Video Capture and multithread

[hucqueime]

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

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

UIComponent.DrawImage(VideoComponent.GetBitmap())

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

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

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.

[SharpCode]

Pino's WebCam Class

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

[hucqueime]

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.

[hucqueime]

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.

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

[hucqueime]

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 ..which would just take an eternity! ..I should post the link and hope to save someone time and frustration

MSDN Article