The program's picMask PictureBox contains a mask of the image to be dragged. Pixels that should be drawn are black and pixels that should let the background show through are white.

In the MouseDown event handler, the program subtracts the overlay picture's current position from the mouse's coordinates to see where on the overlay picture the user clicked. If the corresponding mask pixel is white, the user is pressing where the background should show through so the event is ignored.

If the corresponding mask pixel is black, the user is pressing the mouse on part of the image and the program begins a drag. In that case, the program determines whether it should drag only an outline and saves the result in the Boolean UseOutline.

' See if the mouse is over a point corresponding
' to a black part of the mask.
Private Sub picCanvas_MouseDown(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    OffsetX = PicX - X
    OffsetY = PicY - Y
    If picMask.Point(-OffsetX, -OffsetY) <> vbBlack Then _
        Exit Sub
    
    ' Start dragging.
    UseOutline = (chkUseOutline.Value = vbChecked)
    If UseOutline Then DrawOutline
    Dragging = True
End Sub

The event handler then determines where the picture should be placed and calls DrawOutline or DrawPic to draw the outline or picture as appropriate.

' Drag.
Private Sub picCanvas_MouseMove(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    If Not Dragging Then Exit Sub
    
    If UseOutline Then DrawOutline

    PicX = X + OffsetX
    If PicX < 0 Then
        PicX = 0
    ElseIf PicX > CanvasWid - PicWid Then
        PicX = CanvasWid - PicWid
    End If
    
    PicY = Y + OffsetY
    If PicY < 0 Then
        PicY = 0
    ElseIf PicY > CanvasHgt - PicHgt Then
        PicY = CanvasHgt - PicHgt
    End If
    
    If UseOutline Then
        DrawOutline
    Else
        DrawPic
    End If
End Sub

Subroutine DrawPic restores the background picture by copying it from the hidden PictureBox picBackground onto the visible PictureBox picCanvas. It then uses BitBlt to copy the overlay mask onto picCanvas using the opcode MERGEPAINT. This knocks out the mask's outline on the background so a white image of the overlay appears.

Next the code copies the overlay image onto the background using the SRCAND opcode. This combines the pixel values of the overlay image with the background image. The overlay image is white where it should not show and the background image is white where the overlay should show. The result is the overlay is dropped into its correct position.

' Draw an outline of the picture.
Private Sub DrawOutline()
    picCanvas.Line _
        (PicX, PicY)-Step(PicWid, PicHgt), , B
End Sub

' Draw the picture.
Private Sub DrawPic()
    picCanvas.Picture = picBackground.Picture
    BitBlt picCanvas.hDC, _
        PicX, PicY, PicWid, PicHgt, _
        picMask.hDC, _
        0, 0, MERGEPAINT
    BitBlt picCanvas.hDC, _
        PicX, PicY, PicWid, PicHgt, _
        picImage.hDC, _
        0, 0, SRCAND
    picCanvas.Picture = picCanvas.Image
End Sub

If the user is not dragging only an outline, the picture is already in its new position.

' Stop dragging.
Private Sub picCanvas_MouseUp(Button As Integer, Shift As _
    Integer, X As Single, Y As Single)
    If UseOutline Then DrawPic
    Dragging = False
End Sub

See also:

• Overlay a picture on another and use PaintPicture allow the user to drag it
• Overlay a picture on another and use BitBlt to repaint only the modified parts of the background
• Let the user drag an picture with the mouse or move it with the arrow keys