HOWTO: Retrieving Bitmap from Access and Displaying In Web Page

--------------------------------------------------------------------------------
The information in this article applies to:

Active Server Pages
Microsoft Visual Basic Professional and Enterprise Editions for Windows, versions 5.0, 6.0
ActiveX Data Objects (ADO), versions 1.0, 1.5, 2.0, 2.1 SP2, 2.5
Microsoft Internet Information Server versions 4.0, 5.0
Microsoft Data Access Components version 2.5

--------------------------------------------------------------------------------


SUMMARY
This article shows by example how to extract the bitmap photos in the Microsoft Access 97 Northwind.mdb
database, and view them from a Web browser using Active Server Pages (ASP). In order to accomplish this
task, an ActiveX DLL must be created that strips the Access and OLE headers from the field. This article
shows how to create this ActiveX DLL, and how to implement it.



MORE INFORMATION
WARNING: ANY USE BY YOU OF THE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this
code "as is" without warranty of any kind, either express or implied, including but not limited to the
implied warranties of merchantability and/or fitness for a particular purpose.

This article demonstrates how to use Visual Basic to retrieve a bitmap stored in an OLE Object field.
Because the definition of OLE object storage is not documented, the following code searches the object's
OLE header for characters consistent with the start of the graphic. This method may not work in all
circumstances.

Be aware that Internet Explorer 3.0 is unable to display true color bitmaps. For this reason, the bitmaps
stored in the Access database should be no higher than 256 colors.

Step-by-Step Example to Extract the Photos
Create a new project in Visual Basic and make the project an ActiveX DLL.


Add a reference to ActiveX Data Objects (ADO) by clicking the Project menu and selecting References.
Select "Microsoft OLE DB ActiveX Data Objects 1.0 Library" and click OK.


Add a new module to the project by selecting the Project menu and clicking Add Module. Select Module and
click Open.


Place the following code in the (general) (declarations) section of MODULE1.BAS:

      ' Enter the following Declare statement as one single line:
      Public Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory"
       (lpvDest As Any, lpvSource As Any, ByVal cbCopy As Long)

      Type PT
        Width As Integer
        Height As Integer
      End Type

      Type OBJECTHEADER
        Signature As Integer
        HeaderSize As Integer
        ObjectType As Long
        NameLen As Integer
        ClassLen As Integer
        NameOffset As Integer
        ClassOFfset As Integer
        ObjectSize As PT
        OleInfo As String * 256
      End Type



Place the following code in the (general) (declarations) section of CLASS1.CLS:

        Function DisplayBitmap(ByVal OleField As Variant)
        Dim Arr() As Byte
        Dim ObjHeader As OBJECTHEADER
        Dim Buffer As String
        Dim ObjectOffset As Long
        Dim BitmapOffset As Long
        Dim BitmapHeaderOffset As Integer
        Dim ArrBmp() As Byte
        Dim i As Long

        'Resize the array, then fill it with
        'the entire contents of the field
        ReDim Arr(OleField.ActualSize)
        Arr() = OleField.GetChunk(OleField.ActualSize)

        'Copy the first 19 bytes into a variable
        'of the OBJECTHEADER user defined type.
        CopyMemory ObjHeader, Arr(0), 19

        'Determine where the Access Header ends.
        ObjectOffset = ObjHeader.HeaderSize + 1

        'Grab enough bytes after the OLE header to get the bitmap header.
        Buffer = ""
        For i = ObjectOffset To ObjectOffset + 512
            Buffer = Buffer & Chr(Arr(i))
        Next i

        'Make sure the class of the object is a Paint Brush object
        If Mid(Buffer, 12, 6) = "PBrush" Then
            BitmapHeaderOffset = InStr(Buffer, "BM")
            If BitmapHeaderOffset > 0 Then

                'Calculate the beginning of the bitmap
                BitmapOffset = ObjectOffset + BitmapHeaderOffset - 1

                'Move the bitmap into its own array
                ReDim ArrBmp(UBound(Arr) - BitmapOffset)
                CopyMemory ArrBmp(0), Arr(BitmapOffset), UBound(Arr) -
                 BitmapOffset + 1

                'Return the bitmap
                DisplayBitmap = ArrBmp
            End If
        End If
      End Function



Rename the Project by selecting the Project menu, and clicking on "Project1 Properties" and type your new
name in the "Project Name" field. This example assumes that you named the project "MyProject" and will
refer to that name in future steps.


Make the project Apartment Model Threaded by selecting the "Unattended Execution" check box. Click OK.


Rename the Class in the Property Pane. This example assumes that you named the class "MyClass" and refers
to that name in future steps.


Compile the DLL by clicking the File menu and selecting "Make MyProject.dll."


Create an ASP page named "bitmap.asp" that contains the following code:

      <%@ LANGUAGE="VBSCRIPT" %>
      <%
      '   You need to set up a System DSN named 'NWind' that points to
      '   the Northwind.mdb database
      Set DataConn = Server.CreateObject("ADODB.Connection")
      DataConn.Open "DSN=NWind", "admin", ""
      Set cmdTemp = Server.CreateObject("ADODB.Command")
      Set RS = Server.CreateObject("ADODB.Recordset")
      cmdTemp.CommandText = "SELECT Photo FROM Employees
        WHERE EmployeeID = 1"
      cmdTemp.CommandType = 1
      Set cmdTemp.ActiveConnection = DataConn
      RS.Open cmdTemp, , 0, 1
      Response.ContentType = "image/bmp"
      Set Bitmap = Server.CreateObject("MyProject.MyClass")
      Response.BinaryWrite Bitmap.DisplayBitmap(RS("Photo"))
      RS.Close
      %>



Create an HTML page named "BitmapTest.htm" that contains the following code:

      <HTML>
 &n

[1] [2]  下一页