Accurate 3D Measurement of Complex Texture Objects by Height Compensation Using a Dual-Projector Structure.

Fringe projection profilometry is a widely used technique for 3D measurement due to its high accuracy and speed. However, the accuracy significantly decreases when measuring complex texture objects, especially in the junction of different colors. This paper analyzes the causes of errors resulting from complex textures and proposes a height compensation method to revise the error by employing a dual-projector structure. Moreover, the dual-projector is capable of acquiring a pair of errors with opposite signs, which can be utilized to calculate the accurate 3D information after determining the ratio of this pair of errors. Experiments provide significant improvement in measuring complex texture objects, demonstrating the proposed method's ability.

Super-resolution technique for dense 3D reconstruction in fringe projection profilometry.

Fringe projection profilometry (FPP) is one of the most widely used 3D reconstruction techniques. A higher-resolution fringe pattern produces a more detailed and accurate 3D point cloud, which is critical for 3D sensing. However, there is no effective way to achieve FPP super-resolution except by using greater hardware. Therefore, this Letter proposes a dual-dense block super-resolution network (DdBSRN) to extend the fringe resolution and reconstruct a high-definition 3D shape. Especially, a novel dual-dense block structure is designed and embedded into a multi-path structure to fully utilize the local layers and fuse multiple discrete sinusoidal signals. Furthermore, a fully functional DdBSRN can be obtained even when training with a smaller data sample. Experiments demonstrate that the proposed DdBSRN method is stable and robust, and that it outperforms standard interpolation methods in terms of accuracy and 3D details.

Research on a hole filling algorithm of a point cloud based on structure from motion.

A point cloud can be obtained from three-dimensional measurement reconstruction based on fringe projection. However, there are holes in a point cloud due to objects with complicated shapes and the defect of the method. The holes have a profound impact on the subsequent data processing. A fitting approach to fill the holes based on structure from motion (SFM) is proposed in this paper. First, fringe projection with a two-dimensional phase is used to extract the hole boundary. Second, the registration of the SFM point cloud and the fringe projection point cloud is carried out. Then supplementary points are extracted. Third, the holes are filled based on a radial basis function on the point cloud added with the supplementary points. This method has been proven to be robust by experiments, and information of complex surface holes can be restored sufficiently.

Evidence for an impact of melanopsin activation on unique white perception.

Current models of human color vision only consider cone inputs at photopic light levels, yet it is unclear whether the recently discovered melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs) contribute to color perception. Using a lab-made five-primary photostimulator that can independently control the stimulations of rods, cones, and ipRGCs in human retina, we determined the observer's unique white perception, an equilibrium point for signals arising from the opponent mechanisms of color vision, under different levels of melanopsin activation. We found changing melanopsin activation levels shifts the equilibrium point in the chromatic pathways. Our results suggest potential evidence for an impact of melanopsin activation on unique white perception and the existing color vision model for the periphery may need to be revised by incorporating melanopsin signaling.

Multiple-gamma-value based phase error compensation method for phase measuring profilometry.

Three-dimensional measurement based on fringe projection has been widely used. However, the gamma nonlinearity and system nonlinearities usually result in significant phase error. Furthermore, there are various gamma values due to the non-uniform brightness distribution of the projector and nonlinear factors of the system, which makes the problem more complicated. To solve this problem, a sub-area compensation method based on multiple gamma values is proposed. In the beginning, a uniform image is projected on a standard whiteboard with a smooth surface. The obtained image is partitioned by using histogram statistics. Then, different phase error models are established for different regions. Finally, the phase error is compensated according to the regions. By applying this method, the accuracy of the phase algorithm is greatly improved. The method is simple and convenient compared with the existing methods.

A Novel Camera Calibration Method Based on Polar Coordinate.

A novel calibration method based on polar coordinate is proposed. The world coordinates are expressed in the form of polar coordinates, which are converted to world coordinates in the calibration process. In the beginning, the calibration points are obtained in polar coordinates. By transformation between polar coordinates and rectangular coordinates, the points turn into form of rectangular coordinates. Then, the points are matched with the corresponding image coordinates. At last, the parameters are obtained by objective function optimization. By the proposed method, the relationships between objects and cameras are expressed in polar coordinates easily. It is suitable for multi-camera calibration. Cameras can be calibrated with fewer points. The calibration images can be positioned according to the location of cameras. The experiment results demonstrate that the proposed method is an efficient calibration method. By the method, cameras are calibrated conveniently with high accuracy.

Novel 3D measurement system based on speckle and fringe pattern projection.

