Object Detection with DNN

Object detection identifies and localizes multiple objects in an image, providing both class labels and bounding box coordinates. OpenCV’s DNN module supports popular detection models like YOLO, SSD, and Faster R-CNN.

Supported Models

YOLO
SSD
Faster R-CNN

YOLO (You Only Look Once) - Real-time object detection

YOLOv3, YOLOv4 (Darknet)
YOLOv5 (PyTorch/ONNX)
YOLOv8, YOLOv9, YOLOv10 (Ultralytics)
YOLOX, YOLO-NAS

YOLO models are single-stage detectors optimized for speed.

YOLO Object Detection (Python)

Import Libraries

import cv2 as cv
import numpy as np

Load the Model

# Load YOLOv8 model
model = 'yolov8n.onnx'
net = cv.dnn.readNet(model)
net.setPreferableBackend(cv.dnn.DNN_BACKEND_OPENCV)
net.setPreferableTarget(cv.dnn.DNN_TARGET_CPU)

# Load class names
with open('object_detection_classes_yolo.txt', 'rt') as f:
    classes = f.read().rstrip('\n').split('\n')

Prepare Input

# Read image
frame = cv.imread('image.jpg')
frameHeight, frameWidth = frame.shape[:2]

# Create blob from image
# YOLOv8 uses 640x640 input with scale 1/255
blob = cv.dnn.blobFromImage(frame, 1/255.0, (640, 640), [0, 0, 0], True, crop=False)

Run Detection

# Set input and run forward pass
net.setInput(blob)
outNames = net.getUnconnectedOutLayersNames()
outs = net.forward(outNames)

Post-process Results

def postprocess(frame, outs, confThreshold=0.5, nmsThreshold=0.4):
    frameHeight, frameWidth = frame.shape[:2]
    
    classIds = []
    confidences = []
    boxes = []
    
    # For YOLOv8, output shape is [1, 84, 8400] -> transpose to [1, 8400, 84]
    for out in outs:
        out = out[0].transpose(1, 0)  # [8400, 84]
        
        for detection in out:
            scores = detection[4:]  # class scores
            classId = np.argmax(scores)
            confidence = scores[classId]
            
            if confidence > confThreshold:
                # YOLOv8 uses center format: [cx, cy, w, h]
                center_x = int(detection[0] * frameWidth / 640)
                center_y = int(detection[1] * frameHeight / 640)
                width = int(detection[2] * frameWidth / 640)
                height = int(detection[3] * frameHeight / 640)
                left = int(center_x - width / 2)
                top = int(center_y - height / 2)
                
                classIds.append(classId)
                confidences.append(float(confidence))
                boxes.append([left, top, width, height])
    
    # Apply Non-Maximum Suppression
    indices = cv.dnn.NMSBoxes(boxes, confidences, confThreshold, nmsThreshold)
    
    return classIds, confidences, boxes, indices

classIds, confidences, boxes, indices = postprocess(frame, outs)

Draw Detections

def drawPred(frame, classId, conf, left, top, right, bottom, classes):
    # Draw bounding box
    cv.rectangle(frame, (left, top), (right, bottom), (0, 255, 0), 2)
    
    # Create label
    label = f'{classes[classId]}: {conf:.2f}'
    
    # Draw label background
    labelSize, baseLine = cv.getTextSize(label, cv.FONT_HERSHEY_SIMPLEX, 0.5, 1)
    top = max(top, labelSize[1])
    cv.rectangle(frame, (left, top - labelSize[1]), 
                (left + labelSize[0], top + baseLine), 
                (255, 255, 255), cv.FILLED)
    
    # Draw label text
    cv.putText(frame, label, (left, top), cv.FONT_HERSHEY_SIMPLEX, 0.5, (0, 0, 0))

# Draw all detections
for i in indices:
    box = boxes[i]
    left, top, width, height = box
    drawPred(frame, classIds[i], confidences[i], left, top, 
            left + width, top + height, classes)

# Display result
cv.imshow('Object Detection', frame)
cv.waitKey(0)

YOLO Object Detection (C++)

