YOLO Real time detection on CPU

We’re going to learn in this tutorial how to detect objects in real time running YOLO on a CPU.

If you’re a complete beginner about YOLO I highly suggest to check out my other tutorial about YOLO object detection on images, before proceding with realtime detection, as I’m going to use most of the same code I explained there.

Why did I specify that we’re going to perform the detection using the CPU?

I did specify this as with the deep learning frameworks it’s possible to do the detection using the CPU or the GPU.


I’m going to quickly to compare yolo on a cpu versus yolo on the gpu explaining advantages and disadvantages for both of them.


The big advantage of running YOLO on the CPU is that it’s really easy to set up and it works right away on Opencv withouth doing any further installations. You only need Opencv 3.4.2 or greater.

The disadvantage is that YOLO, as any deep neural network runs really slow on a CPU and we will be able to process only a few frames per second.
Not really good for a realtime detection.


Instead YOLO on a GPU is really fast, and with a good gpu you can process 45 or more frames per seconds.
So we’re not talking about a small speed difference between a CPU and a GPU, but a huge difference where the GPU greatly outperform the CPU by 20 times faster or more.

The disadvantage is that for a beginner setting up a deep neural network on a GPU can be a really harsh process.
Also it doesn’t work with all the GPUs but only with NVIDIA GPUs wich are compatible with CUDA.
For example right now I’m using a laptop with an AMD Radeon GPU, so it won’t work.

We import the libraries and we load the Network.

import cv2
import numpy as np
import time

# Load Yolo
net = cv2.dnn.readNet("weights/yolov3-tiny.weights", "cfg/yolov3-tiny.cfg")
classes = []
with open("coco.names", "r") as f:
    classes = [line.strip() for line in f.readlines()]
layer_names = net.getLayerNames()
output_layers = [layer_names[i[0] - 1] for i in net.getUnconnectedOutLayers()]
colors = np.random.uniform(0, 255, size=(len(classes), 3))

We then load the the camera.

We get the starting time and the frame ID in order to calculate later how many frames per second FPS we are processing.

# Loading camera
cap = cv2.VideoCapture(0)

starting_time = time.time()
frame_id = 0

We run the while loop and we extract the frame from the camera.

while True:
    _, frame = cap.read()
    frame_id += 1

    height, width, channels = frame.shape

We perform the detection.
All this code below is explained in my other tutorial.

    # Detecting objects
    blob = cv2.dnn.blobFromImage(frame, 0.00392, (416, 416), (0, 0, 0), True, crop=False)

    outs = net.forward(output_layers)

    # Showing informations on the screen
    class_ids = []
    confidences = []
    boxes = []
    for out in outs:
        for detection in out:
            scores = detection[5:]
            class_id = np.argmax(scores)
            confidence = scores[class_id]
            if confidence > 0.2:
                # Object detected
                center_x = int(detection[0] * width)
                center_y = int(detection[1] * height)
                w = int(detection[2] * width)
                h = int(detection[3] * height)

                # Rectangle coordinates
                x = int(center_x - w / 2)
                y = int(center_y - h / 2)

                boxes.append([x, y, w, h])

    indexes = cv2.dnn.NMSBoxes(boxes, confidences, 0.4, 0.3)

    for i in range(len(boxes)):
        if i in indexes:
            x, y, w, h = boxes[i]
            label = str(classes[class_ids[i]])
            confidence = confidences[i]
            color = colors[class_ids[i]]
            cv2.rectangle(frame, (x, y), (x + w, y + h), color, 2)
            cv2.rectangle(frame, (x, y), (x + w, y + 30), color, -1)
            cv2.putText(frame, label + " " + str(round(confidence, 2)), (x, y + 30), font, 3, (255,255,255), 3)

We then calculate the FPS by deviding the elapsed time by the number of the frames and we show everything on the screen.

    elapsed_time = time.time() - starting_time
    fps = frame_id / elapsed_time
    cv2.putText(frame, "FPS: " + str(round(fps, 2)), (10, 50), font, 3, (0, 0, 0), 3)
    cv2.imshow("Image", frame)
    key = cv2.waitKey(1)
    if key == 27:


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