How To Retrieve Bitmap from Access and Display It in Web Page

Summary

This article shows by example how to extract the bitmap photos in the Microsoft Access 97 Nwind.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

  1. Create a new project in Visual Basic and make the project an ActiveX DLL.
  2. 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.
  3. Add a new module to the project by selecting the Project menu and clicking Add Module. Select Module and click Open.
  4. 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
  5. 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
  6. 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.
  7. Select the "Unattended Execution" check box. Click OK.
  8. Rename the Class in the Property Pane. This example assumes that you named the class "MyClass" and refers to that name in future steps.
  9. Compile the DLL by clicking the File menu and selecting "Make MyProject.dll."
  10. 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
    %>
  11. Create an HTML page named "BitmapTest.htm" that contains the following code:
          <HTML>
    <HEAD>
    <TITLE>Bitmap Test</TITLE>
    </HEAD>
    <BODY>
    <HR>
    <img src="Bitmap.asp">
    <HR>
    </BODY>
    </HTML>

References

For additional information, please see the following article in the Microsoft Knowledge Base:

173308 How To Displaying Images Stored in a BLOB Field

For the latest Knowledge Base artices and other support information on Visual InterDev and Active Server Pages, see the following page on the Microsoft Technical Support site:


Propiedades

Id. de artículo: 175261 - Última revisión: 05/02/2006 - Revisión: 1

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