 marks the
    epoch with the highest fitness.}
  \label{fig:fitness}
\end{figure}

Overall precision and recall per epoch are shown in
figure~\ref{fig:prec-rec}. The values indicate that neither precision
nor recall change materially during training. In fact, precision
starts to decrease from the beginning, while recall experiences a
barely noticeable increase. Taken together with the box and object
loss from figure~\ref{fig:box-obj-loss}, we speculate that the
pre-trained model already generalizes well to plant detection because
one of the categories in the COCO~\cite{lin2015} dataset is
\emph{potted plant}. Any further training solely impacts the
confidence of detection, but does not lead to higher detection
rates. This conclusion is supported by the increasing
\textsf{mAP}@0.5:0.95 until epoch 133.

\begin{figure}
  \centering
  \includegraphics{graphics/precision_recall.pdf}
  \caption[Object detection precision and recall during
  training.]{Overall precision and recall during training for each
    epoch. The vertical gray line at 133 marks the epoch with the
    highest fitness.}
  \label{fig:prec-rec}
\end{figure}

Further culprits for the flat precision and recall values may be found
in bad ground truth data. The labels from the \gls{oid} are sometimes not
fine-grained enough. Images which contain multiple individual—often
overlapping—plants are labeled with one large bounding box instead of
multiple smaller ones. The model recognizes the individual plants and
returns tighter bounding boxes even if that is not what is specified
in the ground truth. Therefore, it is prudent to limit the training
phase to relatively few epochs in order to not penalize the more
accurate detections of the model. The smaller bounding boxes make more
sense considering the fact that the cutout is passed to the classifier
in a later stage. Smaller bounding boxes help the classifier to only
focus on one plant at a time and to not get distracted by multiple
plants in potentially different stages of wilting.

The box loss decreases slightly during training which indicates that
the bounding boxes become tighter around objects of interest. With
increasing training time, however, the object loss increases,
indicating that less and less plants are present in the predicted
bounding boxes. It is likely that overfitting is a cause for the
increasing object loss from epoch 40 onward. Since the best weights as
measured by fitness are found at epoch 133 and the object loss
accelerates from that point, epoch 133 is probably the correct cutoff
before overfitting occurs.

\begin{figure}
  \centering
  \includegraphics{graphics/val_box_obj_loss.pdf}
  \caption[Object detection box and object loss.]{Box and object loss
    measured against the validation set of 3091 images and 4092 ground
    truth labels. The class loss is omitted because there is only one
    class in the dataset and the loss is therefore always zero.}
  \label{fig:box-obj-loss}
\end{figure}

\subsection{Test Phase}
\label{ssec:yolo-test-phase}

Of the 91479 images around 10\% were used for the test phase. These
images contain a total of 12238 ground truth
labels. Table~\ref{tab:yolo-metrics} shows precision, recall and the
harmonic mean of both (F1-score). The results indicate that the model
errs on the side of sensitivity because recall is higher than
precision. Although some detections are not labeled as plants in the
dataset, if there is a labeled plant in the ground truth data, the
chance is high that it will be detected. This behavior is in line with
how the model's detections are handled in practice. The detections are
drawn on the original image and the user is able to check the bounding
boxes visually. If there are wrong detections, the user can ignore
them and focus on the relevant ones instead. A higher recall will thus
serve the user's needs better than a high precision.

\begin{table}[h]
  \centering
  \begin{tabular}{lrrrr}
    \toprule
    {} &  Precision &    Recall &  F1-score &  Support \\
    \midrule
    Plant        &   0.547571 &  0.737866 &  0.628633 &  12238.0 \\
    \bottomrule
  \end{tabular}
  \caption{Precision, recall and F1-score for the object detection model.}
  \label{tab:yolo-metrics}
\end{table}

Figure~\ref{fig:yolo-ap} shows the \gls{ap} for the \gls{iou}
thresholds of 0.5 and 0.95. Predicted bounding boxes with an \gls{iou}
of less than 0.5 are not taken into account for the precision and
recall values of table~\ref{tab:yolo-metrics}. The lower the detection
threshold, the more plants are detected. Conversely, a higher
detection threshold leaves potential plants undetected. The
precision-recall curves confirm this behavior because the area under
the curve for the threshold of 0.5 is higher than for the threshold of
0.95 ($0.66$ versus $0.41$). These values are combined in COCO's
\cite{lin2015} main evaluation metric which is the \gls{ap} averaged
across the \gls{iou} thresholds from 0.5 to 0.95 in 0.05 steps. This
value is then averaged across all classes and called \gls{map}. The
object detection model achieves a state-of-the-art \gls{map} of 0.5727
for the \emph{Plant} class.

\begin{figure}
  \centering
  \includegraphics{graphics/APpt5-pt95.pdf}
  \caption[Object detection AP@0.5 and AP@0.95.]{Precision-recall
    curves for \gls{iou} thresholds of 0.5 and 0.95. The \gls{ap} of a
    specific threshold is defined as the area under the
    precision-recall curve of that threshold. The \gls{map} across
    \gls{iou} thresholds from 0.5 to 0.95 in 0.05 steps
    \textsf{mAP}@0.5:0.95 is 0.5727.}
  \label{fig:yolo-ap}
\end{figure}

\section{Classification}
\label{sec:resnet-eval}

The classifier receives cutouts from the object detection model and
determines whether the image shows a stressed plant or not. To achieve
this goal, we trained a \gls{resnet} \cite{he2016} on a dataset of 452
images of healthy and 452 stressed plants. We chose the \gls{resnet}
architecture due to its popularity and ease of implementation as well
as its consistently high performance on various classification
tasks. While its classification speed in comparison with networks
optimized for mobile and edge devices (e.g. MobileNet) is
significantly lower, the deeper structure and the additional
parameters are necessary for the fairly complex task at
hand. Furthermore, the generous time budget for object detection
\emph{and} classification allows for more accurate results at the
expense of speed. The architecture allows for multiple different
structures, depending on the amount of layers. The smallest one has 18
and the largest 152 layers with 34, 50 and 101 in-between. The larger
networks have better accuracy in general, but come with trade-offs
regarding training and inference time as well as required space. The
50 layer architecture (\gls{resnet}50) is adequate for our use case.

\subsection{Training Phase}
\label{ssec:resnet-training-phase}

The dataset was split 85/15 into training and validation sets. The
images in the training set were augmented with a random crop to arrive
at the expected image dimensions of 224 pixels. Additionally, the
training images were modified with a random horizontal flip to
increase the variation in the set and to train a rotation invariant
classifier. All images, regardless of their membership in the training
or validation set, were normalized with the mean and standard
deviation of the ImageNet~\cite{deng2009} dataset, which the original
\gls{resnet} model was pre-trained with. Training was done for 50
epochs and the best-performing model as measured by validation
accuracy was selected as the final version.

Figure~\ref{fig:classifier-training-metrics} shows accuracy and loss
on the training and validation sets. There is a clear upwards trend
until epoch 20 when validation accuracy and loss stabilize at around
0.84 and 0.3, respectively. The quick convergence and resistance to
overfitting can be attributed to the model already having robust
feature extraction capabilities.

\begin{figure}
  \centering
  \includegraphics{graphics/classifier-metrics.pdf}
  \caption[Classifier accuracy and loss during training.]{Accuracy and
    loss during training of the classifier. The model converges
    quickly, but additional epochs do not cause validation loss to
    increase, which would indicate overfitting. The maximum validation
    accuracy of 0.9118 is achieved at epoch 27.}
  \label{fig:classifier-training-metrics}
\end{figure}

\subsection{Class Activation Maps}
\label{ssec:resnet-cam}

Neural networks are notorious for their black-box behavior, where it
is possible to observe the inputs and the corresponding outputs, but
the stage in-between stays hidden from view. Models are continuously
developed and deployed to aid in human decision-making and sometimes
supplant it. It is, therefore, crucial to obtain some amount of
interpretability of what the model does \emph{inside} to be able to
explain why a decision was made in a certain way. The research field
of \gls{xai} gained significance during the last few years because of
the development of new methods to peek inside these black boxes.

One such method, \gls{cam}~\cite{zhou2015}, is a popular tool to
produce visual explanations for decisions made by
\glspl{cnn}. Convolutional layers essentially function as object
detectors as long as no fully-connected layers perform the
classification. This ability to localize regions of interest which
play a significant role in the type of class the model predicts, can
be retained until the last layer and used to generate activation maps
for the predictions.

A more recent approach to generating a \gls{cam} via gradients is
proposed by~\textcite{selvaraju2020}. Their \gls{grad-cam} approach
works by computing the gradient of the feature maps of the last
convolutional layer with respect to the specified class. The last
layer is chosen because the authors find that ``[…]  Grad-CAM maps
become progressively worse as we move to earlier convolutional layers
as they have smaller receptive fields and only focus on less semantic
local features.''~\cite[p.5]{selvaraju2020}

Turning to our classifier, figure~\ref{fig:resnet-cam} shows the
\glspl{cam} for \emph{healthy} and \emph{stressed}. While the regions
of interest for the \emph{healthy} class lie on the healthy plant, the
\emph{stressed} plant is barely considered and mostly rendered as
background information (blue). Conversely, when asked to explain the
inputs to the \emph{stressed} classification, the regions of interest
predominantly stay on the thirsty as opposed to the healthy plant. In
fact, the large hanging leaves play a significant role in determining
the class the image belongs to. This is an additional data point
confirming that the model focuses on the \emph{right} parts of the
image during classification.

\begin{figure}
  \centering
  \includegraphics{graphics/classifier-cam.pdf}
  \caption[Classifier \glspl{cam}.]{The top left image shows the
    original image of the same plant in a stressed (left) and healthy
    (right) state. In the top right image, the \gls{cam} for the class
    \emph{healthy} is laid over the original image. The classifier
    draws its conclusion mainly from the healthy plant, which is
    indicated by the red hot spots around the tips of the plant. The
    bottom right image shows the \gls{cam} for the \emph{stressed}
    class. The classifier focuses on the hanging leaves of the thirsty
    plant. The image was classified as \emph{stressed} with a
    confidence of 70\%.}
  \label{fig:resnet-cam}
\end{figure}


\section{Aggregate Model}
\label{sec:aggregate-model}

In this section we turn to the evaluation of the aggregate model. We
have confirmed the performance of the constituent models: the object
detection and the classification model. It remains to evaluate the
complete pipeline from gathering detections of potential plants in an
image and forwarding them to the classifier to obtaining the results
as either healthy or stressed with their associated confidence scores.

The test set contains 640 images which were obtained from a google
search using the terms \emph{thirsty plant}, \emph{wilted plant} and
\emph{stressed plant}. Images which clearly show one or multiple
plants with some amount of visible stress were added to the
dataset. Care was taken to include plants with various degrees of
stress and in various locations and lighting conditions. The search
not only provided images of stressed plants, but also of healthy
plants due to articles, which describe how to care for plants, having
a banner image of healthy plants. The dataset is biased towards potted
plants which are commonly put on display in western
households. Furthermore, many plants, such as succulents, are sought
after for home environments because of their ease of maintenance. Due
to their inclusion in the dataset and how they exhibit water stress,
the test set nevertheless contains a wide variety of scenarios.

After collecting the images, the aggregate model was run on them to
obtain initial bounding boxes and classifications for ground truth
labeling. Letting the model do the work beforehand and then correcting
the labels allowed to include more images in the test set because they
could be labeled more easily. Additionally, going over the detections
and classifications provided a comprehensive view on how the models
work and what their weaknesses and strengths are. After the labels
have been corrected, the ground truth of the test set contains 662
bounding boxes of healthy plants and 488 of stressed plants.

\begin{table}
  \centering
  \begin{tabular}{lrrrr}
    \toprule
    {} &  Precision &  Recall &  F1-score &  Support \\
    \midrule
    Healthy      &      0.824 &   0.745 &     0.783 &    662.0 \\
    Stressed     &      0.707 &   0.783 &     0.743 &    488.0 \\
    micro avg    &      0.769 &   0.761 &     0.765 &   1150.0 \\
    macro avg    &      0.766 &   0.764 &     0.763 &   1150.0 \\
    weighted avg &      0.775 &   0.761 &     0.766 &   1150.0 \\
    \bottomrule
  \end{tabular}
  \caption{Precision, recall and F1-score for the aggregate model.}
  \label{tab:model-metrics}
\end{table}

Table~\ref{tab:model-metrics} shows precision, recall and the F1-score
for both classes \emph{Healthy} and \emph{Stressed}. Both precision
and recall are balanced and the F1-score is high. Unfortunately, these
values do not take the accuracy of bounding boxes into account and
thus have only limited expressive power.

Figure~\ref{fig:aggregate-ap} shows the precision and recall curves
for both classes at different \gls{iou} thresholds. The left plot
shows the \gls{ap} for each class at the threshold of 0.5 and the
right one at 0.95. The \gls{map} is 0.6226 and calculated across all
classes as the median of the \gls{iou} thresholds from 0.5 to 0.95 in
0.05 steps. The difference between \gls{map}@0.5 and \gls{map}@0.95 is
fairly small which indicates that the bounding boxes encapsulate the
objects of interest well. The cliffs at around 0.77 (left) and 0.7
(right) happen at a detection threshold of 0.5. The classifier's last
layer is a softmax layer which necessarily transforms the input into a
probability of showing either a healthy or stressed plant. If the
probability of an image showing a healthy plant is below 0.5, it is no
longer classified as healthy but as stressed. The threshold for
discriminating the two classes lies at the 0.5 value and is therefore
the cutoff for either class.

\begin{figure}
  \centering
  \includegraphics{graphics/APmodel.pdf}
  \caption[Aggregate model AP@0.5 and AP@0.95.]{Precision-recall
    curves for \gls{iou} thresholds of 0.5 and 0.95. The \gls{ap} of a
    specific threshold is defined as the area under the
    precision-recall curve of that threshold. The \gls{map} across
    \gls{iou} thresholds from 0.5 to 0.95 in 0.05 steps
    \textsf{mAP}@0.5:0.95 is 0.6226.}
  \label{fig:aggregate-ap}
\end{figure}

Overall, we believe that the aggregate model shows sufficient
predictive performance to be deployed in the field. The detections are
accurate, especially for potted plants, and the classification into
healthy and stressed is robust.

\backmatter

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