An efficient three-dimensional shape measurement system is proposed, which is based on the combining projection of single digital speckle pattern and phase-shifting fringe patterns. At the beginning, the initial corresponding point for each pixel is obtained by a novel speckle-phase combination method. The initial information can be calculated by the single speckle pattern in a short time, while the phase information is used to ensure the results. Unlike the conventional methods, it is not necessary to obtain the unwrapped phase, therefore the number of projected patterns is reduced greatly. Then accurate corresponding coordinates are obtained according to the wrapped phase. Three cases are analyzed while adjusting the initial corresponding coordinates locally. Thus accuracy coordinates are obtained without missing or incorrect points. Experiments demonstrate that we can achieve accurate reconstruction results with reduced measurement time by the proposed method.

Stimulus-evoked outer segment changes in rod photoreceptors.

Rod-dominated transient retinal phototropism (TRP) has been recently observed in freshly isolated mouse and frog retinas. Comparative confocal microscopy and optical coherence tomography revealed that the TRP was predominantly elicited from the rod outer segment (OS). However, the biophysical mechanism of rod OS dynamics is still unknown. Mouse and frog retinal slices, which displayed a cross-section of retinal photoreceptors and other functional layers, were used to test the effect of light stimulation on rod OSs. Time-lapse microscopy revealed stimulus-evoked conformational changes of rod OSs. In the center of the stimulated region, the length of the rod OS shrunk, while in the peripheral region, the rod OS swung toward the center region. Our experimental observation and theoretical analysis suggest that the TRP may reflect unbalanced rod disc-shape changes due to localized visible light stimulation.

Biophysical mechanism of transient retinal phototropism in rod photoreceptors.

Oblique light stimulation evoked transient retinal phototropism (TRP) has been recently detected in frog and mouse retinas. High resolution microscopy of freshly isolated retinas indicated that the TRP is predominated by rod photoreceptors. Comparative confocal microscopy and optical coherence tomography (OCT) revealed that the TRP predominantly occurred from the photoreceptor outer segment (OS). However, biophysical mechanism of rod OS change is still unknown. In this study, frog retinal slices, which open a cross section of retinal photoreceptor and other functional layers, were used to test the effect of light stimulation on rod OS. Near infrared light microscopy was employed to monitor photoreceptor changes in retinal slices stimulated by a rectangular-shaped visible light flash. Rapid rod OS length change was observed after the stimulation delivery. The magnitude and direction of the rod OS change varied with the position of the rods within the stimulated area. In the center of stimulated region the length of the rod OS shrunk, while in the peripheral region the rod OS tip swung towards center region in the plane perpendicular to the incident stimulus light. Our experimental result and theoretical analysis suggest that the observed TRP may reflect unbalanced disc-shape change due to localized pigment bleaching. Further investigation is required to understand biochemical mechanism of the observed rod OS kinetics. Better study of the TRP may provide a noninvasive biomarker to enable early detection of age-related macular degeneration (AMD) and other diseases that are known to produce retinal photoreceptor dysfunctions.

Fringe image analysis based on the amplitude modulation method.

A novel phase-analysis method is proposed. To get the fringe order of a fringe image, the amplitude-modulation fringe pattern is carried out, which is combined with the phase-shift method. The primary phase value is obtained by a phase-shift algorithm, and the fringe-order information is encoded in the amplitude-modulation fringe pattern. Different from other methods, the amplitude-modulation fringe identifies the fringe order by the amplitude of the fringe pattern. In an amplitude-modulation fringe pattern, each fringe has its own amplitude; thus, the order information is integrated in one fringe pattern, and the absolute fringe phase can be calculated correctly and quickly with the amplitude-modulation fringe image. The detailed algorithm is given, and the error analysis of this method is also discussed. Experimental results are presented by a full-field shape measurement system where the data has been processed using the proposed algorithm.

Flexible three-dimensional measurement technique based on a digital light processing projector.

A new method of 3D measurement based on a digital light processing (DLP) projector is presented. The projection model of the DLP projector is analyzed, and the relationship between the fringe patterns of the DLP and the fringe strips projected into the 3D space is proposed. Then the 3D shape of the object can be obtained by this relationship. Meanwhile a calibration method for this model is presented. Using this calibration method, parameters of the model can be obtained by a calibration plate, and there is no requirement for the plate to move precisely. This new 3D shape measurement method does not require any restrictions as that in the classical methods. The camera and projector can be put in an arbitrary position, and it is unnecessary to arrange the system layout in parallel, vertical, or other stringent geometry conditions. The experiments show that this method is flexible and is easy to carry out. The system calibration can be finished quickly, and the system is applicable to many shape measurement tasks.