Scanning with TWAIN

Hello

I am making an application to scan with TWAIN. I can get to state 4 OK, and scan with the UI. But I need to hide the UI. When I do this the scanner does nothing. I believe it needs some capabilities set (like height and width). So I try to get the physical height and width so I can set them. Herein lies the problem. I get 18874402 back for the physical height and width. (neither can I get the units). I get RC=SUCCESS. I don't think it could be a signed/unsigned issue because what boundary is at 18 million +/- with whatever units? I tried some other code I found but that returns 0. I can't figure out what I am doing wrong. Here's the code:

Code:

---

    Private Function GetOneValue(ByVal twCapability As twsCapability) As String

        Dim rc As twRC
        Dim strTemp As String
        Dim twOneValue As TW_ONEVALUE


        rc = DS_Capability(mtwApp, mtwSource, twDG.Control, twDAT.Capability, twMSG.Get, twCapability)
        If rc = twRC.Success Then
            strTemp = vbNullString
            Select Case twCapability.ConType
                Case twContainerTyoe.One
                    twOneValue = New TW_ONEVALUE
                    Marshal.PtrToStructure(twCapability.Handle, twOneValue)
                    strTemp = CType(twOneValue.Item, String)
                Case Else
                    MsgBox("Error, one value expected; " & twCapability.ConType & " returned.")
            End Select
            Marshal.FreeHGlobal(twCapability.Handle)
            Return strTemp
        Else
            ErrorStatus()
            Return vbNullString
        End If

    End Function

---

And here's how I call it:

Code:

---

            twCapability = New twsCapability
            With twCapability
                .Cap = twCap.IPhysicalWidth
                .ConType = twContainerTyoe.One  'TWON_DONTCARE16
                '.Handle = Nothing
            End With
            strWidth = GetOneValue(twCapability)

---

If I can get it to work I will post the class here for others to use. Any help is appreciated, thanks for looking.

Weclome to the forums! :wave:

When posting code, please make use of [CODE] tags, as explained here :

http://www.codeguru.com/forum/showthread.php?t=403073

Thank you

Hannes

Sorry, didn't see the option for [CODE] tags so I thought I wasn't authorized to use them.


I just searched the forum for marshaling but didn't find anything that matches my situation. I suspect it may be an issue marshaling data between managed code and unmanaged code.

Does anyone know how to get a member of a structure pointed to by a pointer in another structure that is allocated by unmanaged code and returned to managed code via a pointer? Can anyone understand my question? I think that is what I am doing wrong in the above code. I will post the structures.

I allocate this structure; Handle will point to a structure (TW_ONEVALUE) allocated by the unmanaged C dll (TWAIN_32.dll)

Code:

---

    <StructLayout(LayoutKind.Sequential, Pack:=4)> Private Structure twsCapability
        Public Cap As twCap        'short
        Public ConType As twContainerTyoe  'short
        Public Handle As IntPtr    'Pointer to a container
    End Structure

---

TWAIN_32.dll allocates this one

Code:

---

    <StructLayout(LayoutKind.Sequential, Pack:=4)> Friend Class TW_ONEVALUE
        Public ItemType As Short
        Public Item As Integer
    End Class

---

Any help is appreciated. Thanks for looking.

have you seen this?
http://www.planetsourcecode.com/vb/s...1551&lngWId=10

Thanks cimp for your reply. Unfortunately I have seen that. That is the most common solution posted to a number of forums, and rightly so, it does work. However, that solution uses the driver provided with the scanner for a user interface. The application that I am working on needs to hide the implementation from the user (the user will have other things to keep them occupied). As soon as I hide the UI, the scanner refuses to operate. Best I can get from the TWAIN spec is that some scanners require some settings to be set before they will "go". Since the UI requires the user to select the scan area, I presumed that the scanner requires this to be set. That's when I run into problems. I try to get the physical width and height, but get garbage. Structure TW_ONEVALUE is 8 bytes long (added Pack:=4 to make it 8 bytes because it was 6 and C++ reported its structure as 8).

