Stereograms and anaglyphs
A stereogram, also called a stereograph,
is a pair of images with each image designed to be viewed by a single
A stereogram can be viewed using a special instrument called a
stereoscope. The stereoscope shows the left image
to the left eye and the right image to the right eye.
A stereogram can also be viewed with the naked eyes. But then,
most people find it easier when the two images are switched.
The result is called a "cross-eyed stereogram"
by opposition to rhe initial configuration called a
"wide-eyed stereogram". In a cross-eyed stereogram
the lines of sight are crossing each other: the left eye looks
at the right image and the right eye at the left iamge. This is
more like normal viewing when the two lines of sight meet at
some focal point.
Viewing a stereogram
The next illustration is an example of a cross-eyed stereogram.
(It was created using ARBeads and ARPix.)
If you are not used to looking at cross-eyed stereograms, you can
- Put your left index on the left image and your right index on
the left image. Both fingers should be vertical and parallel.
- Now, cross your eyes (like when you are looking at the tip of
your nose) until you see the images of 4 fingers instead of
- Then, let it go a bit until the two central finger images
merge together and you see only 3 images. Keep it that way
and look at the central image.
- The final step is to remove your two fingers while keeping
the same eye position. Keep looking at the central image
and adjust focus.
The 3D effect should appear in the central image. If you are still
having some problems, try varying your distance from the screen.
Remark: This method is only valid for cross-eyed stereograms.
Anaglyphs on a white background
In an anaglyph, the two images are drawn on top of one another but
with different colors: one is in red, the other is in cyan (aqua),
all this on a white background.
To look at an anaglyph, one needs a pair of red-blue glasses.
- The red filter usually covers the left eye. Red gets through,
cyan is filtered out. The left eye then sees the cyan lines
of the image as black lines on a light (red) background.
- The blue filter covers the right eye. Cyan gets through, red
is filtered out. The right eye then sees the red lines of the
image as black lines on a light (blue) background.
Hence, on the anaglyph
- the left view of a scene must be drawn in cyan
- the right view must be drawn in red.
Although the right filter is blue, the drawing must be in cyan,
not blue. Otherwise, it would be visible for both eyes. This
(probably) has to do with the nature of the colors and with the
color sensibility of the human retina.
If, when looking at the anaglyph, you see more red than blue
or more blue than red, you may have a lazy eye problem. Try
focussing your attention equally on both eyes so that your
brain perceives equal amounts of red and blue. Once you got
this, the 3D aspect of the image should appear in all its
splendor. Like anything else, the more often we practice,
the easier this gets.
Looking at stereograms and anaglyphs is not only fun. I find
it to be a great eye exercise against lazy eye and strabismus
problems. This is because to see the full 3D effect, both eyes
have to work equally and both need to get the right orientation.
And the feedback is immediate.
Anaglyphs on a black background
Up to now, we have assumed that the anaglyph was on a white
background. If the background is black, we must have
- the left view of a scene drawn in red
- the right view drawn in cyan.
and both views will appear bright on a dark background.
- the left view will only get through the left (red) filter
- the right view will only get through the right (blue) filter
Advantages of stereograms over anaglyphs
Anaglyphs are fun but are a bit limited compared to stereograms.
- Stereograms can contain shades and even colors. Anaglyphs can
only contain red and cyan (together with their superposition).
Otherwise, the same graph elements would be seen by both eyes
and the 3D effect would be lost.
- Good anaglyphs can only contain dots and lines. For large filled
surfaces, the 3-dimensionality is lost.
- Looking at anaglyphs for a long period of time is a bit of a
strain on the color cones of the eyes (although there is no