  The stair phase coding patterns have been widely used to determine the fringe
order for phase unwrapping of the wrapped phase in 3D shape measurement.
Although the special coding sequence algorithm can achieve with a large number
of codewords, it needs current codeword and its adjacent codewords to jointly
determine the fringe order. If any codeword of the grouped adjacent codewords
is incorrectly recognized, it will result in false fringe order.Therefore, it
is challenging to significantly increase the number of codewords. When it is
necessary to simultaneously measure more than two isolated objects with large
size differences, if the fringe frequency is high, the number of fringes on the
smaller objects is too few to determine the fringe orders. On the other hand,
if the fringe frequency is low, the image can not contain all isolated objects
at the same time. To solve this problem, we propose an absolute phase
measurement method based on bidirectional coding patterns. The wrapped phase of
the object is obtained by fou step phase shifting patterns, and the fringe
orders is obtained by two bidirectional coded patterns. When coding the
bidirectional patterns, we code two groups of stair phase with different
frequency along horizontal direction, which respectively represent local fringe
order information and partition information, then, we alternately repeated the
two groups of stair phase along the vertical direction to obtain the
bidirectional coding patterns in the whole pattern. Each local fringe order
information and the corresponding partition information in small region jointly
determine the fringe order of each position in the wrapped phase. To verify the
effectiveness of our method, we did simulations and three experiments.
Simulation and experimental results show that our method is effective for
complex surfaces and isolated objects with different sizes.
