Welcome to AI View Depth camera repository

4. Opencv Application

4.1. Target Detection

The main problem solved in this section is how to use the dnn module in OpenCV to import a trained target detection network. However, there are requirements for the version of opencv.

Currently, there are three main methods for target detection using deep learning:

• Faster R-CNNs

• You Only Look Once (YOLO)

• Single Shot Detectors (SSDs)

Faster R-CNNs is the most commonly heard neural network based on deep learning. However, this method is technically difficult to understand (especially for newcomers to deep learning), difficult to implement, and difficult to train.

In addition, even if the "Faster" method is used to implement R-CNNs (here R stands for Region Proposal), the algorithm is still relatively slow, about 7FPS.

If we pursue speed, we can turn to YOLO, because it is very fast, reaching 40-90 FPS on TianXGPU, and the fastest version may reach 155 FPS. But the problem with YOLO is that its accuracy needs to be improved.

SSDs was originally developed by Google and can be said to be a balance between the above two. Compared with Faster R-CNNs, its algorithm is more direct. Compared with YOLO, it is more accurate.

4.1.1, Model Structure

The main work of MobileNet is to replace the past standard convolutions with depthwise sparable convolutions to solve the problems of computational efficiency and parameter quantity of convolutional networks. The MobileNets model is based on depthwise sparable convolutions, which can decompose the standard convolution into a depthwise convolution and a point convolution (1 × 1 convolution kernel). Deep convolution applies each convolution kernel to each channel, while 1 × 1 convolution is used to combine the output of channel convolution.

Batch Normalization (BN) is added to the basic components of MobileNet, that is, at each SGD (stochastic gradient descent), the standardization process is performed so that the result (each dimension of the output signal) has a mean of 0 and a variance of 1. Generally, when the convergence speed is very slow or the gradient explodes during neural network training, BN can be tried to solve the problem. In addition, BN can also be added to speed up training and improve model accuracy in general use.

In addition, the model also uses the ReLU activation function, so the basic structure of the depthwise separable convolution is as shown in the figure below:

The MobileNets network is composed of many depthwise separable convolutions shown in the figure above. The specific network structure is shown in the figure below:

4.1.2, Source code analysis

Source code location: ~/orbbec_ws/src/astra_visual/detection/target_detection.py

List of recognizable objects,

xxxxxxxxxx
[person, bicycle, car, motorcycle, airplane, bus, train, truck, boat, traffic light, fire hydrant, street sign, stop sign, parking meter, bench, bird, cat, dog, horse, sheep, cow, elephant, bear, zebra, giraffe, hat, backpack, umbrella, shoe, eye glasses, handbag, tie, suitcase, frisbee, skis, snowboard, sports ball, kite, baseball bat, baseball glove, skateboard, surfboard, tennis racket,bottle, plate, wine glass, cup, fork, knife, spoon, bowl,banana, apple, sandwich, orange, broccoli, carrot, hot dog,pizza, donut, cake, chair, couch, potted plant, bed, mirror,dining table, window, desk, toilet, door, tv, laptop, mouse,remote, keyboard, cell phone, microwave, oven, toaster,sink, refrigerator, blender, book, clock, vase, scissors,teddy bear, hair drier, toothbrush]

Load the category [object_detection_coco.txt], import the model [frozen_inference_graph.pb], specify the deep learning framework [TensorFlow]

xxxxxxxxxx
# load the COCO class names
with open('object_detection_coco.txt', 'r') as f: class_names = f.read().split('\n')
# get a different color array for each of the classes
COLORS = np.random.uniform(0, 255, size=(len(class_names), 3))
# load the DNN modelimage
model = cv.dnn.readNet(model='frozen_inference_graph.pb',config='ssd_mobilenet_v2_coco.txt',framework='TensorFlow')

Import the image, extract the height and width, calculate the 300x300 pixel blob, and pass this blob to the neural network

xxxxxxxxxx
def Target_Detection(image):
image_height, image_width, _ = image.shape
# create blob from image
blob = cv.dnn.blobFromImage(image=image, size=(300, 300), mean=(104, 117, 123), swapRB=True)
model.setInput(blob)
output = model.forward()
# loop over each of the detections
for detection in output[0, 0, :, :]:
# extract the confidence of the detection
confidence = detection[2]
# draw bounding boxes only if the detection confidence is above...
# ... a certain threshold, else skip
if confidence > .4:
# get the class id
class_id = detection[1]
# map the class id to the class
class_name = class_names[int(class_id) - 1]
color = COLORS[int(class_id)]
# get the bounding box coordinates
box_x = detection[3] * image_width
box_y = detection[4] * image_height
# get the bounding box width and height
box_width = detection[5] * image_width
box_height = detection[6] * image_height
# draw a rectangle around each detected object
cv.rectangle(image, (int(box_x), int(box_y)), (int(box_width), int(box_height)), color, thickness=2)
# put the class name text on the detected object
cv.putText(image, class_name, (int(box_x), int(box_y - 5)), cv.FONT_HERSHEY_SIMPLEX, 1, color, 2)
return image

4.1.3, Start

Terminal input,

xxxxxxxxxx
cd ~/orbbec_ws/src/astra_visual/detection
python target_detection.py