请大家帮忙看看 图像局部放大的程序，为什么结果不对呢,该怎么解决

请大家帮忙看看 图像局部放大的程序，为什么结果不对呢
Private   Declare   Function   BitBlt   Lib   "gdi32 "   (ByVal   hDestDC   As   Long,   ByVal   x   As   Long,   ByVal   y   As   Long,   ByVal   nWidth   As   Long,   ByVal   nHeight   As   Long,   ByVal   hSrcDC   As   Long,   ByVal   xSrc   As   Long,   ByVal   ySrc   As   Long,   ByVal   dwRop   As   Long)   As   Long
  Private   Declare   Function   StretchBlt   Lib   "gdi32 "   (ByVal   hdc   As   Long,   ByVal   x   As   Long,   ByVal   y   As   Long,   ByVal   nWidth   As   Long,   ByVal   nHeight   As   Long,   ByVal   hSrcDC   As   Long,   ByVal   xSrc   As   Long,   ByVal   ySrc   As   Long,   ByVal   nSrcWidth   As   Long,   ByVal   nSrcHeight   As   Long,   ByVal   dwRop   As   Long)   As   Long
Dim   exp_x0,   exp_y0,   k,   flag

 
Sub   Form_Load()
 
disppic.AutoRedraw   =   True
 
disppic.AutoSize   =   True
 
disppic.Picture   =   LoadPicture( "C:\Documents   and   Settings\temp.bmp ")
 
disppic.ScaleMode   =   3   -   pixel
 
backpic.AutoRedraw   =   True
 
backpic.AutoSize   =   True
 
backpic.Picture   =   LoadPicture( "C:\temp.bmp ")
 
backpic.ScaleMode   =   3   -   pixel
 
backpic.Visible   =   False
 
End   Sub
 
'当鼠标按下时:
 
Private   Sub   disppic_MouseDown(Button   As   Integer,   Shift   As   Integer,   x   As   Single,   y   As   Single)
 
flag   =   1
 
exp_x0   =   x
 
exp_y0   =   y
 
disppic.DrawStyle   =   4   '（图象框disppic的绘图模式设置为虚线）
 
End   Sub
 
'当鼠标拖动时:
 
Private   Sub   disppic_MouseMove(Button   As   Integer,   Shift   As   Integer,   x   As   Single,   y   As   Single)
 
If   flag   =   1   Then
 
k   =   BitBlt(disppic.hdc,   0,   0,   disppic.Width,   disppic.Height,   backpic.hdc,   0,   0,   &HCC0020)
'（把前一次的虚线框擦掉）
 
disppic.Line   (exp_x0,   exp_y0)-(x,   y),   RGB(0,   0,   0),   B   '（化虚线框）
 
End   If
 
End   Sub
 
'当鼠标被放开时:
 
Private   Sub   disppic_MouseUp(Button   As   Integer,   Shift   As   Integer,   x   As   Single,   y   As   Single)
 
flag   =   0
 
k   =   StretchBlt(disppic.hdc,   0,   0,   disppic.Width,   disppic.Height,   backpic.hdc,   exp_x0,   exp_y0,   x   -   exp_x0,   y   -   exp_y0,   &HCC0020)   '（将虚线框内的图象放大到整个图象框）
 
disppic.Refresh
 
k   =   BitBlt(backpic.hdc,   0,   0,   backpic.Width,   backpic.Height,   disppic.hdc,   0,   0,   &HCC0020)