' Convert a color image to gray scale.
Private Sub MakeGray(ByVal picColor As PictureBox)
Dim bitmap_info As BITMAPINFO
Dim pixels() As Byte
Dim bytes_per_scanLine As Integer
Dim pad_per_scanLine As Integer
Dim X As Integer
Dim Y As Integer
Dim ave_color As Byte

    ' Prepare the bitmap description.
    With bitmap_info.bmiHeader
        .biSize = 40
        .biWidth = picColor.ScaleWidth
        ' Use negative height to scan top-down.
        .biHeight = -picColor.ScaleHeight
        .biPlanes = 1
        .biBitCount = 32
        .biCompression = BI_RGB
        bytes_per_scanLine = ((((.biWidth * .biBitCount) + _
            31) \ 32) * 4)
        pad_per_scanLine = bytes_per_scanLine - (((.biWidth _
            * .biBitCount) + 7) \ 8)
        .biSizeImage = bytes_per_scanLine * Abs(.biHeight)
    End With

    ' Load the bitmap's data.
    ReDim pixels(1 To 4, 1 To picColor.ScaleWidth, 1 To _
        picColor.ScaleHeight)
    GetDIBits picColor.hdc, picColor.Image, _
        0, picColor.ScaleHeight, pixels(1, 1, 1), _
        bitmap_info, DIB_RGB_COLORS

    ' Modify the pixels.
    For Y = 1 To picColor.ScaleHeight
        For X = 1 To picColor.ScaleWidth
            ave_color = CByte((CInt(pixels(pixR, X, Y)) + _
                pixels(pixG, X, Y) + _
                pixels(pixB, X, Y)) \ 3)
            pixels(pixR, X, Y) = ave_color
            pixels(pixG, X, Y) = ave_color
            pixels(pixB, X, Y) = ave_color
        Next X
    Next Y

    ' Display the result.
    SetDIBits picColor.hdc, picColor.Image, _
        0, picColor.ScaleHeight, pixels(1, 1, 1), _
        bitmap_info, DIB_RGB_COLORS
    picColor.Picture = picColor.Image
End Sub