Although text stereograms do not offer as rich a 3D image as pixel-based stereograms, they continue to enjoy some popularity because they are easy to create without the need for special software. Text stereograms can be created manually (if your penmanship is good), using a typewriter, or by using a simple text editor such as Notepad.

Whether text or pixel based, stereograms are created one line at a time. Each line is in fact a complete stereogram but whereas a line of pixels is too small for the human eye to detect the 3D content, the 3D content of a single line text stereogram can be seen by most people.

Text Stereogram Example
Consider the following line consisting of just the repetitive text "Gary Beene".

    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene 

There is no 3D information in the line above. But by applying the algorithm described below, the following single line stereogram can be created:

    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao

Even though the line above is a complete stereogram, most people can more easily view the 3D content when viewing multiple lines of the stereogram, as in the the following 12-line text stereogram (same line as above, just repeated 12 times):

    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene
    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene
    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Ga Beene Ga Beene Gaor Beene Gaor Beene Gao
    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene 
    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene 
    Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene Gary Beene 

In this example, a rectangle can be seen floating above the background.

Pattern Algorithm
The following source code implements what I call the "pattern" algorithm for creating a text stereogram. I wrote the code based on algorithms descriptions from several Internet sites, as well as the algorithm described in the book "Hidden Images: Making Random Dot Stereograms" by Bob Hankinson and Alfonso Hermida (1994).

In general, the algorithm involves creating a stereogram line by line. Each line consists of a text pattern which is repeated left to right. The repeating pattern is modified in accordance with a depth mask which describes the 3D image content. As image levels increase text characters are dropped from the pattern and as image levels decrease text characters are added to the pattern. Additional discussion is provided following the source code.

 1 Dim Pattern As String, Stereogram As String, PatternWidth as Long
 2 Dim PL As Long, NL As Long, Ptr As Long, i As Long, Stereogram as String
 3 maxX = picOut.ScaleWidth / picOut.TextWidth("a")
 4 maxY = picOut.ScaleHeight / picOut.TextHeight("a")
 5 ReDim hArray(maxX, maxY): CreateShapeDepthArray: PatternWidth = 12
 6 For y = 0 To maxY - 1
 7    Pattern = "": Ptr = 1: PL = 0
 8    For i = 1 To PatternWidth: Pattern = Pattern & Chr(Int(Rnd * 95 + 32)): Next i
 9    Stereogram = Stereogram & Pattern
10    For X = 0 To maxX - 1
11        NL = hArray(X, y)
12        If NL > PL Then
13            For i = 1 To NL - PL
14                Pattern = Left(Pattern, Ptr - 1) & Mid$(Pattern, Ptr + 1)
15                If Ptr > Len(Pattern) Then Ptr = 1
16            Next i
17            PL = NL
18        ElseIf NL < PL Then
19            For i = 1 To PL - NL
20                Pattern = Left(Pattern, Ptr - 1) & Chr(Int(Rnd * 95 + 32)) & Mid$(Pattern, Ptr)
21            Next i
22            PL = NL
23        End If
24        Stereogram = Stereogram & Mid$(Pattern, Ptr, 1)
25        Ptr = Ptr + 1
26        If Ptr > Len(Pattern) Then Ptr = 1
27    Next X
28    Stereogram = Stereogram & vbCrLf
29 Next y
30 picOut.Cls: picOut.Picture = LoadPicture("")
31 picOut.CurrentX = 0: picOut.CurrentY = 0: picOut.Print Stereogram

The following source code implements the same pattern algorithm. It is more cryptic but takes only half the number of lines of code. This code is based on work published by A. Fontana. It uses a very clever technique for handling the pattern modifications.

 1 Dim iLevel As Long, minString As Long, Pattern As String, Diff As Long
 2 maxX = picOut.ScaleWidth / picOut.TextWidth("a")
 3 maxY = picOut.ScaleHeight / picOut.TextHeight("a")
 4 ReDim hArray(maxX, maxY): CreateShapeDepthArray: PatternWidth = 12
 5 For y = 0 To maxY - 1
 6     iLevel = 0: Diff = PatternWidth
 7     For i = 1 To PatternWidth: Pattern = Pattern & Chr(Int(Rnd * 95 + 32)): Next i
 8     For X = 0 To maxX - 1
 9         If hArray(X, y) <> iLevel Then
10             Diff = minString - hArray(X, y)
11             iLevel = hArray(X, y)
12         End If
13         Pattern = Pattern & Left(Right(Pattern, Diff), 1)
14     Next X
15    Stereogram = Stereogram & Pattern & vbCrLf
16 Next y

Source Code Discussion
... in work