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4 vbMedia &nbsp
 Using DIB Sections in VB
 An Easy to Use Class for manipulating DIBSections plus a blindingly quick technique for updating the bits

DIB Section Graphic

Download the cDIBSection Class (4kb)

&nbsp UpdatedUpdated! 17 February 1999 &nbsp
&nbsp The previous version of this code crashed when run under NT4.0. This was because the code was not always clearing up the SAFEARRAY pointer after it had finished using it. Under Win9x, this did not cause a problem but NT is stricter. All the samples and the code are now updated to fix this problem. &nbsp

This is a supporting article describing in more detail the DIB Section techniques used in the vbAccelerator Image Processor. It describes what DIB Sections are, how to use them and provides a class I wrote to wrap up the DIB Section.

What is a DIB Section?
A DIB (Device Independent Bitmap) Section is a GDI object like a standard DIB but which additionally provides a pointer to the memory used to store the bitmap bits to which the creating application has direct access. This allows ultimate flexibility if you want to modify the bits of a bitmap.

Using DIB Sections
A DIB section is created using the GDI CreateDIBSection call. You need to modify the declare provided for this in the VB API guide because this declare assumes you cannot use the pointer to the bitmap returned by the function and simply discards it. Here are the declares you need:

Private Type BITMAPINFOHEADER '40 bytes
&nbsp &nbsp biSize As Long
&nbsp &nbsp biWidth As Long
&nbsp &nbsp biHeight As Long
&nbsp &nbsp biPlanes As Integer
&nbsp &nbsp biBitCount As Integer
&nbsp &nbsp biCompression As Long
&nbsp &nbsp biSizeImage As Long
&nbsp &nbsp biXPelsPerMeter As Long
&nbsp &nbsp biYPelsPerMeter As Long
&nbsp &nbsp biClrUsed As Long
&nbsp &nbsp biClrImportant As Long
End Type
&nbsp &nbsp bmiHeader As BITMAPINFOHEADER
&nbsp &nbsp bmiColors As RGBQUAD
End Type

' Note - this is not the declare in the API viewer - modify lplpVoid to be
' Byref so we get the pointer back:
Private Declare Function CreateDIBSection Lib "gdi32" _
&nbsp &nbsp (ByVal hdc As Long, _
&nbsp &nbsp pBitmapInfo As BITMAPINFO, _
&nbsp &nbsp ByVal un As Long, _
&nbsp &nbsp lplpVoid As Long, _
&nbsp &nbsp ByVal handle As Long, _
&nbsp &nbsp ByVal dw As Long) As Long

To create the DIB Section, you initialise a BITMAPINFO structure with the required fields, which are all in the bmiHeader sub-structure:

Member Required Value
biSize Size of the BITMAPINFO structure
biWidth Width of the DIBSection in pixels
biHeight Height of the DIBSection in pixels
biPlanes Number of colour planes. Set to 1
biBitCount Bits per pixel. Set to 24 for true colour
biCompression Whether to use compression. If you want to work on the bits, set this to BI_RGB so the image is uncompressed
biSizeImage The size of the image in bytes. This is worked out from the width, height, number of bits per pixel. In a 24 bit image there are three bytes per pixel. Additionally, GDI requires that every horizontal line in the image aligns on a four byte boundary. So for a 24 bit image the ImageSize is biWidth*biHeight*3 rounded up to the nearest four bytes. You can round up to the width to the nearest four bytes as follows:
&nbsp &nbsp (.biWidth * 3 + 3) And &HFFFFFFFC

This allows you to create a DIB Section. You call CreateDIBSection like this:

&nbsp &nbsp hDib = CreateDIBSection( _
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp lHDC, _
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp m_tBI, _
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp DIB_RGB_COLORS, _
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp m_lPtr, _
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp 0, 0)


  • hDib is a variable to receive the GDI handle to the DIB Section
  • lHDC is a valid DC, for example a Form's DC or the desktop DC
  • m_tBI is a the BITMAPINFO structure
  • m_lPtr is a variable to receive the pointer to the memory containing the bitmap bits.

To actually display a DIB Section, you must select it into a DC.

&nbsp &nbsp m_hDC = CreateCompatibleDC(0)
&nbsp &nbsp If (m_hDC 0) Then
&nbsp &nbsp &nbsp &nbsp If (CreateDIB(m_hDC, lWidth, lHeight, m_hDIb)) Then
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp m_hBmpOld = SelectObject(m_hDC, m_hDIb)
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp Create = True
&nbsp &nbsp &nbsp &nbsp Else
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp DeleteObject m_hDC
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp m_hDC = 0
&nbsp &nbsp &nbsp &nbsp End If
&nbsp &nbsp End If

Once it is in a DC you can then use BitBlt to paint it to another device context or to transfer the contents of another device context into it. Remember you must keep track of all the handles created in GDI so you can clear them up again when the DIBSection is no longer needed. To clear up, you need to:

  • Select the old bitmap (m_hBmpOld) back into the DC.
  • Delete the DIB section.
  • Delete the DC.
So far this has created a DIB which you can load with a graphic and display on the screen, but this achieves no more than you can do with a standard bitmap. The good stuff starts when you start modifying the bitmap bits.

