- freely available
Sensors 2016, 16(10), 1740; https://doi.org/10.3390/s16101740
2. Sensor Overview
- The image acquisition procedure uses specialized hardware (comprised of a laser projector and a high-speed camera) to project a structured light pattern onto the target scene with the goal of capturing an image of the distorted pattern (i.e., a spatial distortion map). The procedure is based on the recently-introduced concept of modulated pattern projection [31,34], which ensures that spatial distortion maps of good quality can be captured in challenging conditions; for example, in the presence of strong incident sunlight or under mutual interference caused by other similar sensors directed at the same scene.
- The light plane-labeling procedure establishes the correspondence between all parts of the projected light pattern and the detected pattern that has been distorted due to the interaction with the target scene. The procedure uses loopy-belief-propagation inference over probabilistic graphical models (PGMs) as proposed in  to solve the correspondence problem and, differently from other existing techniques in the literature, exploits spatial relationships between parts of the projected pattern, as well as temporal information from several consecutive frames to establish correspondence.
- The 3D reconstruction procedure reconstructs the depth image of the target scene based on (i) the reference frames of the light pattern projected onto a planar surface at different distances from the camera and (ii) the established correspondence between parts of the projected pattern and the detected distortion map.
3. The Acquisition Procedure
- Noise suppression: The modulated acquisition procedure is robust for various types of noise. If information related to the visual appearance of the target scene is treated as “background noise,” then the procedure presented obviously removes the background noise as long as the control/modulation sequence employed in the FPGA register is balanced (a balanced modulation sequence is defined as a sequence with an equal number of zero- and one-valued bits). Because demodulation is a pixel-wise operation, the acquisition procedure presented also suppresses sensor noise (typically assumed to be Gaussian) caused, for instance, by poor illumination or high temperatures, where a simple pair-wise sub-frame subtraction would not suffice.
- Operation under exposure to incident sunlight: Even if the illumination of the target scene by incident sunlight is relatively strong, the modulation sequence is capable of raising the level of “signal” pixels sufficiently to recover a good-quality image of the projected pattern. This characteristic is related to the noise suppression property discussed above, because incident sunlight behaves very much like background noise under the assumption that the intensity level of the sunlight is reasonably stable.
- Mutual interference compensation: With the modulated acquisition procedure, it is possible to compensate for the mutual interference typically encountered when two or more similar sensors operate on the same target scene. This can be done by constructing the control/modulation sequences based on cyclic orthogonal (Walsh–Hadamard) codes, in which the cross-correlation properties of the modulation codes are exploited to compensate the mutual interference (see  for more information). Similar concepts are used in other areas, as well; for example, for synchronized CDMA (code division multiple access) systems  or sensor networks , for which mutual interference also represents a major problem.
4. Light-Plane Labeling
4.1. Problem Statement
4.2. Labeling with Graphical Models
4.2.1. Graph Construction
4.2.2. Factor Definition
|Algorithm 1 Calculating prior factors|
|1:||for all pixel-segments (i.e., random variables ) in the image I do|
|2:||Init: Initialize as an M-dimensional vector of all zeros|
|3:||Result: Normalized distribution (prior factor)|
|4:||for all x-coordinates of the pixel-segment corresponding to do|
|5:||▹ find (at most) M biggest line fragments in I having a pixel segment at the current x-coordinate|
|6:||▹ record the position, k, of the pixel-segment (corresponding to ) among the found m line fragments counting from the bottom of image I up|
|7:||if the number of found line fragments m equals M then|
|8:||▹ increase the k-th element of by some positive constant q|
|10:||▹ increase all elements of from position k to by some positive constant q|
|13:||▹ normalize the vector to unit norm;|
5. Depth Image Reconstruction
6.1. Characteristics of the Acquisition Procedure
6.2. Characteristics of the Light-Plane Labeling Technique
- The naive labeling approach (NLA), which assigns light plane labels to the detected non-zero pixels in a consecutive manner. The first non-zero pixel at the given x-coordinate (looking from the bottom of the image up) is assigned the label 1; the second detected non-zero pixel at the given x-coordinate is assigned the label 2, and so on; until all 11 labels have been assigned.
- The labeling approach based on prior information (PR), which assigns light plane labels to the detected non-zero pixels by constructing a PGM based on prior factors only. This approach represents a refined version of the naive labeling technique introduced above.
- The reference approach from Ulusoy et al. (RUL) , which also exploits probabilistic graphical models, but relies only on spatial information to assign light plane labels to the detected non-zero pixels in the distortion map.
6.3. Constructing Depth Maps: 3D Reconstruction
7. Conclusions and Future Work
Conflicts of Interest
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|Method||Outdoor Dataset (Noisy)|
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