Camera Capture

From Emgu CV: OpenCV in .NET (C#, VB, C++ and more)
Jump to: navigation, search

Camera Capture




EMGU Reference
EMGU CV_PROP Reference
OpenCV Reference


Source Code V2.0


The following example shows the use of the Capture function within EMGU. The function of this library is to allow video streaming for web camera type devices and video files. This example will show an example of web camera capture as well as reading and setting properties of the camera. Only some function is available in this version to demonstrate the methods available to the user. A video file example is available here. The key difference from this example and the Capture: Camera - Legacy example is that the acquisition of frames is controlled by it's own thread. This limits the methods that can be called to acquire frames see the Methods Available section bellow.




The code provided should run straight out of the Emgu.Example folder (V2.4.2), extract it to this location. If the code fails to execute re-reference the EMGU libraries and include the required opencv dlls in the bin directory. Note that the project is set to build to the output path "..\..\..\bin\" you may wish to change this if you don't extract to the EMGU.Example folder.

A Web camera is required for this example.

EMGU Coding Level: While the coding is not advanced the rated level for this example is Intermediate/Beginner. This is not designed as a full on tutorial and general knowledge of the EMGU is expected. While the coding is basic the are several methods involved that may be of putting the newcomers of EMGU.

The Code

The code provided in this sample is basic there is only someo error checking. Support is available through the Forums but please try and examine the code before saying it doesn't work for you. The code is not optimised instead is better formatted to provided an understanding of the stages involved.

Each method is split up to do it's own function while there are several methods in the code the code is not advanced and is often repetitive. The new features introduced in this code is the ability to select the camera source through Direct Show (dll supplied). The code shows the use of two functions for acquiring the frame the others are legacy based and will not work see the Methods Available section. This example also shows how to get and set camera features such as Brightness, Contrast, and Sharpness. While other settings are available only these three attributes are changeable from the form interface. It is easy to add additional support for other settings, these will be made available in future iterations of the example.

The Code: Variables

There are only a few variables in the example separated into two regions the first Camera Capture Variables contains the variables for camera information and the capture device. The Video_Device[] array is a custom structure to store video information this is discussed bellow.

        #region Camera Capture Variables
        private Capture _capture = null; //Camera
        private bool _captureInProgress = false; //Variable to track camera state
        int CameraDevice = 0; //Variable to track camera device selected
        Video_Device[] WebCams; //List containing all the camera available

The second region Camera Settings simply stores the original setting of the camera when it is loaded this is useful in case we need to reset the values to the way we had them.

        #region Camera Settings
        int Brightness_Store = 0;
        int Contrast_Store = 0;
        int Sharpness_Store = 0;

The Code: Methods

As discussed there are several method in the code, they will be decribed in the order that they appear in the code.

The method CameraCapture() is the form initialiser here we disable all our sliders on the form with the method call of Slider_Enable(false). We gather all the video devices available on the system using the Direct Show library (Supplied in the 'Lib' Folder). We populate both an array of Video_Devices and the combo box on the form to enable user selection. Finally we set the combo box to the first index '0' which will be the default device of the OS.

        public CameraCapture()
            Slider_Enable(false); //Disable sliders untill capturing

            //-> Find systems cameras with DirectShow.Net dll
            //thanks to carles lloret
            DsDevice[] _SystemCamereas = DsDevice.GetDevicesOfCat(FilterCategory.VideoInputDevice);
            WebCams = new Video_Device[_SystemCamereas.Length];
            for (int i = 0; i < _SystemCamereas.Length; i++)
                WebCams[i] = new Video_Device(i, _SystemCamereas[i].Name, _SystemCamereas[i].ClassID); //fill web cam array
            if (Camera_Selection.Items.Count > 0)
                Camera_Selection.SelectedIndex = 0; //Set the selected device the default
                captureButton.Enabled = true; //Enable the start

The method ProcessFrame() simply switches between what type of capture event is used either RetrieveBgrFrame() or RetrieveGrayFrame() according to the selected checkbox on the form. The DisplayImage() method is used to display the image in a picture box.

