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16.5. COLORFIELD STEREOGRAMS
259
Figure 16.18: A Coloreld Image of Houses
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260
CHAPTER 16. RANDOM DOT STEREOGRAMS
Figure 16.19: A Depth Image
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16.5. COLORFIELD STEREOGRAMS
261
Figure 16.20: The Stereogram from Figures 16.18 and 16.19
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262
CHAPTER 16. RANDOM DOT STEREOGRAMS
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111111111110000000000000000000000000000000
00000000000001111111112222222222222220000000000000000000000
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00000000000000000000002222222222222220000000000000000000000
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Figure 16.21: A Character Depth File
the letters \writecode" as the special repeating pattern (just like the boy and
house above). This has a pattern length of 9, so if printed at 10 characters
per inch, it is easy to view and see the depth.
Figure 16.22 illustrates the dierences in coloreld stereograms. Shorten-
ing a pattern is simple and is the same as in random dot stereograms. The
algorithm removes a letter of the pattern. The change is when lengthening
the pattern. The inserted element cannot be just any character (in earlier
examples an A was inserted into the 0 though 9 pattern). The inserted el-
ement must be from the special pattern (in this case a letter in the pattern
writecode).
Listings 16.3 and 16.4 shows the scstereo and spstereo programs. These
dier fromcstereo and pstereo shown earlier onlyin theroutinesthatlengthen
the pattern and make the nal substitutions. These new programs use a
s
lengthen
patten and aspecial
substitution routine vice the former lengthen
pattern
and random
substitution routines. These new routines ensure that only ele-
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16.5. COLORFIELD STEREOGRAMS
263
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcodewriitcodewriitcodewriitcodewriitco
writecodewritcodewritcdewritcdewritcdodewritcdodewritcdodew
writecodewritcodewritcdewritcdewritcdodewritcdodewritcdodew
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writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritodewritodewritocodewritocodewritocode
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
writecodewritecodewritecodewritecodewritecodewritecodewrite
Figure 16.22: A Character Colored Stereogram
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264
CHAPTER 16. RANDOM DOT STEREOGRAMS
ments from the special pattern are used.
16.6 Conclusions
This chapter has described how stereograms work and shown how to make
your own. There are many ways to experiment with these. Filter the depth
les to blur or round o the edges of objects. Try making depth les with
dierent commercial packages. Use dierent pattern lengths and output de-
vices. This is a fun topic and making stereograms can become more addic-
tive than playing Doom. The cstereo program allows you to make character
stereograms that you can e-mail. Send hidden messages through the mail,
experiment, and have fun.
16.7 Reference
16.1 \Hidden Images: Making Random Dot Stereograms," Bob Hankinson,
Alfonso Hermida, Que Corporation, 1994.
Chapter 17
Steganography: Hiding
Information
17.1 Introduction
Steganography is the art of hidinginformation. It includes techniques to hide
an image, a text le, and even an executable program inside a \cover" image
without distorting the cover image. This paper will discuss the basic ideas of
steganography and show how to hide text on an image via watermarking and
hide an image in an image. Source code will be presented to implement these
techniques. Extensions to these ideas are also available for those interested
in augmenting the code shown. Further information on steganography is
available in [17.1] and related web sites.
17.2 Hidden Writing
The word steganography comes from the Greek and literally means \hidden
writing." People have used steganography through the centuries to hide
messages. The messages are hidden in plain sight, because they are visible
to people who know where to look.
Consider the sentence \Where real interesting technical exchanges can
overcome dull entertainment."
The rst letter of each word spells the message \write code." This is not
hidden well. Better hiding methods use the second or third letter of each
word or the rst letter of the rst word, second letter of the second word,
265
266
CHAPTER 17. STEGANOGRAPHY: HIDING INFORMATION
etc.
Steganography and cryptography are closely related. Cryptography scram-
bles a message to produce something that looks scrambled. The \write code"
example could be scrambled to be \xsjuf dpef" (replace each letter with the
letter that follows it in the alphabet). The scramble sometimes encourages
prying eyes who see it as a challenge to unscramble. Steganography instead
hides a message in a cover message. The result looks like something in-
nocent, so prying eyes often dismiss it. Lawyers and libertarians debate if
steganography is close enough to cryptography to regulate its use. To date,
steganography remains unregulated.
17.3 Watermarking
Awatermark adds information to a document or image by placing a logo or
seal in plain sight. The watermark protects the owner’s rights by showing
ownership. TV broadcasters commonly do this by placing their logo in a
corner of the broadcast picture. A watermark can be hidden in an image.
Hiding the watermark does not change the appearance of the image. This
protects the owner’s rights, without disturbing the image.
Figures 17.1through 17.4 show an example of hiding a watermark. Figure
17.1 shows aboy and Figure 17.2showsa watermark. The watermark is white
words on a black background. It is possible touse more complex watermarks,
but white on black simplies the program.
Figure 17.3 is the result of laying the watermark on top of the boy image.
Avalue of 20 was added to each pixel of the boy image where the watermark
image was white. This example did not hide the watermark.
Figure 17.4 shows the result of hiding the watermark on the boy image.
Avalue of 2 was added to each pixel of the boy image where the watermark
image was white. This small increase is not visible to the casual observer.
It is simple to recover the watermark by subtracting the original boy
image (Figure 17.1) from Figure 17.4.
Listing 17.1 shows the source code that hides a watermark in an image
and recovers it. The rst part of listing 17.1 is the hiding program. After
interpreting the command line, the code ensures the images are the same
size, and allocates two arrays to hold the images. The hiding operation adds
afactor to the image when the watermark image is non-zero. The last few
lines of code write the result to a le and free the memory allocated for the
17.3. WATERMARKING
267
Figure 17.1: The Original Boy Image
Figure 17.2: The Watermark Image
268
CHAPTER 17. STEGANOGRAPHY: HIDING INFORMATION
Figure 17.3: Overlaying the Watermark on the Boy Image
Figure 17.4: Hiding the Watermark on the Boy Image
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