#dotnet – GoogleNet detection from the 🎦 camera feed using #OpenCV and #net5. Bonus: C++ to C# time!

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Hi !

So I was browsing in the OpenCV documentation and I find a nice sample that uses opencv_dnn module for image classification by using GoogLeNet trained network from Caffe model zoo.

So I give it a try, and get a decent .Net 5 Winforms App running at ~30 FPS.

opencv net5 load and analyze camera frames with googlenet

The model was trained with 1000 classes, and once you get the main focus on the camera it work great with objects like a machine, mug, bottle, etc. There is a nice amount of code here, and because the DNN analysis is performed in a separated thread, I need to update the label details using PInvoke functions.

using System;
using System.IO;
using System.Linq;
using System.Threading;
using System.Windows.Forms;
using OpenCvSharp;
using OpenCvSharp.Dnn;
using OpenCvSharp.Extensions;
using Point = OpenCvSharp.Point;
using Size = OpenCvSharp.Size;
namespace Demo11_WinFormGoogleNet
public partial class Form1 : Form
private bool _run = true;
private bool _useGoogleNet = false;
private VideoCapture _capture;
private Mat _image;
private Thread _cameraThread;
private bool _fps = false;
private Net _netGoogleNet;
private string[] _classNames;
const string ProtoTxt = @"models\bvlc_googlenet.prototxt";
const string CaffeModel = @"models\bvlc_googlenet.caffemodel";
const string SynsetWords = @"models\synset_words.txt";
private delegate void SafeCallDelegate(string text);
public Form1()
Load += Form1_Load;
Closed += Form1_Closed;
private void Form1_Closed(object sender, EventArgs e)
private void btnStart_Click(object sender, EventArgs e)
_run = true;
private void btnStop_Click(object sender, EventArgs e)
_run = false;
private void btnGoogleNet_Click(object sender, EventArgs e)
_useGoogleNet = !_useGoogleNet;
private void buttonFPS_Click(object sender, EventArgs e)
_fps = !_fps;
private void Form1_Load(object sender, EventArgs e)
_classNames = File.ReadAllLines(SynsetWords)
.Select(line => line.Split(' ').Last())
_netGoogleNet = CvDnn.ReadNetFromCaffe(ProtoTxt, CaffeModel);
_capture = new VideoCapture(0);
_image = new Mat();
_cameraThread = new Thread(new ThreadStart(CaptureCameraCallback));
private void CaptureCameraCallback()
while (true)
if (!_run) continue;
var startTime = DateTime.Now;
if (_image.Empty()) return;
var imageRes = new Mat();
Cv2.Resize(_image, imageRes, new Size(320, 240));
if (_useGoogleNet)
// Convert Mat to batch of images
using var inputBlob = CvDnn.BlobFromImage(imageRes, 1, new Size(224, 224), new Scalar(104, 117, 123));
_netGoogleNet.SetInput(inputBlob, "data");
using var prob = _netGoogleNet.Forward("prob");
// find the best class
GetMaxClass(prob, out int classId, out double classProb);
var msg = @$"Best class: #{classId} '{_classNames[classId]}' – Probability: {classProb:P2}";
// display output
if (_fps)
var diff = DateTime.Now startTime;
var fpsInfo = $"FPS: Nan";
if (diff.Milliseconds > 0)
var fpsVal = 1.0 / diff.Milliseconds * 1000;
fpsInfo = $"FPS: {fpsVal:00}";
Cv2.PutText(imageRes, fpsInfo, new Point(10, 20), HersheyFonts.HersheyComplexSmall, 1, Scalar.White);
var bmpWebCam = BitmapConverter.ToBitmap(imageRes);
pictureBoxWebCam.Image = bmpWebCam;
private void WriteTextSafe(string text)
if (lblOutputAnalysis.InvokeRequired)
var d = new SafeCallDelegate(WriteTextSafe);
lblOutputAnalysis.Invoke(d, new object[] { text });
lblOutputAnalysis.Text = text;
private static void GetMaxClass(Mat probBlob, out int classId, out double classProb)
// reshape the blob to 1×1000 matrix
using (var probMat = probBlob.Reshape(1, 1))
Cv2.MinMaxLoc(probMat, out _, out classProb, out _, out var classNumber);
classId = classNumber.X;

Super fun ! and check the references for the model and support files download location.

Happy coding!


El Bruno

More posts in my blog ElBruno.com.

More info in https://beacons.ai/elbruno


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