VB Forum / General 2 / June 2004

I'll try to keep this as short and to the point as I can...
I'm doing a rewrite of some classes I downloaded from VBAdvance, image
processing...
I'm not all that familure using the CopyMemory API (RtlMoveMemory) and have
gotten confused as to just what in the hex is happening here...

First: Here is one (shortest) of the subs/functions that use CopyMemory...
After the code I have layed out the question(s)...

***Start Code***
Public Sub PerformCompositeClear()
' Both the color and the alpha of the destination are cleared.
' Neither the source nor the destination are used as input.
'  Dc' = 0
'  Da ' = 0
 
Dim srcX As Long
Dim srcY As Long
Dim srcWidth As Long
Dim srcHeight As Long
Dim bDibSrc() As Byte
Dim bDibDst() As Byte
Dim tSASrc As SAFEARRAY2D
Dim tSADst As SAFEARRAY2D
Dim x As Long
Dim y As Long
Dim xEnd As Long
Dim yEnd As Long
 
  If getSourceDimensions(srcX, srcY, srcWidth, srcHeight) Then
 
     ' Get the bits in the from DIB section:
     With tSASrc
         .cbElements = 1
         .cDims = 2
         .Bounds(0).lBound = 0
         .Bounds(0).cElements = m_cDibSrc.height
         .Bounds(1).lBound = 0
         .Bounds(1).cElements = m_cDibSrc.BytesPerScanLine
         .pvData = m_cDibSrc.DIBSectionBitsPtr
     End With
     CopyMemory ByVal VarPtrArray(bDibSrc()), VarPtr(tSASrc), 4
 
     ' Get the bits in the from DIB section:
     With tSADst
         .cbElements = 1
         .cDims = 2
         .Bounds(0).lBound = 0
         .Bounds(0).cElements = m_cDibDst.height
         .Bounds(1).lBound = 0
         .Bounds(1).cElements = m_cDibDst.BytesPerScanLine()
         .pvData = m_cDibDst.DIBSectionBitsPtr
     End With
     CopyMemory ByVal VarPtrArray(bDibDst()), VarPtr(tSADst), 4
 
     xEnd = srcX + srcWidth - 1
     yEnd = srcY + srcHeight - 1
     
     For x = srcX To xEnd Step 4
        For y = srcY To yEnd
           bDibDst(x, y) = 0
           bDibDst(x + 1, y) = 0
           bDibDst(x + 2, y) = 0
           bDibDst(x + 3, y) = 0
        Next y
     Next x
     
     CopyMemory ByVal VarPtrArray(bDibDst), 0&, 4
     CopyMemory ByVal VarPtrArray(bDibSrc), 0&, 4
 
  End If
 
End Sub
***End Code***

1) m_cDibSrc is another Class that holds the DIBSection... Not a problem...
2) Doesn't 'CopyMemory ByVal VarPtrArray(bDibDst()), VarPtr(tSADst), 4' just
copy the contents of the DIBSection into bDibDst()??? ie: Different memory
location...
3) The For...Next... loops modifies this new area of the memory, correct???
4) Doesn't 'CopyMemory ByVal VarPtrArray(bDibDst), 0&, 4' just clear this newly
created memory area???

Ok so what am I missing here??? How does the modified data get back to the
original memory location???

Have a good day...

Don