Black and white picture

[aamir55]

hi
i have written program to trun a picture in black and white
but problem with this code is that it is very slow specialy for high resolution picture
i have a written a function which gives black and white
i m running a loop through image height and width and setting each pixel and also getting each pixel
i think that is problem why it is slow
when i run my program , i have to wait for minutes and quality of image is not so good also
code for black and white function

Public Function PureBW(ByVal image As System.Drawing.Bitmap, Optional ByVal Mode As BWMode = BWMode.By_Lightness, Optional ByVal tolerance As Single = 0) As System.Drawing.Bitmap
        image = Me.PictureBox1.Image
        Dim x As Integer

        Dim y As Integer

        If tolerance > 1 Or tolerance < -1 Then

            Throw New ArgumentOutOfRangeException

            Exit Function

        End If

        For x = 0 To image.Width - 1 Step 1

            For y = 0 To image.Height - 1 Step 1

                Dim clr As Color = image.GetPixel(x, y)

                If Mode = BWMode.By_RGB_Value Then

                    If (CInt(clr.R) + CInt(clr.G) + CInt(clr.B)) > 383 - (tolerance * 383) Then

                        image.SetPixel(x, y, Color.White)

                    Else

                        image.SetPixel(x, y, Color.Black)

                    End If

                Else

                    If clr.GetBrightness > 0.5 - (tolerance / 2) Then

                        image.SetPixel(x, y, Color.White)

                    Else

                        image.SetPixel(x, y, Color.Black)

                    End If

                End If

            Next

        Next

        Return image

    End Function

i want it to change, i want that it should work quickly
what changes this need and any alternative method available to do that??

i have an other function for inverting the picture
it is very fast, here i m using a matrix

Public Function InvertImage(ByVal img As Image) As Boolean
        Dim cm As ColorMatrix = New ColorMatrix(New Single()() _
                                   {New Single() {-1, 0, 0, 0, 0}, _
                                    New Single() {0, -1, 0, 0, 0}, _
                                    New Single() {0, 0, -1, 0, 0}, _
                                    New Single() {0, 0, 0, 1, 0}, _
                                    New Single() {1, 1, 1, 1, 1}})
        Return DrawImage(img, cm)
    End Function

can i get black and white image from above function,if yes then how??
what changes i need in above function to get desire result?
thanks