Special-Draw transparent picture - vb6

Question

got a PictureBox (called i_MC) and i draw a simple image (m_ImgMCN) on it doing:
Call i_MC.PaintPicture(m_ImgMCN, 0, 0, i_MC.width, i_MC.height)
now i would like to put a transparent image on this picture, on a specific position. i found a sample code, which does the job quite well with one problem: parts of the image that shouldn't be overdrawn with the 2nd (transparent) image are overdrawn with plain black.
the algo works perfectly if the background image from above is drawn by setting the Picture-property. cannot do this because this does not allow any stretching.
the transparent image is a simple image smaller than the box containing a color that is masked. i've used the following sample code (.AutoRedraw=true for all boxes and .ScaleMode=3 'Pixel):
Option Explicit
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 CreateBitmap Lib "gdi32" (ByVal nWidth _
As Long, ByVal nHeight As Long, ByVal nPlanes As Long, _
ByVal nBitCount As Long, lpBits As Any) As Long
Private Declare Function SetBkColor Lib "gdi32" (ByVal hdc As _
Long, ByVal crColor As Long) As Long
Private Declare Function SelectObject Lib "gdi32" (ByVal hdc As _
Long, ByVal hObject 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 CreateCompatibleDC Lib "gdi32" (ByVal hdc _
As Long) As Long
Private Declare Function DeleteDC Lib "gdi32" (ByVal hdc As Long) _
As Long
Private Declare Function DeleteObject Lib "gdi32" (ByVal hObject _
As Long) As Long
Private Type RECT
Left As Long
Top As Long
Right As Long
Bottom As Long
End Type
Dim R As RECT
Private Sub TranspPic(OutDstDC&, DstDC&, SrcDC&, SrcRect _
As RECT, ByVal DstX&, ByVal DstY&, _
TransColor&)
Dim Result&, W&, H&
Dim MonoMaskDC&, hMonoMask&, MonoInvDC&, hMonoInv&
Dim ResultDstDC&, hResultDst&, ResultSrcDC&, hResultSrc&
Dim hPrevMask&, hPrevInv&, hPrevSrc&, hPrevDst&
W = SrcRect.Right - SrcRect.Left
H = SrcRect.Bottom - SrcRect.Top
'Generieren einer Monochromen & einer inversen Maske
MonoMaskDC = CreateCompatibleDC(DstDC)
MonoInvDC = CreateCompatibleDC(DstDC)
hMonoMask = CreateBitmap(W, H, 1, 1, ByVal 0&)
hMonoInv = CreateBitmap(W, H, 1, 1, ByVal 0&)
hPrevMask = SelectObject(MonoMaskDC, hMonoMask)
hPrevInv = SelectObject(MonoInvDC, hMonoInv)
'Puffer erstellen
ResultDstDC = CreateCompatibleDC(DstDC)
ResultSrcDC = CreateCompatibleDC(DstDC)
hResultDst = CreateCompatibleBitmap(DstDC, W, H)
hResultSrc = CreateCompatibleBitmap(DstDC, W, H)
hPrevDst = SelectObject(ResultDstDC, hResultDst)
hPrevSrc = SelectObject(ResultSrcDC, hResultSrc)
'Sourcebild in die monochrome Maske kopieren
Dim OldBC As Long
OldBC = SetBkColor(SrcDC, TransColor)
Result = BitBlt(MonoMaskDC, 0, 0, W, H, SrcDC, _
SrcRect.Left, SrcRect.Top, vbSrcCopy)
TransColor = SetBkColor(SrcDC, OldBC)
'Inverse Maske erstellen
Result = BitBlt(MonoInvDC, 0, 0, W, H, _
MonoMaskDC, 0, 0, vbNotSrcCopy)
'Hintergrund des Zielbildes auslesen
Result = BitBlt(ResultDstDC, 0, 0, W, H, _
DstDC, DstX, DstY, vbSrcCopy)
'AND mit der Maske
Result = BitBlt(ResultDstDC, 0, 0, W, H, _
MonoMaskDC, 0, 0, vbSrcAnd)
'Überlappung des Sourcebildes mit dem Zielbild auslesen
Result = BitBlt(ResultSrcDC, 0, 0, W, H, SrcDC, _
SrcRect.Left, SrcRect.Top, vbSrcCopy)
'AND mit der invertierten, monochromen Maske
Result = BitBlt(ResultSrcDC, 0, 0, W, H, _
MonoInvDC, 0, 0, vbSrcAnd)
'XOR mit beiden
Result = BitBlt(ResultDstDC, 0, 0, W, H, _
ResultSrcDC, 0, 0, vbSrcInvert)
'Ergebnis in das Zielbild kopieren
Result = BitBlt(OutDstDC, DstX, DstY, W, H, _
ResultDstDC, 0, 0, vbSrcCopy)
'Erstellte Objekte & DCs wieder freigeben
hMonoMask = SelectObject(MonoMaskDC, hPrevMask)
DeleteObject hMonoMask
DeleteDC MonoMaskDC
hMonoInv = SelectObject(MonoInvDC, hPrevInv)
DeleteObject hMonoInv
DeleteDC MonoInvDC
hResultDst = SelectObject(ResultDstDC, hPrevDst)
DeleteObject hResultDst
DeleteDC ResultDstDC
hResultSrc = SelectObject(ResultSrcDC, hPrevSrc)
DeleteObject hResultSrc
DeleteDC ResultSrcDC
End Sub
Private Sub MovePicTo(ByVal X&, ByVal Y&)
i_MC.Cls
picSrc.Picture = m_ImgMCN
With R
.Left = 0
.Top = 0
.Right = Picture2.ScaleWidth
.Bottom = Picture2.ScaleHeight
End With
Call TranspPic(i_MC.hdc, i_MC.hdc, picSrc.hdc, R, X, Y, vbWhite)
i_MC.Refresh
DoEvents
End Sub
this code originally resides on activevb.de, i modified it a little bit without changing the algorithm or functionality. i may post a link to an original article.
without success, I've tried to modify the sizes for the different intermediate pictures, but it keeps painting the image wrong:
the part of the image where the transparent picture is drawn is correct, the background is included. the rest of the picture (which shouldn't be touched by the algo) is overwritten with black.
any idea is appreciated. an algorithm to paint 24-bit alphablended images would be fine as well! I've googled quite long and didn't find a working piece of code.
PS: this is plain old VB6, moving to .NET or any other language is unfortunately not an option.
thanks in advance and best regards

Answer 1

damn. a friend of mine gave me the tip using the TransparentBlt (MSDN)-Function from WinAPI. works now quite well. thanks to those who took a look at it.