关于图形转动

Const SRCCOPY = &HCC0020
               Const Pi = 3.14159265359
               Private Declare Function SetPixel Lib "GDI32" (ByVal hDC As Integer, ByVal X As Integer, ByVal Y As Integer, ByVal crColor As Long) As Long
               Private Declare Function GetPixel Lib "GDI32" (ByVal hDC As Integer, ByVal X As Integer, ByVal Y As Integer) As Long
               Private Declare Function StretchBlt% Lib "GDI32" (ByVal hDC%, ByVal X%, ByVal Y%, ByVal nWidth%, ByVal nHeight%, ByVal hSrcDC%, ByVal XSrc%, ByVal YSrc%, ByVal nSrcWidth%, ByVal nSrcHeight%, ByVal dwRop&)               Sub Form_Load()
                Picture1.ScaleMode = 3
                Picture2.ScaleMode = 3
               End Sub
               Sub Command1_Click()
               'flip horizontal
                Picture2.Cls
                px% = Picture1.ScaleWidth
                py% = Picture1.ScaleHeight
                retval% = StretchBlt(Picture2.hDC, px%, 0, -px%, py%, Picture1.hDC, 0, 0, px%, py%, SRCCOPY)
               End Sub
               Sub Command2_Click()
                'flip vertical
                Picture2.Cls
                px% = Picture1.ScaleWidth
                py% = Picture1.ScaleHeight
                retval% = StretchBlt(Picture2.hDC, 0, py%, px%, -py%, Picture1.hDC, 0, 0, px%, py%, SRCCOPY)
               End Sub               Sub Command3_Click()
                'rotate 45 degrees
                Picture2.Cls
                Call bmp_rotate(Picture1, Picture2, 3.14 / 4)
               End Sub
               Sub bmp_rotate(pic1 As PictureBox, pic2 As PictureBox, ByVal theta!)
                ' bmp_rotate(pic1, pic2, theta)
                ' Rotate the image in a picture box.
                '   pic1 is the picture box with the bitmap to rotate
                '   pic2 is the picture box to receive the rotated bitmap
                '   theta is the angle of rotation
                Dim c1x As Integer, c1y As Integer
                Dim c2x As Integer, c2y As Integer
                Dim a As Single
                Dim p1x As Integer, p1y As Integer
                Dim p2x As Integer, p2y As Integer
                Dim n As Integer, r   As Integer                c1x = pic1.ScaleWidth \ 2
                c1y = pic1.ScaleHeight \ 2
                c2x = pic2.ScaleWidth \ 2
                c2y = pic2.ScaleHeight \ 2                If c2x < c2y Then n = c2y Else n = c2x
                n = n - 1
                pic1hDC% = pic1.hDC
                pic2hDC% = pic2.hDC                For p2x = 0 To n
                  For p2y = 0 To n
                    If p2x = 0 Then a = Pi / 2 Else a = Atn(p2y / p2x)
                    r = Sqr(1& * p2x * p2x + 1& * p2y * p2y)
                    p1x = r * Cos(a + theta!)
                    p1y = r * Sin(a + theta!)
                    c0& = GetPixel(pic1hDC%, c1x + p1x, c1y + p1y)
                    c1& = GetPixel(pic1hDC%, c1x - p1x, c1y - p1y)
                    c2& = GetPixel(pic1hDC%, c1x + p1y, c1y - p1x)
                    c3& = GetPixel(pic1hDC%, c1x - p1y, c1y + p1x)
                    If c0& <> -1 Then xret& = SetPixel(pic2hDC%, c2x + p2x, c2y + p2y, c0&)
                    If c1& <> -1 Then xret& = SetPixel(pic2hDC%, c2x - p2x, c2y - p2y, c1&)
                    If c2& <> -1 Then xret& = SetPixel(pic2hDC%, c2x + p2y, c2y - p2x, c2&)
                    If c3& <> -1 Then xret& = SetPixel(pic2hDC%, c2x - p2y, c2y + p2x, c3&)
                  Next
                  t% = DoEvents()
                Next
               End Sub
运行时说retval% = StretchBlt(Picture2.hDC, px%, 0, -px%, py%, Picture1.hDC, 0, 0, px%, py%, SRCCOPY)这条语句溢出，请问是怎么回事