大家知道在VB下利用API函数Bitblt可以将屏幕或者窗口上的图象拷贝到VB中的PictureBox对象中,但是如果简单的利用PictureBox的SavePicture函数来保存图象，会发现什么也保存不了。这篇文章就是介绍如何捕获并利用Windows下的OLE API函数保存图象。
首先来看源程序，首先建立一个新的工程文件，然后在Form1中加入5个CommandButton对象和一个PictureBox对象，然后在Form1中加入以下代码：
Option Explicit
Option Base 0
Private Type PALETTEENTRY
peRed As Byte
peGreen As Byte
peBlue As Byte
peFlags As Byte
End Type
Private Type LOGPALETTE
palVersion As Integer
palNumEntries As Integer
palPalEntry(255) As PALETTEENTRY
End Type
Private Type GUID
Data1 As Long
Data2 As Integer
Data3 As Integer
Data4(7) As Byte
End Type
Private Const RASTERCAPS As Long = 38
Private Const RC_PALETTE As Long = &H100
Private Const SIZEPALETTE As Long = 104
Private Type RECT
Left As Long
Top As Long
Right As Long
Bottom As Long
End Type
Private Declare Function CreateCompatibleDC Lib "GDI32" (ByVal hDC As Long) As Long
Private Declare Function CreateCompatibleBitmap Lib "GDI32" (ByVal hDC As Long, _
ByVal nWidth As Long, ByVal nHeight As Long) As Long
Private Declare Function GetDeviceCaps Lib "GDI32" (ByVal hDC As Long, ByVal _
iCapabilitiy As Long) As Long
Private Declare Function GetSystemPaletteEntries Lib "GDI32" (ByVal hDC As Long, _
ByVal wStartIndex As Long, ByVal wNumEntries As Long, lpPaletteEntries _
As PALETTEENTRY) As Long
Private Declare Function CreatePalette Lib "GDI32" (lpLogPalette As LOGPALETTE) _
As Long
Private Declare Function SelectObject Lib "GDI32" (ByVal hDC As Long, ByVal hObject _
As Long) As Long
Private Declare Function BitBlt Lib "GDI32" (ByVal hDCDest As Long, ByVal XDest As _
Long, ByVal YDest As Long, ByVal nWidth As Long, ByVal nHeight As Long, _
ByVal hDCSrc As Long, ByVal XSrc As Long, ByVal YSrc As Long, ByVal dwRop _
As Long) As Long
Private Declare Function DeleteDC Lib "GDI32" (ByVal hDC As Long) As Long
Private Declare Function GetForegroundWindow Lib "USER32" () As Long
Private Declare Function SelectPalette Lib "GDI32" (ByVal hDC As Long, ByVal hPalette _
As Long, ByVal bForceBackground As Long) As Long
Private Declare Function RealizePalette Lib "GDI32" (ByVal hDC As Long) As Long
Private Declare Function GetWindowDC Lib "USER32" (ByVal hWnd As Long) As Long
Private Declare Function GetDC Lib "USER32" (ByVal hWnd As Long) As Long
Private Declare Function GetWindowRect Lib "USER32" (ByVal hWnd As Long, lpRect As _
RECT) As Long
Private Declare Function ReleaseDC Lib "USER32" (ByVal hWnd As Long, ByVal hDC As _
Long) As Long
Private Declare Function GetDesktopWindow Lib "USER32" () As Long
Private Type PicBmp
Size As Long
Type As Long
hBmp As Long
hPal As Long
Reserved As Long
End Type
Private Declare Function OleCreatePictureIndirect Lib "olepro32.dll" (PicDesc As _
PicBmp, RefIID As GUID, ByVal fPictureOwnsHandle As Long, IPic As IPicture) As Long
注释：捕捉整个屏幕
Private Sub Command1_Click()
Set Picture1.Picture = CaptureScreen()
End Sub
注释：在两秒钟后捕捉当前的活动窗口
Private Sub Command2_Click()
MsgBox "当你关闭这个对话框两秒钟之后程序会捕捉处于活动状态的窗口."
