Video Capture and multithread

Hi,
I am new to VB and programming on Windows as such. Mostly c++/gcc/linux.
I am trying to build this VB app that shows a streaming video from a camera and that needs to be overlaid with some custom graphics and text.

I used the code published here:
http://www.vb-helper.com/howto_net_video_capture.html
to get the capture part to work. It essentially uses the avicap32.dll that most of you would probably know.

My plan:
1) Capture frames using

Code:

---

SendMessage(windowHandle, WM_CAP_EDIT_COPY, 0, 0)

---

and then copy from clipboard.

2) Do whatever graphics/text overlay on the obtained bitmap

3) send it to some picturebox on some form that is being shown

4) Do step 1 to 3 in a loop by a spawned worker thread.


Firstly is this a reasonable way to do what I want? or is there a much more efficient/easier way to about?

Now the specific problem:
From my main thread directly if I do step 1 to 3 once

Code:

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UIComponent.DrawImage(VideoComponent.GetBitmap())

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It shows me once captured image as it should and all is good.
Just to clarify:
UIComponent is an object/instance that launches the Form and owns the picturebox I want to display on.
VideoComponent is an object/instance that has started the videocapture and owns the picturebox to which the video is being captured

But if I wrap the same call above in a function and launch a thread that would start with that function as its starting point, I see a blank screen.
Again, for testing purpose, I am only doing step 1 through 3 just once.
The thread part of the code is like this

Code:

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Private drawthread As Thread
public sub main()   
  drawthread = New Thread(AddressOf threadfunc)
  drawthread.Start()
  drawthread.Join()
end sub

Private Sub threadfunc()
        'While True
        Try
            Console.WriteLine("DBG: Should be drawing image in " & Thread.CurrentThread.GetHashCode)
            UIComponent.DrawImage(VideoComponent.GetBitmap())
            'Thread.Sleep(500)
        Catch ex As Exception
            'Exit While
        End Try
        'End While
End Sub

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I would appreciate if you would let me know what is messed up. I guess I am doing something conceptually wrong somewhere. Also I see the console print messages fine.

Pino's WebCam Class

Have a look at this thread posted by Pino in the CodeBank it is very helpful!

Yes I have seen that class and the link i posted is pretty much doing the same thing. My problem is not with the capture itself. That seems to be working reliably. Problem is when i run it from a worker thread.

alright so I narrowed it down a bit by stepping in through the code. I saw that the getbitmap function is returning Nothing when called from the thread and "accessibilityObject" is showing some message like invalid access. Object created in a thread different from the current one. (which is indeed the case, My objects VideoComponent and UIComponent are created as global objects before I launch the threads). But by the threading model that I understand, these should be shared by all the threads.

I did read some about the apartment business in windows but I gather that multi-thread apartment is default now in .Net and so my global objects should be shared indeed. So whats the catch.

I also tried to write a small testcase to understand this. Again a couple global objects created from the main thread but accessed by the child threads as shown below. But this seems to work fine and I get console messages as I would expect.

Code:

---

Imports System.Threading

Public Class test
    Private var As Integer = 0
    Public Sub setvar(ByVal newvar As Integer)
        var = newvar
    End Sub
    Public Function getvar()
        getvar = var
    End Function
End Class

Module MainModule

    Private myobj1 As New test
    Private myobj2 As New test

    Sub Main()
        Console.WriteLine("Initially var is : " & myobj1.getvar())
        For i As Integer = 1 To 5
            Dim t As Thread = New Thread(AddressOf threadfunc)
            't.SetApartmentState(ApartmentState.MTA)
            myobj2.setvar(i)
            t.Start()
            t.Join()
        Next i
        Console.WriteLine("Finally var is : " & myobj1.getvar())
    End Sub

    Public Sub threadfunc()
        myobj1.setvar(2 * myobj2.getvar())
        Console.WriteLine("Thread made var : " & myobj1.getvar())
    End Sub

End Module

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ok so I get it finally. Regular stuff is in multi-thread apartment but stupid windows form/control stays in single-thread apartments. No wonder my test code worked while the main guy didnt. I mean! this is the most obnoxious thing ever!!! At the least, I would have expected to see a compile/runtime error message about it!!

Anyway, instead of ranting away on my woes with marvelous Windows :p ..which would just take an eternity! ..I should post the link and hope to save someone time and frustration

MSDN Article