Complete Example
Video Processing

#include <opencv2/dnn.hpp>
#include <opencv2/imgproc.hpp>
#include <opencv2/highgui.hpp>
#include <fstream>
#include <iostream>

using namespace cv;
using namespace cv::dnn;

std::vector<std::string> classes;

void getClasses(std::string classesFile) {
    std::ifstream ifs(classesFile.c_str());
    if (!ifs.is_open())
        CV_Error(Error::StsError, "File " + classesFile + " not found");
    std::string line;
    while (std::getline(ifs, line))
        classes.push_back(line);
}

void drawPrediction(int classId, float conf, int left, int top, 
                   int right, int bottom, Mat& frame) {
    rectangle(frame, Point(left, top), Point(right, bottom), Scalar(0, 255, 0));
    
    std::string label = format("%.2f", conf);
    if (!classes.empty()) {
        CV_Assert(classId < (int)classes.size());
        label = classes[classId] + ": " + label;
    }
    
    int baseLine;
    Size labelSize = getTextSize(label, FONT_HERSHEY_SIMPLEX, 0.5, 1, &baseLine);
    top = max(top, labelSize.height);
    rectangle(frame, Point(left, top - labelSize.height),
             Point(left + labelSize.width, top + baseLine), 
             Scalar::all(255), FILLED);
    putText(frame, label, Point(left, top), FONT_HERSHEY_SIMPLEX, 0.5, Scalar());
}

void yoloPostProcessing(std::vector<Mat>& outs,
                       std::vector<int>& keep_classIds,
                       std::vector<float>& keep_confidences,
                       std::vector<Rect2d>& keep_boxes,
                       float conf_threshold,
                       float iou_threshold,
                       const std::string& model_name,
                       const int nc = 80) {
    std::vector<int> classIds;
    std::vector<float> confidences;
    std::vector<Rect2d> boxes;
    
    // For YOLOv8/v9/v10, transpose output
    if (model_name == "yolov8" || model_name == "yolov10" || model_name == "yolov9") {
        cv::transposeND(outs[0], {0, 2, 1}, outs[0]);
    }
    
    for (auto preds : outs) {
        preds = preds.reshape(1, preds.size[1]);
        for (int i = 0; i < preds.rows; ++i) {
            // Filter out non-objects
            float obj_conf = (model_name == "yolov8" || model_name == "yolov9" || 
                             model_name == "yolov10") ? 1.0f : preds.at<float>(i, 4);
            if (obj_conf < conf_threshold)
                continue;
            
            Mat scores = preds.row(i).colRange(
                (model_name == "yolov8" || model_name == "yolov9" || 
                 model_name == "yolov10") ? 4 : 5, preds.cols);
            double conf;
            Point maxLoc;
            minMaxLoc(scores, 0, &conf, 0, &maxLoc);
            
            conf = (model_name == "yolov8" || model_name == "yolov9" || 
                   model_name == "yolov10") ? conf : conf * obj_conf;
            if (conf < conf_threshold)
                continue;
            
            // Get bbox coordinates
            float* det = preds.ptr<float>(i);
            double cx = det[0];
            double cy = det[1];
            double w = det[2];
            double h = det[3];
            
            if (model_name == "yolov10") {
                boxes.push_back(Rect2d(cx, cy, w, h));
            } else {
                boxes.push_back(Rect2d(cx - 0.5 * w, cy - 0.5 * h,
                                      cx + 0.5 * w, cy + 0.5 * h));
            }
            classIds.push_back(maxLoc.x);
            confidences.push_back(static_cast<float>(conf));
        }
    }
    
    // Apply NMS
    std::vector<int> keep_idx;
    NMSBoxes(boxes, confidences, conf_threshold, iou_threshold, keep_idx);
    
    for (auto i : keep_idx) {
        keep_classIds.push_back(classIds[i]);
        keep_confidences.push_back(confidences[i]);
        keep_boxes.push_back(boxes[i]);
    }
}

int main(int argc, char** argv) {
    // Load model
    std::string weightPath = "yolov8n.onnx";
    Net net = readNet(weightPath);
    net.setPreferableBackend(DNN_BACKEND_OPENCV);
    net.setPreferableTarget(DNN_TARGET_CPU);
    
