Graphics Reference
In-Depth Information
Space
Time
(b)
(a)
Figure 1.5
(a) Binary-coded patterns projection for 3D acquisition, (b)
n
-ary-coded coded patterns
projection for 3D acquisition
view. Correspondences between image points and points of the projected pattern can be easily
found. Finally the decoded points can be triangulated, and depth is recovered. The patterns are
designed so that code words are assigned to a set of pixels.
A code word is assigned to a coded pixel to ensure a direct mapping from the code words to
the corresponding coordinates of the pixel in the pattern. The code words are numbers and they
are mapped in the pattern by using gray levels, color or geometrical representations. Pattern
projection techniques can be classified according to their coding strategy: time-multiplexing,
neighborhood codification, and direct codification. Time-multiplexing consists in projecting
code words as sequence of patterns along time, so the structure of every pattern can be very
simple. In spite of increased complexity, neighborhood codification represents the code words
in a unique pattern. Finally, direct codification defines a code word for every pixel; equal to
the pixel gray level or color.
One of the most commonly exploited strategies is based on temporal coding. In this case,
a set of patterns are successively projected onto the measuring surface. The code word for a
given pixel is usually formed by the sequence of illumination values for that pixel across the
projected patterns. Thus, the codification is called
temporal
because the bits of the code words
are multiplexed in time. This kind of pattern can achieve high accuracy in the measurements.
This is due to two factors: First, because multiple patterns are projected, the code word basis
tends to be small (usually binary) and hence a small set of primitives is used, being easily
distinguishable among each other. Second, a coarse-to-fine paradigm is followed, because the
position of a pixel is encoded more precisely while the patterns are successively projected.
During the three last decades, several techniques based on time-multiplexing have appeared.
These techniques can be classified into three categories: binary codes (Figure 1.5
a
),
n
-ary codes
(Fig. 1.5
b
), and phase-shifting techniques.
Binary codes.
In binary code, only two illumination levels are used. They are coded as
0 and 1. Each pixel of the pattern has its code word formed by the sequence of 0 and
1 corresponding to its value in every projected pattern. A code word is obtained once
the sequence is completed. In practice, illumination source and camera are assumed to be
strongly calibrated and hence only one of both pattern axes is encoded. Consequently, black
and white strips are used to compose patterns - black corresponding to 0 and white 1,
m
patterns encode 2
m
stripes. The maximum number of patterns that can be projected is the
resolution in pixels of the projector device; however, because the camera cannot always
perceive such narrow stripes, reaching this value is not recommended. It should be noticed
that all pixels belonging to a similar stripe in the highest frequency pattern share the same
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