注释：等待两秒钟
Dim EndTime As Date
EndTime = DateAdd("s", 2, Now)
Do Until Now > EndTime
DoEvents
Loop
Set Picture1.Picture = CaptureActiveWindow()
Me.SetFocus
End Sub
Private Sub Command3_Click()
Set Picture1.Picture = Nothing
End Sub
Public Function CreateBitmapPicture(ByVal hBmp As Long, ByVal hPal As Long) As Picture
Dim r As Long
Dim Pic As PicBmp
Dim IPic As IPicture
Dim IID_IDispatch As GUID
注释：填充IDispatch界面
With IID_IDispatch
.Data1 = &H20400
.Data4(0) = &HC0
.Data4(7) = &H46
End With
注释：填充Pic
With Pic
.Size = Len(Pic)注释： Pic结构长度
.Type = vbPicTypeBitmap 注释： 图象类型
.hBmp = hBmp注释： 位图句柄
.hPal = hPal注释： 调色板句柄
End With
注释：建立Picture图象
r = OleCreatePictureIndirect(Pic, IID_IDispatch, 1, IPic)
注释：返回Picture对象
Set CreateBitmapPicture = IPic
End Function
Public Function CaptureWindow(ByVal hWndSrc As Long, ByVal Client As Boolean, ByVal _
LeftSrc As Long, ByVal TopSrc As Long, ByVal WidthSrc As Long, ByVal HeightSrc _
As Long) As Picture
Dim hDCMemory As Long
Dim hBmp As Long
Dim hBmpPrev As Long
Dim r As Long
Dim hDCSrc As Long
Dim hPal As Long
Dim hPalPrev As Long
Dim RasterCapsScrn As Long
Dim HasPaletteScrn As Long
Dim PaletteSizeScrn As Long
Dim LogPal As LOGPALETTE
If Client Then
hDCSrc = GetDC(hWndSrc)
Else
hDCSrc = GetWindowDC(hWndSrc)
End If
hDCMemory = CreateCompatibleDC(hDCSrc)
hBmp = CreateCompatibleBitmap(hDCSrc, WidthSrc, HeightSrc)
hBmpPrev = SelectObject(hDCMemory, hBmp)
注释：获得屏幕属性
RasterCapsScrn = GetDeviceCaps(hDCSrc, RASTERCAPS)
HasPaletteScrn = RasterCapsScrn And RC_PALETTE
PaletteSizeScrn = GetDeviceCaps(hDCSrc, SIZEPALETTE)
注释：如果屏幕对象有调色板则获得屏幕调色板
If HasPaletteScrn And (PaletteSizeScrn = 256) Then
注释：建立屏幕调色板的拷贝
LogPal.palVersion = &H300
LogPal.palNumEntries = 256
r = GetSystemPaletteEntries(hDCSrc, 0, 256, LogPal.palPalEntry(0))
hPal = CreatePalette(LogPal)
注释：将新建立的调色板选如建立的内存绘图句柄中
hPalPrev = SelectPalette(hDCMemory, hPal, 0)
r = RealizePalette(hDCMemory)
End If
注释：拷贝图象
r = BitBlt(hDCMemory, 0, 0, WidthSrc, HeightSrc, hDCSrc, LeftSrc, TopSrc, vbSrcCopy)
hBmp = SelectObject(hDCMemory, hBmpPrev)
If HasPaletteScrn And (PaletteSizeScrn = 256) Then
hPal = SelectPalette(hDCMemory, hPalPrev, 0)
End If
注释：释放资源
r = DeleteDC(hDCMemory)
r = ReleaseDC(hWndSrc, hDCSrc)
Set CaptureWindow = CreateBitmapPicture(hBmp, hPal)
End Function
注释：capturescreen函数捕捉整个屏幕图象
Public Function CaptureScreen() As Picture
Dim hWndScreen As Long
注释：获得桌面的窗口句柄
hWndScreen = GetDesktopWindow()