This is the C struc that gets allocated by the dumb dll and gets a pointer to it put in the handle property of twsCapability

Code:

---

typedef struct {
  TW_UINT16  ItemType;
  TW_UINT32  Item;
} TW_ONEVALUE, FAR * pTW_ONEVALUE;

---

I tried viewing the bytes returned (for the width) using this:

Code:

---

BitConverter.GetBytes(Marshal.ReadInt64(twCapability.Handle))

---

and I got this:
{232, 28, 35, 0, 48, 1, 198, 3}
ran it again and got this:
{216, 252, 34, 0, 56, 1, 198, 3}

So either I am getting garbage back from the call to TWAIN, or I am doing something wrong with the Marshaling (I used Int64 to view the eight bytes belonging to the structure), I use this call to get a structure that I can use in VB:

Code:

---

Marshal.PtrToStructure(twCapability.Handle, twOneValue)

---

I get the same data back with either call to Marshal.whatever. Is there a different way to get the data?

I presume bytes 3 and 4 are the padding bytes? Does anyone know how VB.NET will store the structure in memory? How about C? Shouldn't they store the structures the same way? Can they be doing it different and that be the problem?

Thanks for looking

Never played too much with pointers, but to get the pointer of a structure
you might try

Code:

---

Dim ptr As IntPtr = Marshal.AllocHGlobal(Marshal.SizeOf(YourStructure))

---

while YourStructure.Handle is not a pointer for a structure, but only for a window

Code:

---

Public Handle As IntPtr    'Pointer to a container

---

and if I understood it correctly, the window is what you do not have here. This is a shot in the dark, of course...

Thanks Cimp, a shot in the dark is better than no shot at all :)

This is the first time I had to really deal with pointers myself, hence the difficulty. The C++ people don't know VB so they can't help and the VB people look up at the sky when I mention pointers.

But to respond to your suggestion (maybe you may have another one) I don't allocate the structure, twain_32.dll allocates TW_ONEVALUE. Here's the steps:

1) Call twain_32.dll passing it a MSG_Get to inform it that you are retrieving a value and in the same call send it a twsCapability structure with its Cap member set to the value of the capability you wish to retrive.
2) twain_32.dll will allocate a structure of the correct type, set the member ConType to indicate what kind of structure it allocated and set Handle to a reference to this container.
3) When the call returns, ConType should be examined to determine what kind of container you got, then you just get the data.

Sounds simple. Could there be any overhead bytes? Do I need to add an offset to the Handle?
I sure don't know!

I found out that a pointer to a structure is not the same as a pointer to the memory that a structure occupies. This won't work:

Code:

---

Marshal.PtrToStructure(twCapability.Handle, twOneValue)

---

But this works (explicitly copying members manually)

Code:

---

Private Function GetOneValueZ(ByVal twCapability As twsCapability) As String

        Dim rc As twRC
        Dim pOneValue As IntPtr
        Dim pPosition As Int32 = 0
        Dim strValue As String
        Dim twFix32 As twsFix32
        Dim twOneValue As TW_ONEVALUE


        rc = DS_Capability(mtwApp, mtwSource, twDG.Control, twDAT.Capability, twMSG.Get, twCapability)
        If rc = twRC.Success Then

            twOneValue = New TW_ONEVALUE
            pOneValue = GlobalLock(twCapability.Handle)
            twOneValue.ItemType = CType(Marshal.ReadInt16(pOneValue, pPosition), ItemType)
            pPosition += 2

            Select Case twOneValue.ItemType


                Case ItemType.Fix32
                    Dim origFrac As Int32
                    twFix32 = New twsFix32

                    twOneValue.Item = Marshal.ReadInt16(pOneValue, pPosition)
                    twFix32.Whole = twOneValue.Item
                    pPosition += 2


                    origFrac = Marshal.ReadInt16(pOneValue, pPosition)
                    If origFrac < 0 Then
                        origFrac += 65536
                    End If
                    pPosition += 2

                    'twFix32.Frac = CUShort(origFrac)
                    strValue = (twFix32.Whole + (origFrac / 65536)).ToString

                Case Else
                    MsgBox("Unexpected One Value.  Ignored")

            End Select

            GlobalUnlock(twCapability.Handle)

            Return strValue

        End If

    End Function

---

Yeah!!!

Quote:

---

Originally Posted by Marman

I found out that a pointer to a structure is not the same as a pointer to the memory that a structure occupies.!

---

oh, yes: structures are not like array: they are value type, not reference ones; thus a pointer to a structure means you should first box it, and then get the boxer pointer...