        private void ProcessFrame(object sender, EventArgs arg)
            //***If you want to access the image data the use the following method call***/
            //Image<Bgr, Byte> frame = new Image<Bgr,byte>(_capture.RetrieveBgrFrame().ToBitmap());

            if (RetrieveBgrFrame.Checked)
                Image<Bgr, Byte> frame = _capture.RetrieveBgrFrame();
                //because we are using an autosize picturebox we need to do a thread safe update
            else if (RetrieveGrayFrame.Checked)
                Image<Gray, Byte> frame = _capture.RetrieveGrayFrame();
                //because we are using an autosize picturebox we need to do a thread safe update

The method DisplayImage() is a thread safe method to display the image in a picture box. We don't need this is we set the Sizemode property of the picturebox to 'Normal' or 'StretchMode'. Since we are using the 'Autosize' Sizemode parameter the size of the picture box is managed on the main thread. Without using a thread safe call a cross thread exception will be thrown as an invoke will be required.

        private delegate void DisplayImageDelegate(Bitmap Image);
        private void DisplayImage(Bitmap Image)
            if (captureBox.InvokeRequired)
                    DisplayImageDelegate DI = new DisplayImageDelegate(DisplayImage);
                    this.BeginInvoke(DI, new object[] { Image });
                catch (Exception ex)
                captureBox.Image = Image;

The method captureButtonClick() looks complex but it's mainly for error checking purposes. As we are allowing the user to set/change the camera according to form settings we need to look for this. This can be done within the SelectedIndexChanged method as an alternative. First we see if the _capture variable has been initiated yet. If not then we call the SetupCapture(); method to set the Camera_Identifier and initialise the capture event. We then recall the same event to initialise the the camera as it will instantly fall down to the RetrieveCaptureInformation() method call.

If the _capture variable has been initialised the we either stop the camera acquiring or check to see if camera selection has changed before restarting the camera. If the camera selection has changed then SelectedIndexChanged is called again to dispose of the old _capture and re-initialise a new one.

        private void captureButtonClick(object sender, EventArgs e)
            if (_capture != null)
                if (_captureInProgress)
                    //stop the capture
                    captureButton.Text = "Start Capture"; //Change text on button
                    _capture.Pause(); //Pause the capture
                    _captureInProgress = false; //Flag the state of the camera
                    //Check to see if the selected device has changed
                    if (Camera_Selection.SelectedIndex != CameraDevice)
                        SetupCapture(Camera_Selection.SelectedIndex); //Setup capture with the new device
                    RetrieveCaptureInformation(); //Get Camera information
                    captureButton.Text = "Stop"; //Change text on button
                    StoreCameraSettings(); //Save Camera Settings
                    Slider_Enable(true);  //Enable User Controls
                    _capture.Start(); //Start the capture
                    _captureInProgress = true; //Flag the state of the camera
                //set up capture with selected device
                //Be lazy and Recall this method to start camera
                captureButtonClick(null, null);

The method SetupCapture() is a simple method it records the selected index for the chosen camera provided to the method. Then it disposes of the existing _capture variable if it exists and then attempts to re-initialise a new one.

        private void SetupCapture(int Camera_Identifier)
            //update the selected device
            CameraDevice = Camera_Identifier;

           //Dispose of Capture if it was created before
            if (_capture != null) _capture.Dispose();
                //Set up capture device
                _capture = new Capture(CameraDevice);
                _capture.ImageGrabbed += ProcessFrame;
            catch (NullReferenceException excpt)

The method FlipHorizontalButtonClick() simply sets the FlipHorizontal flag of the _capture variable. Similarly the FlipVerticalButtonClick() method sets the FlipVertical flag. Each one flips the produced frame according to the flag direction.

