Game Development Reference
In-Depth Information
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Two dimensions
: Contrary to one-dimensional barcodes, which should be
placed either horizontally or vertically in order to be read properly, AR mark-
ers can be perceived at any angle. Thus, they should be two-dimensional.
This is why in most cases, the markers have square placeholders.
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Lack of rotational symmetry
: This property is pretty obvious. For the vision
system to easily determine where the horizontal and vertical parts of a mark-
er are, there should be no hint of rotational symmetry. After being rotated to
90 degrees, the angle of the marker should not be equal to that of its initial
state. This is why a plain square cannot be a fiducial marker; it must have
some asymmetric elements inside.
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Contrast with environment
: Markers should be significantly different from
the ambiance around them so the image recognition algorithm won't be con-
fused by other patterns or textures. Markers should be like the help symbol
in a desert: a triangle with straight lines, which looks very unnatural against
the space around it so it cannot be missed by someone looking down at it.
As a rule, a contrast-continuous border is required around a marker.
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Achromatic
: In most cases, this is not about the chromatic colors. Symbols
must be made of black-and-white elements (bi-tonal); image scanning al-
gorithms usually transform images turning any shades into their black-and-
white representations. Such design also provides very high visual contrast,
guaranteeing that the marker will be perceived even in poor lighting condi-
tions.
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Low density of elements
: Despite the fact that cameras in modern hand-
held devices have a high resolution, patterns inside markers should not in-
clude very small elements. Because of bad lighting, distance, distorted per-
spective, and other factors, the image-recognition system may lose these
patterns.
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Diversity
: The design should include the opportunity to generate severalver-
sions of markers, so the patterns or images inside must be changeable. That
will help us use only a few fiducials for a single scene.
Oneofthemajordifficultieswithfiducialmarkersisthelackofstrongstandardization.
There is no single format to create geometrical shapes and data coding; each de-
veloper of AR platforms tries to invent something unique. Maybe this is quite natural
because the technology is pretty young and not that popular yet, so they are only try-
ing to find the right direction. AR technology also gives you some freedom to choose
the marker you think is the most effective for your task. Among the popular designs
that can be highlighted are as follows:
