This chapter explains how to use object detection based on deep learning.
With object detection we want to find the different instances in an image and assign them to a class. The instances can partially overlap and still be distinguished as distinct. This is illustrated in the following schema.
Object detection leads to two different tasks: Finding the instances and classifying them. In order to do so, we use a combined network consisting of three main parts. The first part, called backbone, consists of a pretrained classification network. Its task is to generate various feature maps, so the classifying layer is removed. These feature maps encode different kinds of information at different scales, depending how deep they are in the network. See also the chapter Deep Learning. Thereby, feature maps with the same width and height are said to belong to the same level. In the second part, we take different feature maps providing features of different levels and combine them. As a result we obtain feature maps containing information of lower and higher levels. These are the feature maps we will use in the third part. This second part is also called feature pyramid and together with the first part it constitutes the feature pyramid network. The third part consists of additional networks, called heads, for every selected level. They get the corresponding feature maps as input and learn how to localize and classify, respectively, potential objects. Additionally this third part includes the reduction of overlapping predicted bounding boxes. An overview of the three parts is shown in the following figure.
Let us have a look what happens in this third part.
In object detection, the location in the image of an instance is given by a
rectangular bounding box.
Hence, the first task is to find a suiting bounding box for every single
instance.
To do so, the network generates reference bounding boxes and learns, how to
modify them to fit the instances best possible.
These reference bounding boxes are called anchors.
The better these anchors represent the shape of the different ground truth
bounding boxes, the easier the network can learn them.
For this purpose the network generates a set of anchors
on every anchor point, thus on every pixel of the used feature maps of the
feature pyramid.
Such a set consists of anchors of all combinations of shapes, sizes, and
for instance type 'rectangle2' (see below) also orientations.
The shape of those boxes is affected by the parameter
'anchor_aspect_ratios' the size by the parameter
'anchor_num_subscales', and the orientation by the parameter
'anchor_angles', see the illustration below and
get_dl_model_param
'anchor_num_subscales'.
(3) Anchors of different shapes are created setting
'anchor_aspect_ratios'.
(4) Anchors of different orientations are created setting
'anchor_angles' (only for instance type
'rectangle2').
The network predicts offsets how to modify the anchors in order to obtain bounding boxes fitting the potential instances better. The network learns this with its bounding box heads, which compare the anchors generated for their level with the corresponding ground truth bounding boxes, thus the information where in the image the single instances are. An illustration is shown in the figure below.
| (1) | (2) |
'rectangle1'.
(1) The network modifies the anchor (light blue) in order
to predict a better fitting bounding box (orange).
(2) During training, the predicted bounding box (orange) gets compared
with the most overlapping ground truth bounding box (blue) so
the network can learn the necessary modifications.
As mentioned before, feature maps of different levels are used.
Depending on the size your instances have in comparison to the total image
it is beneficial to include early feature maps (where the feature map is not
very compressed and therefore small features are still visible) and deeper
feature maps (where the feature map is very compressed and only large
features are visible) or not.
This can be controlled by the parameters 'min_level' and
'max_level', which determine the levels of the feature pyramid.
With these bounding boxes we have the localization of a potential instance,
but the instance is not classified yet.
Hence, the second task consists of classifying the content of the image
part within the bounding boxes. This is done by the class heads.
For more information about classification
in general, see the chapter Deep Learning / Classification and the
“Solution Guide on Classification”.
Most probably the network will find several promising bounding boxes for a
single object. The reduction of overlapping predicted bounding boxes is
done by non-maximum suppression, set over the parameters
'max_overlap' and 'max_overlap_class_agnostic' when
creating the model or using set_dl_model_param
| (1) | (2) | (3) |
'rectangle1'.
(1) The network finds several promising instances for the class apple
(orange) and lemon (blue).
(2) The suppression of overlapping instances assigned to the same class
is set by 'max_overlap'.
Overlapping instances of different classes are not suppressed.
(3) Using the parameter 'max_overlap_class_agnostic', also
strongly overlapping instances of different classes get
suppressed.
As output you get bounding boxes proposing possible localizations of objects and confidence values, expressing the affinity of this image part to one of the classes.
In HALCON, object detection with deep learning is implemented within the more general deep learning model. For more information to the latter one, see the chapter Deep Learning / Model. Two different instance types of object detection models are implemented, differing in the orientation of the bounding boxes:
'rectangle1'The rectangular bounding boxes are axis-aligned.
'rectangle2'The rectangular bounding boxes have an arbitrary orientation.
The following sections are introductions to the general workflow needed for object detection, information related to the involved data and parameters, and explanations to the evaluation measures.
In this paragraph, we describe the general workflow for an object
detection task based on deep learning.
Thereby we assume, your dataset is already labeled, see also the section
“Data” below.
Have a look at the HDevelop example series
detect_pills_deep_learning for an application.
Note, this example is split into the four parts 'Preparation',
'Training', 'Evaluation', and 'Inference', which give guidance on
possible implementations.
This part covers the creation of a DL object detection model and the
adaptation the data for this model.
The single steps are also shown in the HDevelop example
detect_pills_deep_learning_1_prepare.hdev.
Create a model using the operator
Thereby you will have to specify at least the backbone and the number
of classes to be distinguished. Further parameters can be set over
the dictionary DLModelDetectionParam'DLModelHandle'.
Alternatively you can also use
to read in a model you have already saved with write_dl_model
The information what is to be found on which image of your training dataset needs to be read in. This can be done reading the data out of
a DLDatasetread_dict
a file in the COCO data format using the procedure
read_dl_dataset_from_coco,
whereby a dictionary DLDataset
The dictionary DLDataset
Split the dataset represented by the dictionary
DLDataset
split_dl_dataset.
The resulting split will be saved over the key splitDLDataset
The network imposes requirements on the images, as e.g., the image width and height. You can retrieve every single value using the operator
or you can retrieve all necessary parameter using the procedure
create_dl_preprocess_param_from_model.
Now you can preprocess your dataset. For this, you can use the procedure
preprocess_dl_dataset.
In case of custom preprocessing, this procedure offers guidance on the implementation. We recommend to preprocess and store all images used for the training before starting the training, since this speeds up the training significantly.
To visualize the preprocessed data, the procedure
dev_display_dl_data
is available.
This part covers the training of a DL object detection model.
The single steps are also shown in the HDevelop example
detect_pills_deep_learning_2_train.hdev.
Set the training parameters and store them
in the dictionary TrainingParam
the hyperparameters, for an overview see the section “Model Parameters and Hyperparameters” below and the chapter Deep Learning.
parameters for possible data augmentation
Train the model. This can be done using the procedure
train_dl_model.
The procedure expects:
the model handle DLDetectionHandle
the dictionary with the data information DLDataset
the dictionary with the training parameter
TrainingParam
the information, over how many epochs the training shall run.
During the training you should see how the total loss minimizes.
In this part we evaluate the object detection model.
The single steps are also shown in the HDevelop example
detect_pills_deep_learning_3_evaluate.hdev.
Set the model parameters which may influence the evaluation.
The evaluation can conveniently be done using the procedure
evaluate_dl_model.
This procedure expects a dictionary
GenParamEvaldetailed_evaluation'true' to
get the data necessary for the visualization.
You can visualize your evaluation results using the procedure
dev_display_detection_detailed_evaluation.
This part covers the application of a DL object detection model.
The single steps are also shown in the HDevelop example
detect_pills_deep_learning_4_infer.hdev.
Request the requirements the network imposes on the images using the operator
or the procedure
create_dl_preprocess_param_from_model.
Set the model parameter described in the section “Model Parameters and Hyperparameters” below, using the operator
The batch_sizeapply_dl_model
Generate a data dictionary DLSample
gen_dl_samples_from_images.
Every image has to be preprocessed as done for the training. For this, you can use the procedure
preprocess_dl_samples.
Apply the model using the operator
Retrieve the results from the dictionary
'DLResultBatch'.
We distinguish between data used for training and evaluation, consisting of images with their information about the instances, and data for inference, which are bare images. For the first ones, you provide the information defining for each instance to which class it belongs and where it is in the image (via its bounding box).
As a basic concept, the model handles data over dictionaries, meaning it
receives the input data over a dictionary DLSampleDLResultDLTrainResult
The dataset consists of images and corresponding information. They have to be provided in a way the model can process them. Concerning the image requirements, find more information in the section “Images” below.
The training data is used to train and evaluate a network for your
specific task. With the aid of this data the network can learn which
classes are to be distinguished, how such examples look like, and how to
find them.
The necessary information is provided by telling for each object in
every image to which class this object belongs to and where it is
located. This is done by providing a class label and an enclosing
bounding box for every object.
There are different ways possible, how to store and retrieve this
information.
How the data has to be formated in HALCON for a DL model is explained
in the chapter Deep Learning / Model.
In short, a dictionary DLDatasetDLDatasetcreate_dl_dataset_from_coco
(for 'instance_type' = 'rectangle1' only).
It formats the data and creates a dictionary DLDataset
You also want enough training data to split it into three subsets, used for training, validation and testing the network. These subsets are preferably independent and identically distributed, see the section “Data” in the chapter Deep Learning.
Note, that in object detection the network has to learn how to find possible locations and sizes of the instances. That is why also the later important instance locations and sizes need to appear representatively in your training dataset.
Regardless of the application, the network poses requirements on the
images regarding e.g., the image dimensions.
The specific values depend on the network itself and can be queried with
get_dl_model_parampreprocess_dl_dataset and
in preprocess_dl_samples for a single sample, respectively.
In case of custom preprocessing these procedures offer guidance on the
implementation.
Depending on the instance type of object detection model, the bounding boxes are parametrized differently:
'rectangle1'
The bounding boxes are defined over the coordinates of upper left
corner ('bbox_row1', 'bbox_col1') and the lower right
corner ('bbox_row2', 'bbox_col2').
This is consistent with gen_rectangle1
'rectangle2'
The bounding boxes are defined over the coordinates of
their center ('bbox_row', 'bbox_col'), the
orientation 'Phi' and the half edge lengths
'bbox_length1' and 'bbox_length2'. The orientation
is given in arc measure and indicates the angle between the horizontal
axis and 'bbox_length1' (mathematically positive).
This is consistent with gen_rectangle2
'rectangle2' you are interested in the
oriented bounding box, but without considering the direction of the
object within the bounding box, the parameter
'ignore_direction' can be set to 'true'.
This is illustrated in the figure below.
| (1) | (2) | (3) |
'rectangle1'.
(2) Instance type 'rectangle2', where the bounding box is
oriented towards the banana end.
(3) Instance type 'rectangle2', where the oriented bounding
box is of interest without considering the direction of the
banana within the bounding box.
As training output, the operator train_dl_model_batchDLTrainResult
As inference and evaluation output, the operator apply_dl_modelDLResult'instance_type') and given in
pixel centered sub-pixel accurate coordinates.
For more information to the coordinate system, see the chapter
Transformations / 2D Transformations.
Further information on the output dictionary can be found in the chapter
Deep Learning / Model.
Next to the general DL hyperparameters explained in Deep Learning, there are further hyperparameters relevant for object detection:
'bbox_heads_weight'
'class_heads_weight'
'class_weights'
These hyper parameters are explained in more detail in
get_dl_model_paramcreate_dl_model_detection
For an object detection model, there are two different types of model parameters:
Parameter defining your architecture. They can not be
changed anymore once your model is created. These parameters are all
set using the operator create_dl_model_detection
Parameter influencing your predictions and as a consequence the evaluation results. Those only relevant for object detection are
'class_ids_no_orientation'
(only applicable for 'instance_type' =
'rectangle2')
'max_num_detections'
'max_overlap'
'max_overlap_class_agnostic'
'min_confidence'
They are explained in more detail in get_dl_model_paramcreate_dl_model_detectionset_dl_model_param
For object detection, the following evaluation measures are supported in HALCON. Note that for computing such a measure for an image, the related ground truth information is needed.
Mean average precision, mAPap_iou_classname
The AP value is an average of maximum precisions at different recall values. In simple words it tells us, if the objects predicted for this class are generally correct detections or not. Thereby we pay more attention to the predictions with high confidence values. The higher the value, the better.
To count a prediction as a hit, we want both correct, its top-1 classification and its localization. The measure, telling us the correctness of the localization is the intersection over union, IoU: an instance is localized correctly if the IoU is higher than the demanded threshold. The IoU is explained in more detail below. For this reason, the AP value depends on the class and on the IoU threshold.
You can obtain the specific AP values, the averages over the classes, the averages over the IoU thresholds, and the average over both, the classes and the IoU thresholds. The latter one is the mean average precision, mAP, a measure to tell us how well instances are found and classified.
True Positives, False Positives, False Negatives
The concept of true positive, false positives, and false negatives is explained in Deep Learning. It applies for object detection with the exception that there are different kinds of false positives, as e.g.:
An instance got classified wrongly.
An instance was found where there is only background.
An instance was localized badly, meaning the IoU between the instance and its ground truth is lower than the evaluation IoU threshold.
There is a duplicate, thus at least two instances overlap mainly
with the same ground truth bounding box, but they overlap not more
than 'max_overlap' with each other, so none of them got
suppressed.
Note, these values are only available from the detailed evaluation.
This means, in evaluate_dl_model the parameter
detailed_evaluation'true'.
Score of Angle Precision (SoAP)
The SoAP value is a score for the precision of the inferred
orientation angles. This score is determined by the angle differences
between the inferred instances (I) and the corresponding ground
truth annotations (GT):
where the index runs over all inferred instances.
This score only applies for detection models of 'instance_type'
'rectangle2'.
| (1) | (2) |
'rectangle1'.
(1) The input image with the ground truth bounding box (orange)
and the predicted bounding box (light blue).
(2) The IoU is the ratio between the area intersection and the
area overlap.
create_dl_model_detection