    // Load classes
    getClasses("object_detection_classes_yolo.txt");
    
    // Read image
    Mat img = imread("image.jpg");
    
    // Preprocess
    Mat blob;
    Size size(640, 640);
    Scalar mean(0, 0, 0);
    Scalar scale(1.0/255.0, 1.0/255.0, 1.0/255.0);
    blobFromImage(img, blob, 1.0/255.0, size, mean, true, false);
    
    // Forward pass
    net.setInput(blob);
    std::vector<Mat> outs;
    net.forward(outs, net.getUnconnectedOutLayersNames());
    
    // Postprocess
    std::vector<int> keep_classIds;
    std::vector<float> keep_confidences;
    std::vector<Rect2d> keep_boxes;
    yoloPostProcessing(outs, keep_classIds, keep_confidences, keep_boxes,
                      0.5, 0.4, "yolov8", 80);
    
    // Draw results
    std::vector<Rect> boxes;
    for (auto box : keep_boxes) {
        boxes.push_back(Rect(cvFloor(box.x), cvFloor(box.y), 
                           cvFloor(box.width - box.x), cvFloor(box.height - box.y)));
    }
    
    for (size_t idx = 0; idx < boxes.size(); ++idx) {
        Rect box = boxes[idx];
        drawPrediction(keep_classIds[idx], keep_confidences[idx], 
                     box.x, box.y, box.width + box.x, box.height + box.y, img);
    }
    
    imshow("YOLO Object Detection", img);
    waitKey(0);
    
    return 0;
}

// Open video capture
VideoCapture cap;
cap.open(0); // or video file path

Mat img;
while (waitKey(1) < 0) {
    cap >> img;
    if (img.empty())
        break;
    
    // Preprocess
    Mat inp = blobFromImageWithParams(img, imgParams);
    
    // Forward
    net.setInput(inp);
    std::vector<Mat> outs;
    net.forward(outs, net.getUnconnectedOutLayersNames());
    
    // Postprocess
    std::vector<int> keep_classIds;
    std::vector<float> keep_confidences;
    std::vector<Rect2d> keep_boxes;
    yoloPostProcessing(outs, keep_classIds, keep_confidences, keep_boxes,
                      confThreshold, nmsThreshold, yolo_model, nc);
    
    // Draw boxes
    for (size_t idx = 0; idx < keep_boxes.size(); ++idx) {
        Rect2d box = keep_boxes[idx];
        drawPrediction(keep_classIds[idx], keep_confidences[idx], 
                     box.x, box.y, box.width + box.x, box.height + box.y, img);
    }
    
    imshow("YOLO Object Detector", img);
}

SSD Object Detection

MobileNet-SSD (Caffe)

import cv2 as cv

# Load MobileNet-SSD model
model = 'MobileNetSSD_deploy.caffemodel'
config = 'MobileNetSSD_deploy.prototxt'
net = cv.dnn.readNetFromCaffe(config, model)

# Prepare input
frame = cv.imread('image.jpg')
blob = cv.dnn.blobFromImage(frame, 0.007843, (300, 300), [127.5, 127.5, 127.5], False)

# Run detection
net.setInput(blob)
detections = net.forward()

# Process detections
for i in range(detections.shape[2]):
    confidence = detections[0, 0, i, 2]
    if confidence > 0.5:
        # Extract bounding box
        box = detections[0, 0, i, 3:7] * np.array([width, height, width, height])
        (left, top, right, bottom) = box.astype("int")
        
        # Draw detection
        cv.rectangle(frame, (left, top), (right, bottom), (0, 255, 0), 2)

Model Configurations

YOLOv8 (ONNX)

yolov8:
  model: "yolov8n.onnx"
  mean: [0, 0, 0]
  scale: 0.00392  # 1/255
  width: 640
  height: 640
  rgb: true
  classes: "object_detection_classes_yolo.txt"