The method RetrieveCaptureInformation() demonstrates the use of the GetCaptureProperty(). A majority of the variables gathered from this method are simply displayed in the richtextbox on the form. Only Brightness, Contrast, and Sharpness are saved to variables and slider values for adjustment. If a method is invalid it will usually return a '-1'. The commented GetCaptureProperty() calls are used for specific devices. Not all properties are writeable through the SetCaptureProperty(). For more reference see

        private void RetrieveCaptureInformation()
            richTextBox1.AppendText("Camera: " + WebCams[CameraDevice].Device_Name + " (-1 = Unknown)\n\n");

            richTextBox1.AppendText("Brightness: " + _capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_BRIGHTNESS).ToString() + "\n"); //get the value and add it to richtextbox
            Brigtness_SLD.Value = (int)_capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_BRIGHTNESS);  //Set the slider value
            Brigthness_LBL.Text = Brigtness_SLD.Value.ToString(); //set the slider text

            richTextBox1.AppendText("Contrast: " + _capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_CONTRAST).ToString() + "\n");//get the value and add it to richtextbox
            Contrast_SLD.Value = (int)_capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_CONTRAST);  //Set the slider value
            Contrast_LBL.Text = Contrast_SLD.Value.ToString(); //set the slider text

            richTextBox1.AppendText("Sharpness: " + _capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_SHARPNESS).ToString() + "\n");
            Sharpness_SLD.Value = (int)_capture.GetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_CONTRAST);  //Set the slider value
            Sharpness_LBL.Text = Sharpness_SLD.Value.ToString(); //set the slider text


The method StoreCameraSettings() is called after the _capture variable is initialised, it simply records the values that have been assigned to the sliders by the GetCaptureProperty() method call. It does does as it is more efficient that invoking the GetCaptureProperty() method again. It simply stores the values that can be adjusted by the user in global variables allowing the camera properties to be reset to their original state. This is useful in case adjustments are made that the user does not want to save.

The method Refresh_BTN_Click() simply allows the user to re-read the camera setting from the RetrieveCaptureInformation() method. This allows the user to see some of the limitations of the values they can use and ensure that the camera setting have been updated.

The method Reset_Cam_Settings_Click() resets the overwritten properties to those initially read in when the camera was initialised. It does this using the SetCaptureProperty() method call.

        private void Reset_Cam_Settings_Click(object sender, EventArgs e)
            if (_capture != null)
                _capture.SetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_BRIGHTNESS, Brightness_Store);
                _capture.SetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_CONTRAST, Contrast_Store);
                _capture.SetCaptureProperty(Emgu.CV.CvEnum.CAP_PROP.CV_CAP_PROP_SHARPNESS, Sharpness_Store);
                RetrieveCaptureInformation(); // This will refresh the settings

The region Sliders contains four methods the 'Scroll' call simply update the associated label and sets the camera variable using the SetCaptureProperty(). The Slider_Enable method simply allows the 'Enable' state of the scroll bars to be set all together rather than individually.

The method OnClosing() will reset the camera to the default settings using the Reset_Cam_Settings_Click() method unless the user has stopped capturing from the device. If a user stops the capture the setting will be saved by default. This method will dispose of the _capture object to release the camera.

Methods Available


  • Capture(Int32)
  • Dispose()
  • RetrieveBgrFrame()
  • RetrieveGrayFrame()
  • GetCaptureProperty(CAP_PROP)
  • SetCaptureProperty(CAP_PROP, Double)
  • Start()
  • Stop()
  • Wait()


  • Capture()
  • Capture(String)
  • RetrieveBgrFrame(Int32)
  • RetrieveGrayFrame(Int32)
  • DuplexQueryFrame() WCF dependant
  • DuplexQuerySmallFrame() WCF dependant
  • QueryFrame() Legacy Method
  • QueryGrayFrame() Legacy Method
  • QuerySmallFrame() Legacy Method
  • Equals(Object)
  • GetHashCode()
  • GetType()
  • MemberwiseClone()
  • ToString()


  1. There was a problem when changing the device to acquire from and restarting the capture. This has been addressed in the new source see the captureButtonClick() method description where an else statement has been removed.