这篇文章谈谈图像的二值化，也就是将彩色图像转为黑白图像。

步骤是：

彩色图像的灰度化，根据灰度和阈值来确定颜色是黑色还是白色。

通常情况下将阈值设置为128：

    '黑白1
    Private Sub btn2Color_Click(sender As Object,e As EventArgs) Handles btn2Color1.Click
        Dim pSourceColor As Color
        Dim pDestColor As Color
 
        Dim destImg As New Bitmap(sourceImg.Width,sourceImg.Height)
        Dim R,G,B As Integer
        Dim AvgColor As Integer
        For i As Integer = 0 To sourceImg.Width - 1
            For j As Integer = 0 To sourceImg.Height - 1
                pSourceColor = sourceImg.GetPixel(i,j)
                R = pSourceColor.R
                G = pSourceColor.G
                B = pSourceColor.B
                AvgColor = (R + G + B) / 3
                If AvgColor >= 128 Then AvgColor = 255 Else AvgColor = 0
                pDestColor = Color.FromArgb(AvgColor,AvgColor,AvgColor)
                destImg.SetPixel(i,j,pDestColor)
            Next
        Next
        picDest.Image = destImg
    End Sub

处理后的图像如下：

可以看出选择128作为阈值实际并不是很好，网上有大量的求阈值的方法，比如大律法等，下面我采用的是一种比较简单的算法：

    Private Sub btn2Color2_Click(sender As Object,e As EventArgs) Handles btn2Color2.Click
        Dim pSourceColor As Color
        Dim pDestColor As Color
 
        Dim destImg As New Bitmap(sourceImg.Width,B As Integer
        Dim HistGram(255) As Integer
 
        For i As Integer = 0 To sourceImg.Width - 1
            For j As Integer = 0 To sourceImg.Height - 1
                pSourceColor = sourceImg.GetPixel(i,j)
                HistGram(pSourceColor.R) += 1
            Next
        Next
 
        Dim threshold As Integer
 
        Dim allSum,allCount As Integer
        For k As Integer = 0 To 255
            allCount += HistGram(k)
            allSum += k * HistGram(k)
        Next
        threshold = allSum / allCount
 
        For i As Integer = 0 To sourceImg.Width - 1
            For j As Integer = 0 To sourceImg.Height - 1
                pSourceColor = sourceImg.GetPixel(i,j)
                R = pSourceColor.R
 
                If R >= threshold Then R = 255 Else R = 0
                pDestColor = Color.FromArgb(R,R,R)
                destImg.SetPixel(i,pDestColor)
            Next
        Next
 
        picDest.Image = destImg
 
    End Sub

运行如下：

由于本人数学不是很好，更多的算法以后有空研究后再发出来，大家也可以网上搜索一下相应的算法。

由于.net平台下C#和vb.NET很相似，本文也可以为C#爱好者提供参考。

学习更多vb.net知识，请参看 vb.net 教程 目录