Programming video for Windows by E. J. Bantz
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Step 1 - Creating The Windowstep 2 - Connecting The WindowPassing Strings To SendMessagePassing Structurement To sendMessageProcessing images from the video stream
Step 1 - Creating the WindowThe video capturing processes starts with the capCreateCaptureWindowA function When calling this function, a handle to a new window will be returned The handle is a 32-bit number that is used to reference an object (in this case,.. A Window. This Handle Is The Foundation for The Rest of Your Program, And Should Be Kept It in A Safe Place.
You Can Customize The Look and Feel of this New Window by Changing The Style Parameters. In This Example, The Window Will Be a child and visible upon creation.
LWNDC = CapcreatecaptureWindowa ("My Capture Window", WS_CHILD or WS_VISIBLE, 0, 0, 160, 120, Me.hWnd, 0)
All future commands are issued by sending a Windows Message to that new window. A Windows Message is sent with the SendMessage function that is built into the Win32 API. You pass this function the Handle of the window you would like to send a command to, the 32-bit message you are sending, a 16-bit parameter, and finally a 32-bit parameter. The 32-bit messages are replaced with an easy to understand constant. I've defined all of the messages that are used for Capturing Video INSIDE VBAVICAP.BAS.
Step 2 - Connecting the WindowOnce the Capture Window is created, you can connect the window to the video driver The video driver is installed with your video camera The connection is made with the WM_CAP_DRIVER_CONNECT message This will "bind" the window to first... Video driver it finds (index 0). Keep in mind that could send this message to any window, but only this special window created with the capCreateCaptureWindow will understand what you are talking about.SendMessage lwnd, WM_CAP_DRIVER_CONNECT, 0, 0
Aside from Defining All The Possible Message Constants, Vbavicap.bas Provides Functions to make sending Windows Messages A Bit Easier. Making The Following Call Is Exactly The Same As Above:
CapdriverConnect LWND, 0
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Passing Strings to SendMessageThe SendMessage API is always passed four things: a handle to a window (hwnd), some message to send to that window (wMsg), a short parameter (wParam), and a long parameter (lParam) The w in. Word and long area. Word and long area. Word and long are c data type. A Word IS 16bit Number. in VB, THESE CALLED INTEGER AND LONG. THE Declaration Looks Like this:
Declare function sendMessage Lib "User32" Alias "SendMessagea" (Byval HWnd As Long, Byval WParam As Integer, BYVAL LPARAM As Long)
In order to pass a string to this function, you need to redefine lParam to be a String instead of a Long. Now, one might think that sending the wrong type of information to a DLL would cause big problems, and that's right, it normally would. Actually, we are sending the function exactly what it wants. you see, in Visual Basic, when you pass a STRING by Value to an external function, it will actually send a pointer to the STRING instead. A pointer is a 32bit number , which is a LONG, which is back to what it expected. So we did pass the right type of data after all. One more thing, we can not just change the declaration of SendMessage because some calls NEED lParam to be a LONG. So, you can just create a new declaration and call it something slightly different Here is my new declaration:. Declare Function SendMessageS Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Integer, ByVal LPARAM As String) As long
Passing Structures to SendMessageStructures (User Defined Types) are not directly passed to the functions either. Passing a structure is a bit easier that passing a string though. This time, instead of letting Visual Basic pass the pointer for you, use just to it by Hand. Varptr () IS A BUILT IN FUNCTION THAT WHER RETURN THE LOCATION IN MEMORY (A POINTER) Where The Variable You Pass, The Variable Will Be a user defined Type. Again, a Pointer IS A 32bit Number, Or Long, Which is what we can pass as lparam.
Here is a quick example of how to retrieve the capture parameters.
DIM CAP_PARAMS AS CAPTUREPARMSCAPCAPTUREGETSETUP LWNDC, VARPTR (CAP_PARAMS), LEN (CAP_PARAMS)
Processing Images from the Video StreamThe most common question I have been receiving is, "How do I get at the video capture data during streaming?" The answer is in the call back functions. Use capSetCallbackonFrame to process frames during preview, and capSetCallbackOnVideoStream to process Frames During Capture. Here IS An Example: CapsetCallbackonframe LWNDC, Addressof Myframecallback
MyFrameCallback is a function that you create for your application. It should be located in a module, not a form. This is the function that will be called every time a new frame has been received from the video driver. It gets passed a handle the A videohdr Which should be declared as the folload:
Type videohdr
lpData As Long '// address of video bufferdwBufferLength As Long' // size, in bytes, of the Data bufferdwBytesUsed As Long '// see belowdwTimeCaptured As Long' // see belowdwUser As Long '// user-specific datadwFlags As Long' // See BelowdWReServed (3) as long '// reserved; do not use
End Type
lpData is the actual data from the device. You'll have have to manipulate the VIDEOHDR structure to get to it. Here is some sample code to copy the structure from memory in a real Visual Basic User Defined Type.
Function myframecallback (byval lwnd as long, byval lpvhdr as long) AS Long
Debug.Print "FrameCallBack" Dim VideoHeader As VIDEOHDRDim VideoData () As Byte '// Fill VideoHeader with data at lpVHdrRtlMoveMemory VarPtr (VideoHeader), lpVHdr, Len (VideoHeader)' // Make room for dataReDim VideoData (VideoHeader.dwBytesUsed) '/ / Copy Data Into the ArrayrtLmoveMemory Varptr (VideoData (0)), VideoHeader.dwbytesuseddebug.print VideoHeader.dwbytesuseddebug.print VideoDetaend Function
In this Example, RTLMovememory Should Be Declared As the Following:
Declare Sub RTLMoveMemory Lib "kernel32" (Byval HPVSource As Long, Byval CBCopy As Long)
The end ...
