Grid letters with rectangle OCR - vb.net

Question

I have been trying to grid each letter with rectangle on an image (on that image there is only colors white and black). I've started like this:
Dim bmp As Bitmap = Label1.Image
Dim xmax As Integer
Dim ymax As Integer
xmax = Label1.Width - 1
ymax = Label1.Height - 1
Dim top, bottom, left, right As Integer
Dim first As Boolean = True
Dim last As Boolean = False
Dim first2 As Boolean = True
Dim last2 As Boolean = False
Dim pen As Pen = Pens.Red
For x = 0 To xmax
For y = 0 To ymax
With bmp.GetPixel(x, y)
If .R = 0 And .G = 0 And .B = 0 And first = True Then
left = x
first = False
last = True
ElseIf .R = 0 And .G = 0 And .B = 0 And last = True Then
right = x
End If
End With
Next y
Next x
For y = 0 To ymax
For x = 0 To xmax
With bmp.GetPixel(x, y)
If .R = 0 And .G = 0 And .B = 0 And first2 = True Then
top = y
first2 = False
last2 = True
ElseIf .R = 0 And .G = 0 And .B = 0 And last2 = True Then
bottom = y
End If
End With
Next x
Next y
first = True
first2 = True
last = False
last2 = False
Dim rect As New Rectangle(left, top, right - left, bottom - top)
Dim g As Graphics = Graphics.FromImage(bmp)
g.DrawRectangle(pen, rect)
Label1.Image = bmp
It'll check all of the pixels on the image and it'll get the black ones, I could draw rectangle around only one letter but I couldn't more then one at the time.
Still can't use it for seperate letters.
I'll be thankful for all of your responses.