Modifying the Bitmap Bits Directly
CreateDIBSection returns an address to the memory containing the bitmap. You can manipulate this directly through VB using a cool technique to make the memory look like a VB Byte array. This technique was originally presented in VBPJ article (Note the original article is no longer easily available). It uses a hidden VB call exposed by MSVBVM50.DLL (which is also available in VB6 - thanks to Paul Wilde for pointing this out) and the ubiquitous CopyMemory call. In my opinion, “CopyMemory” is the best language feature in VB (except that it isn't VB at all!)

Here are the declares you need:

Private Type SAFEARRAY2D
&nbsp &nbsp cDims As Integer
&nbsp &nbsp fFeatures As Integer
&nbsp &nbsp cbElements As Long
&nbsp &nbsp cLocks As Long
&nbsp &nbsp pvData As Long
&nbsp &nbsp Bounds(0 To 1) As SAFEARRAYBOUND
End Type
Private Declare Function VarPtrArray Lib "msvbvm50.dll" Alias "VarPtr" (Ptr() As Any) As Long
Private Declare Sub CopyMemory Lib "kernel32" Alias "RtlMoveMemory" ( _
&nbsp &nbsp lpvDest As Any, lpvSource As Any, ByVal cbCopy As Long)

To make the byte array point to the memory, you have to fill in the SAFEARRAY2D structure and then use CopyMemory as follows:

Dim bDib() As Byte

&nbsp &nbsp
' Get the bits in the from DIB section:
&nbsp &nbsp With tSA
&nbsp &nbsp &nbsp &nbsp .cbElements = 1
&nbsp &nbsp &nbsp &nbsp .cDims = 2
&nbsp &nbsp &nbsp &nbsp .Bounds(0).lLbound = 0
&nbsp &nbsp &nbsp &nbsp
' Height of the bitmap
&nbsp &nbsp &nbsp &nbsp .Bounds(0).cElements = m_tBI.bmiHeader.biHeight
&nbsp &nbsp &nbsp &nbsp .Bounds(1).lLbound = 0
&nbsp &nbsp &nbsp &nbsp
' Width of the bitmap in bits (see earlier):
&nbsp &nbsp &nbsp &nbsp .Bounds(1).cElements = BytesPerScanLine()
&nbsp &nbsp &nbsp &nbsp .pvData = m_lPtr
&nbsp &nbsp End With
&nbsp &nbsp
' Make the bDib() array point to the memory addresses:
&nbsp &nbsp CopyMemory ByVal VarPtrArray(bDib()), VarPtr(tSA), 4

Now the bDib() array is a two dimensional array with the first dimension being the x values and the second being the y values. A 24 bit DIB section is arranged so the bytes run Blue, Green, Red and remember that since the array is padded to a DWORD boundary there may be up to three unused bytes at the end of each row. So, for example, to set the top left pixel to purple you would write this:

&nbsp &nbsp &nbsp &nbsp bDib(0,0) = 255 ' Blue
&nbsp &nbsp &nbsp &nbsp bDib(1, 0) = 0 ' Green
&nbsp &nbsp &nbsp &nbsp bDib(2, y) = 255 ' Red

Once you have finished with the bDib array, you need to be sure to clear up the SAFEARRAY pointer you have created. If you fail to do this, your code will work under Win9x but will crash NT4.0:

&nbsp &nbsp CopyMemory ByVal VarPtrArray(bDib), 0&, 4

Enough of That, I Want to Use It
That covers the theory of using DIB Sections. To make it easy, I include a self-contained class (cDibSection) which you can include. The implementation is as follows:

Method Description
BytesPerScanLine Returns the number of bytes horizontally, taking into account the bits per pixel and 4 byte boundary padding.
ClearUp Frees up any GDI objects held by the class. Called automatically when the class terminates.
CopyToClipboard Does as it says!
Create Creates a DIB section of the specified width and height in pixels.
CreateFromPicture Creates a DIB section the same size as a VB picture object and copies the bitmap in it into the DIB section.
DIBSectionBitsPtr Returns the address of the DIBSection's bits in memory.
hdc Returns the memory device context used by the class to hold the DIB Section. You can use this in GDI operations, but do not call DeleteObject on it.
hDib Returns a handle to the DIBSection held by the class. You can use this in GDI operations, but do not call DeleteObject on it.
Height Returns the Height of the DIBSection in pixels.
LoadPictureBlt Copies all or a part of a picture from another Device Context into the DIB section.
PaintPicture Similar to the VB paint picture method, this copies all or part of the DIB section to another device context using the specified Raster Operation.
RandomiseBits Randomises the pixels in the DIB Section, either to random colours or gray scaled.
Resample Resizes the DIB using linear interpolation to create a smoother resized version than you would get if you used StretchBlt.
Width Returns the width of the DIB in pixels.