Download: https://github.com/ultralytics/ultralytics

YOLOv4 (Darknet)

yolov4:
  model: "yolov4.weights"
  config: "yolov4.cfg"
  mean: [0, 0, 0]
  scale: 0.00392
  width: 416
  height: 416
  rgb: true
  classes: "object_detection_classes_yolo.txt"

Download: https://github.com/AlexeyAB/darknet/releases

MobileNet-SSD (Caffe)

ssd_caffe:
  model: "MobileNetSSD_deploy.caffemodel"
  config: "MobileNetSSD_deploy.prototxt"
  mean: [127.5, 127.5, 127.5]
  scale: 0.007843
  width: 300
  height: 300
  rgb: false
  classes: "object_detection_classes_pascal_voc.txt"

Faster R-CNN (TensorFlow)

faster_rcnn_tf:
  model: "faster_rcnn_inception_v2_coco_2018_01_28.pb"
  config: "faster_rcnn_inception_v2_coco_2018_01_28.pbtxt"
  mean: [0, 0, 0]
  scale: 1.0
  width: 800
  height: 600
  rgb: true

Download: http://download.tensorflow.org/models/object_detection/

Non-Maximum Suppression (NMS)

NMS removes duplicate detections by suppressing boxes with high overlap.

# Apply NMS
confidence_threshold = 0.5
nms_threshold = 0.4

indices = cv.dnn.NMSBoxes(boxes, confidences, confidence_threshold, nms_threshold)

for i in indices:
    box = boxes[i]
    # Draw detection
    drawPrediction(frame, classIds[i], confidences[i], box[0], box[1], 
                  box[0] + box[2], box[1] + box[3])

Performance Tips

Use GPU Acceleration

net.setPreferableBackend(cv.dnn.DNN_BACKEND_CUDA)
net.setPreferableTarget(cv.dnn.DNN_TARGET_CUDA)

Use FP16 for Faster Inference

net.setPreferableTarget(cv.dnn.DNN_TARGET_CUDA_FP16)

Choose Appropriate Model Size

YOLOv8n (nano): Fastest, lowest accuracy
YOLOv8s (small): Balanced
YOLOv8m (medium): Higher accuracy
YOLOv8l/x (large/xlarge): Best accuracy, slowest

Different YOLO versions (v3, v4, v5, v8) have different output formats and require different post-processing.

Source Code

Complete source code for object detection:

Python: samples/dnn/object_detection.py
C++ (YOLO): samples/dnn/yolo_detector.cpp
C++ (Generic): samples/dnn/object_detection.cpp

Basic Operations

Advanced Applications

Deep Learning

Object Detection with DNN

Supported Models

YOLO Object Detection (Python)

YOLO Object Detection (C++)

SSD Object Detection

MobileNet-SSD (Caffe)

Model Configurations

YOLOv8 (ONNX)

YOLOv4 (Darknet)

MobileNet-SSD (Caffe)

Faster R-CNN (TensorFlow)

Non-Maximum Suppression (NMS)

Performance Tips

Source Code

Build docs developers (and LLMs) love

Basic Operations

Advanced Applications

Deep Learning

​Supported Models

​YOLO Object Detection (Python)

​YOLO Object Detection (C++)

​SSD Object Detection

​MobileNet-SSD (Caffe)

​Model Configurations

​YOLOv8 (ONNX)

​YOLOv4 (Darknet)

​MobileNet-SSD (Caffe)

​Faster R-CNN (TensorFlow)

​Non-Maximum Suppression (NMS)

​Performance Tips

​Source Code

Build docs developers (and LLMs) love

Supported Models

YOLO Object Detection (Python)

YOLO Object Detection (C++)

SSD Object Detection

MobileNet-SSD (Caffe)

Model Configurations

YOLOv8 (ONNX)

YOLOv4 (Darknet)

MobileNet-SSD (Caffe)

Faster R-CNN (TensorFlow)

Non-Maximum Suppression (NMS)

Performance Tips

Source Code