Implementing YoloV3 for object detection

3 minute read

Yolo is one of the greatest algorithm for real-time object detection. In its large version, it can detect thousands of object types in a quick and efficient manner. I this article, I won’t cover the technical details of YoloV3, but I’ll jump straight to the implementation. We will learn to build a simple web application with Streamlit that detects the objects present in an image. This implementation is a simplified version of Streamlit’s yolo demo.

Download the weights and config

Create a separate folder for your web application, and download the weights via your command line:

wget https://pjreddie.com/media/files/yolov3.weight

Then, download the configuration file:

wget https://pjreddie.com/media/files/yolov3.cfg

Finally, download the classes predicted by the YoloV3 algorithm in a text file:

wget https://pjreddie.com/media/files/classes.txt

App layout

# Add a title and sidebar
st.title("Object Detection")
st.sidebar.markdown("# Model")
confidence_threshold = st.sidebar.slider("Confidence threshold", 0.0, 1.0, 0.5, 0.01)

There are now 3 main steps:

  • a function to download the image from the selected file
  • a function to apply the object detection on the image and plot the boxes
  • a selector on the sidemenu to pick the input image

The first function is quick to implement :

@st.cache(show_spinner=False)
def read_img(img):
    image = cv2.imread(img, cv2.IMREAD_COLOR)
    image = image[:, :, [2, 1, 0]] # BGR -> RGB
    return image

Then, build the function to identify the bounding boxes. The code is commented in order to understandd the key steps.

def yolo_v3(image, confidence_threshold=0.5, overlap_threshold=0.3):

	# Load model architecture
    net = cv2.dnn.readNetFromDarknet("yolov3.cfg", "yolov3.weights")
    output_layer_names = net.getLayerNames()
    output_layer_names = [output_layer_names[i[0] - 1] for i in net.getUnconnectedOutLayers()]

    # Set input and get output
    blob = cv2.dnn.blobFromImage(image, 1 / 255.0, (416, 416), swapRB=True, crop=False)
    net.setInput(blob)
    layer_outputs = net.forward(output_layer_names)

    boxes, confidences, class_IDs = [], [], []
    H, W = image.shape[:2]

    # For each detected object, compute the box, find the score, ignore if below
    for output in layer_outputs:
        for detection in output:
            scores = detection[5:]
            classID = np.argmax(scores)
            confidence = scores[classID]
            if confidence > confidence_threshold:
                box = detection[0:4] * np.array([W, H, W, H])
                centerX, centerY, width, height = box.astype("int")
                x, y = int(centerX - (width / 2)), int(centerY - (height / 2))
                boxes.append([x, y, int(width), int(height)])
                confidences.append(float(confidence))
                class_IDs.append(classID)

    # Write the name of detected objects above image
    f = open("classes.txt", "r")
    f = f.readlines()
    f = [line.rstrip('\n') for line in list(f)]
    try:
    	st.subheader("Detected objects: " + ', '.join(list(set([f[obj] for obj in class_IDs]))))
    except IndexError:
    	st.write("Nothing detected")

    # Apply non-max suppression to identify best bounding box
    indices = cv2.dnn.NMSBoxes(boxes, confidences, confidence_threshold, overlap_threshold)
    xmin, xmax, ymin, ymax, labels = [], [], [], [], []
    
    if len(indices) > 0:
        for i in indices.flatten():
            x, y, w, h = boxes[i][0], boxes[i][1], boxes[i][2], boxes[i][3]
            xmin.append(x)
            ymin.append(y)
            xmax.append(x+w)
            ymax.append(y+h)
    boxes = pd.DataFrame({"xmin": xmin, "ymin": ymin, "xmax": xmax, "ymax": ymax})

    # Add a layer on top on a detected object 
	LABEL_COLORS = [0, 255, 0]
	image_with_boxes = image.astype(np.float64)
	for _, (xmin, ymin, xmax, ymax) in boxes.iterrows():
		image_with_boxes[int(ymin):int(ymax),int(xmin):int(xmax),:] += LABEL_COLORS
		image_with_boxes[int(ymin):int(ymax),int(xmin):int(xmax),:] /= 2

	# Display the final image
	st.image(image_with_boxes.astype(np.uint8), use_column_width=True)

Finally, let the user choose from several inout images such as car images, people, animals or a meeting, and run your pipeline on top of it.

img_type = st.sidebar.selectbox("Select image type?", ['Cars', 'People', 'Animals', "Meeting"])

if img_type == 'People':
    image_url = "images/people.jpg"
elif img_type == 'Cars':
    image_url = "images/cars.jpg"
elif img_type == 'Animals':
    image_url = "images/animal.jpg"
elif img_type == 'Meeting':
    image_url = "images/meeting.jpg"

image = read_img(image_url)

# Get the boxes for the objects detected by YOLO by running the YOLO model.
yolo_v3(image, confidence_threshold)

Finally, to run your Streamlit app, supposing that you called your app “app.py”, simply run :

streamlit run app.py

And you should be able to see this:

image

Conclusion : This project is adapted from Streamlit’s demo of Yolo. I tried to make the overall steps easier to understand, and executable on your own images.


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