Is there a faster method to convert bitmap pixels to greyscale?

Question

At the moment, I am using the SetPixel() method to change the colour of every pixel in a bitmap. This works fine on small images with small dimensions, but when I test it on large images it does take a while.

I haven't worked with images in VB.Net before, so I might just be overlooking something obvious. I'm doing this to make a program which converts an image to grey scale. This produces the right result but at a low speed, and during this time the UI freezes, so I'm keen to maximize the speed of conversion.

This is my code at the moment:

Dim tmpImg As New Bitmap(img) '"img" is a reference to the original image 
For x As Integer = 0 To tmpImg.Width - 1
    For y As Integer = 0 To tmpImg.Height - 1
        Dim clr As Byte
        With tmpImg.GetPixel(x, y)
            clr = ConvertToGrey(.R, .G, .B)
        End With
        tmpImg.SetPixel(x, y, Color.FromArgb(clr, clr, clr))
    Next
Next

Private Function ConvertToGrey(ByVal R As Byte, ByVal G As Byte, ByVal B As Byte) As Byte
    Return (0.2126 * R) + (0.7152 * B) + (0.0722 * G)
End Function

Answer 1

Fast is a relative term, but this will convert a 480x270 image to greyscale in 10-12 ms (obviously system dependent) which does not seem unduly long. I'm quite sure it will be faster than SetPixel.

Private Function GrayedImage(orgBMP As Bitmap) As Bitmap

    Dim grayscale As New Imaging.ColorMatrix(New Single()() _
        {New Single() {0.3, 0.3, 0.3, 0, 0},
         New Single() {0.59, 0.59, 0.59, 0, 0},
         New Single() {0.11, 0.11, 0.11, 0, 0},
         New Single() {0, 0, 0, 1, 0},
         New Single() {0, 0, 0, 0, 1}})

    Dim bmpTemp As New Bitmap(orgBMP)
    Dim grayattr As New Imaging.ImageAttributes()
    grayattr.SetColorMatrix(grayscale)

    Using g As Graphics = Graphics.FromImage(bmpTemp)
        g.DrawImage(bmpTemp, New Rectangle(0, 0, bmpTemp.Width, bmpTemp.Height), 
                    0, 0, bmpTemp.Width, bmpTemp.Height, 
                    GraphicsUnit.Pixel, grayattr)
    End Using

    Return bmpTemp
End Function

Values are rounded from .299, .587, .114