A Simple Fade Example
This demonstrates how to fade out a bitmap. It should run as a real-time animation, provided the image size isn't too big. I've found that images which are too large don't show as a smooth animation even when the fade code runs quickly enough because BitBlt tends to "tear". This occurs because BitBlt doesn't completely display the bitmap during a single screen refresh and therefore the image is partially displayed before the refresh occurs. To get round this problem you need to use DirectX.

This sample is simplified version of the static and fade example available for download on the Image Processing using DIB Sections page.

To try this sample, create a new project and add the cDIBSection class to it. Copy the declares for CopyMemory, VarPtrArray and SAFEARRAY2D into the project's form.

Then add this sub:

Private Sub Fade( _
&nbsp &nbsp ByRef cTo As cDIBSection, _
&nbsp &nbsp ByVal lAmount As Long _
&nbsp &nbsp )
Dim bDib() As Byte
Dim x As Long, y As Long
Dim xMax As Long, yMax As Long
Dim lB As Long, lG As Long, lR As Long
Dim lA As Long, lA2 As Long
Dim lTIme As Long
&nbsp &nbsp
&nbsp &nbsp
' have the local matrix point to bitmap pixels
&nbsp &nbsp With tSA
&nbsp &nbsp &nbsp &nbsp .cbElements = 1
&nbsp &nbsp &nbsp &nbsp .cDims = 2
&nbsp &nbsp &nbsp &nbsp .Bounds(0).lLbound = 0
&nbsp &nbsp &nbsp &nbsp .Bounds(0).cElements = cTo.Height
&nbsp &nbsp &nbsp &nbsp .Bounds(1).lLbound = 0
&nbsp &nbsp &nbsp &nbsp .Bounds(1).cElements = cTo.BytesPerScanLine
&nbsp &nbsp &nbsp &nbsp .pvData = cTo.DIBSectionBitsPtr
&nbsp &nbsp End With
&nbsp &nbsp CopyMemory ByVal VarPtrArray(bDib), VarPtr(tSA), 4
&nbsp &nbsp &nbsp &nbsp
&nbsp &nbsp yMax = cTo.Height - 1
&nbsp &nbsp xMax = cTo.Width - 1
&nbsp &nbsp
&nbsp &nbsp For x = 0 To (xMax * 3) Step 3
&nbsp &nbsp &nbsp &nbsp For y = 0 To yMax
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp lB = lAmount * bDib(x, y) \ 255
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp lG = lAmount * bDib(x + 1, y) \ 255
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp lR = lAmount * bDib(x + 2, y) \ 255
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp bDib(x, y) = lB
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp bDib(x + 1, y) = lG
&nbsp &nbsp &nbsp &nbsp &nbsp &nbsp bDib(x + 2, y) = lR
&nbsp &nbsp &nbsp &nbsp Next y
&nbsp &nbsp Next x

&nbsp &nbsp CopyMemory ByVal VarPtrArray(bDib), 0&, 4
&nbsp &nbsp
End Sub

Add a Command Button to the Form, and put this code behind it:

Private Sub Command1_Click()

Dim cDib As New cDibSection
Dim cDibBuffer as New cDibSection
Dim i As Long

&nbsp &nbsp
' Load the picture to fade:
&nbsp &nbsp Set sPic = LoadPicture("Put Your File Here!")
&nbsp &nbsp cDib.CreateFromPicture sPic

&nbsp &nbsp
' Create a copy of it:
&nbsp &nbsp cDibBuffer.Create cDib.Width, cDib.Height
&nbsp &nbsp cDib.PaintPicture cDibBuffer.HDC

&nbsp &nbsp
' Fade Loop:
&nbsp &nbsp For i = 0 To 255 Step 4
&nbsp &nbsp &nbsp &nbsp
' Fade the dib by amount i:
&nbsp &nbsp &nbsp &nbsp Fade cDib, i
&nbsp &nbsp &nbsp &nbsp
' Draw it:
&nbsp &nbsp &nbsp &nbsp cDib.PaintPicture Form1.hDC
&nbsp &nbsp &nbsp &nbsp
' Breathe a little. You may have to put a slowdown here:
&nbsp &nbsp &nbsp &nbsp DoEvents
&nbsp &nbsp &nbsp &nbsp
' Reset for next fade:
&nbsp &nbsp &nbsp &nbsp cDibBuffer.PaintPicture cDib.HDC
&nbsp &nbsp Next i

End Sub

Now run the code. When you click the button, the image will be faded as fast as your system allows. The code will run slowly in the IDE but will go much much quicker if you compile it to native code. Also, checking all the Advanced Optimisation settings will make it run about 60% faster again! On my machine (PII 266 with 8Mb Xpert@Work card) it does 40fps when working on a 256x256 pixel image.

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Copyright © 1998-1999, Steve McMahon ( All Rights Reserved.
Last updated: 25 August 1999