Dxsnap does not display video properly after first opening

Question

I am using DirectShowLib-2005 - DxSnap example to display and capture images from a webcam.
Everything works fine with an example.
But when I try to combine it with my application (I tried to call this form from my main form), it works for the first time. When I close and open the capture window, it does not display the video properly.
But image capture works fine all the time.

public partial class frmMain : Form { public frmMain() { InitializeComponent(); } /// <summary> /// The main entry point for the application. /// </summary> [STAThread] private static void Main() { Application.Run(new frmMain()); } private void button1_Click(object sender, EventArgs e) { frmdxSnap frmdxSnap = new frmdxSnap(); frmdxSnap.ShowDialog(this); } } 

Even after rebooting the PC its still the same.
I have not changed anything in the DxSnap form.

Answer 1

While DxSnap is a good introductory sample, it cuts a pair of angles making artifacts like the ones mentioned possible. The problem is assuming this in the following line:

m_stride = m_videoWidth * (videoInfoHeader.BmiHeader.BitCount / 8);

The actual step may be different, and this is a well-known effect of video equipment that offers great success. When you copy an image from the Sample Grabber buffer, it would be more accurate to list the step as BufferLen / m_videoHeight (see the code snippet below, and also note the statement there - perhaps you ignore it or run release builds). It would be even better to just check the current media type and get from there.

You may not have a problem with the first instance of the video pipeline, as it may use the video overlay and a different code path. You may not have a problem with well-aligned frame sizes (widths) such as 640, 1024, etc.

 /// <summary> buffer callback, COULD BE FROM FOREIGN THREAD. </summary> int ISampleGrabberCB.BufferCB( double SampleTime, IntPtr pBuffer, int BufferLen ) { // Note that we depend on only being called once per call to Click. Otherwise // a second call can overwrite the previous image. Debug.Assert(BufferLen == Math.Abs(m_stride) * m_videoHeight, "Incorrect buffer length"); if (m_WantOne) { m_WantOne = false; Debug.Assert(m_ipBuffer != IntPtr.Zero, "Unitialized buffer"); // Save the buffer CopyMemory(m_ipBuffer, pBuffer, BufferLen); //////////////////////////////////////////// // HOTFIX: Let have the stride re-computed for the case it was changed dynamically or otherwise m_stride = BufferLen / m_videoHeight; //////////////////////////////////////////// // Picture is ready. m_PictureReady.Set(); } return 0; }