How to build Bitmap's Alpha map?

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I made a function that returning Bitmap's alpha channel.

VB

Private Function CreateAlphaMap(ByVal Image As Bitmap) As Byte()
    Dim TempBytes(Image.Width * Image.Height - 1) As Byte

    For W As Int32 = 0 To Image.Width - 1
        For H As Int32 = 0 To Image.Height - 1
            TempBytes(H * Image.Width + W) = Image.GetPixel(W, H).A
        Next
    Next
    Return TempBytes
End Function

But I don't think this way is the best way.
Is there have any better ways to do this work more efficiently?

Posted 9-Dec-13 4:33am

SlaneR

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Sergey Alexandrovich Kryukov · Accepted Answer · 2013-12-09T04:48:00

You are quite right in your concern about efficiency. This way is no-go, but it's easy to fix.

I did not pay attention for the algorithm itself, only to one big problem: the use of GetPixel. This method is prohibitively slow. Instead, you should use lock the bitmap data into an array and work with the whole bitmap data in memory. Use System.Drawing.Bitmap.LockBits:
http://msdn.microsoft.com/en-us/library/5ey6h79d%28v=vs.110%29.aspx[^].

In these two MDSN articles, you will find a simple code sample.

—SA