Set CaptureScreen = CaptureWindow(hWndScreen, False, 0, 0, Screen.Width _
\ Screen.TwipsPerPixelX, Screen.Height \ Screen.TwipsPerPixelY)
End Function
Public Function CaptureActiveWindow() As Picture
Dim hWndActive As Long
Dim r As Long
Dim RectActive As RECT
hWndActive = GetForegroundWindow()
r = GetWindowRect(hWndActive, RectActive)
Set CaptureActiveWindow = CaptureWindow(hWndActive, False, 0, 0, _
RectActive.Right - RectActive.Left, RectActive.Bottom - RectActive.Top)
End Function
Public Sub PrintPictureToFitPage(Prn As Printer, Pic As Picture)
Const vbHiMetric As Integer = 8
Dim PicRatio As Double
Dim PrnWidth As Double
Dim PrnHeight As Double
Dim PrnRatio As Double
Dim PrnPicWidth As Double
Dim PrnPicHeight As Double
If Pic.Height >= Pic.Width Then
Prn.Orientation = vbPRORPortrait
Else
Prn.Orientation = vbPRORLandscape
End If
PicRatio = Pic.Width / Pic.Height
PrnWidth = Prn.ScaleX(Prn.ScaleWidth, Prn.ScaleMode, vbHiMetric)
PrnHeight = Prn.ScaleY(Prn.ScaleHeight, Prn.ScaleMode, vbHiMetric)
PrnRatio = PrnWidth / PrnHeight
If PicRatio >= PrnRatio Then
PrnPicWidth = Prn.ScaleX(PrnWidth, vbHiMetric, Prn.ScaleMode)
PrnPicHeight = Prn.ScaleY(PrnWidth / PicRatio, vbHiMetric, Prn.ScaleMode)
Else
PrnPicHeight = Prn.ScaleY(PrnHeight, vbHiMetric, Prn.ScaleMode)
PrnPicWidth = Prn.ScaleX(PrnHeight * PicRatio, vbHiMetric, Prn.ScaleMode)
End If
Prn.PaintPicture Pic, 0, 0, PrnPicWidth, PrnPicHeight
End Sub
Private Sub Command4_Click()
CommonDialog1.DefaultExt = ".BMP"
CommonDialog1.Filter = "Bitmap Image (*.bmp)|*.bmp"
CommonDialog1.ShowSave
If CommonDialog1.FileName <> "" Then
SavePicture Picture1.Picture, CommonDialog1.FileName
End If
End Sub
Private Sub Command5_Click()
PrintPictureToFitPage Printer, Picture1.Picture
Printer.EndDoc
End Sub
Private Sub Form_Load()
Command1.Caption = "捕捉整个屏幕"
Command2.Caption = "两秒钟后捕捉活动窗口"
Command3.Caption = "清除图象"
Command4.Caption = "保存图象"
Command5.Caption = "打印图象"
End Sub
运行程序，点击command1或者Command2就可以捕捉成个屏幕或者窗口到Picture1中，然后点击Command4或者Command5就可以保存或打印图象。
上面的程序中最重要的是CaptureWindow函数以及CreateBitmapPicture函数，CaptureWindow函数建立与要捕捉的窗口的绘图设备（Device Context）句柄相兼容的绘图设备（Device Context）句柄，然后建立相应的调色板，最后将绘图设备（Device Context）中的图象拷贝到一个hBitmap对象句柄中。CreateBitmapPicture函数则根据传递过来的hBitmap对象句柄和调色板句柄建立一个Picture对象。在将这个对象赋予PictureBox的Picture属性，然后就可以使用SavePicture函数来保存图象了。
OleCreatePictureIndirect函数支持的不仅有BMP图象，同时也支持Ico图标，所以利用该函数和ExtractIcon函数也可以提取并保存Windows文件中的图标。有兴趣的读者可以到我的主页http://www.nease.net/~blackcat上下载源程序。