Yuankun Liu

Local stereo matching with adaptive support-weight, rank transform and disparity calibration

In this paper, a new window-based method for stereo matching is proposed. Differing with the existing local approaches, our algorithm divides the matching process into two steps, initial matching and disparity calibration. Initial disparity is first approximated by an adaptive support-weight and a rank transform method, and then a compact disparity calibration approach is designed to refine the initial disparity, so an accurate result can be acquired. The experimental results are evaluated on the Middlebury dataset, showing that our method is better than other local methods on standard stereo benchmarks.

3D shape measurement of the aspheric mirror by advanced phase measuring deflectometry

An advanced Phase Measuring Deflectometry(PMD) is proposed to measure the three dimensional (3D) shape of the aspheric mirror. In the measurement process, a liquid crystal display(LCD)screen displaying sinusoidal fringe patterns and a camera observing the fringe patterns reflected via the tested mirror, are moved along the tested mirror optical axis, respectively. At each movement position, the camera records the fringe patterns of the screen located at two different positions. Using these fringe patterns, every camera pixels can find a corresponding point on the tested mirror and gets its coordinate and slope. By integrating, the 3D shape of the tested mirror can be reconstructed. Compared with the traditional PMD, this method doesn???t need complex calibration and can measure the absolute height of the aspheric mirror which has large range of surface geometries unambiguously. Furthermore, this method also has strong anti-noise ability. Computer simulations and preliminary experiment validate the feasibility of this method.

Testing an aspheric mirror based on phase measuring deflectometry

A method based on basic phase measuring deflectometry is proposed for testing the aspherical mirror. The method uses a reference screen in two different distances from the mirror under test. The sinu- soidal, intensity-modulated patterns generated by the computer are dis- played on the LCD screen, and the camera observes the patterns re- flected off the testing mirror. The observed pattern appears distorted depending on the shape of the mirror. Using the phase-shifting tech- nique, the original ray of every image point and its corresponding de- flected ray can be constructed. Their intersection points and the surface normal are obtained. Then the mirror surface is reconstructed with high accuracy by numerically integrating the surface normals. The proposed method is robust against noise and can test the mirror full field. In this work, the method is introduced, and computer simulation and experi- mental results are shown.

A novel encoded-phase technique for phase measuring profilometry

Three-dimensional (3-D) shape measurement using a novel encoded-phase grating is proposed. The projected sinusoidal fringe patterns are designed with wrapped and encoded phase instead of monotonic and unwrapped phase. Phase values of the projected fringes on the surface are evaluated by phase-shift technique. The absolute phase is then restored with reference to the encoded information, which is extracted from the differential of the wrapped phase. To solve the phase errors at some phase-jump areas, Hilbert transform is employed. By embedding the encoded information in the wrapped phase, there is no extra pattern that needs to be projected. The experimental results identify its feasibility and show the possibility to measure the spatially isolated objects. It will be promising to analyze dynamic objects.

Wavefront measurement based on active deflectometry

In this paper, a new wavefront measurement is proposed, which is based on active deflectormetry and phase-shift technique. The deflections of imaging rays caused by a phase object could be measured accurately with the phase-shift technique and a removable TFT flat panel to display both the horizontal and vertical sinusoidal intensity patterns respectively, and then the wavefront distribution could be calculated. When a phase object is placed between a display and a calibrated CCD camera, the intensity patterns will be distorted. The distortion can be measured, and another different distortion can be got by moving the display. Then the ray deflections can be measured as well as the gradients of phase shift caused by the object. Therefore the wavefront can be reconstructed. Experimental results show the feasibility of this method. Compared with other techniques, this technique is simpler, cheaper and more flexible.

Temporal-spatial encoding binary fringes toward three-dimensional shape measurement without projector nonlinearity

Abstract. Projector nonlinearity is a common problem for digital structured light-based three-dimensional (3-D) shape measurement techniques. A temporal-spatial binary encoding method is presented for the purpose of eluding it. We build a 3-D shape measurement scheme by combining our proposed method with phase measurement profiling. A standard sinusoidal fringe pattern is divided into more than two binary fringe patterns using specially designed temporal and spatial binary encoding rule based on intensity hierarchic quantification, and then are in-focus projected onto the measured object at a time sequence to reconstruct a frame phase-shifting fringe image. On account of the projected binary fringe pattern strictly consisting of zeros and ones, the influence of the projector nonlinearity on the measurement result can be effectively ruled out and simultaneously enables high-quality sinusoidality. In-depth investigations by theoretical analysis and experiments are conducted to demonstrate the performance of this method.

Shape measurement and vibration analysis of moving speaker cone

Surface three-dimensional (3-D) shape information is needed for many fast processes such as structural testing of material, standing waves on loudspeaker cone, etc. Usually measurement is done from limited number of points using electrical sensors or laser distance meters. Fourier Transform Profilometry (FTP) enables fast shape measurement of the whole surface. Method is based on angled sinusoidal fringe pattern projection and image capturing. FTP requires only one image of the deformed fringe pattern to restore the 3-D shape of the measured object, which makes real-time or dynamic data processing possible. In our experiment the method was used for loudspeaker cone distortion measurement in dynamic conditions. For sound quality issues it is important that the whole cone moves in same phase and there are no partial waves. Our imaging resolution was 1280x1024 pixels and frame rate was 200 fps. Using our setup we found unwanted spatial waves in our sample cone.

Precision Displacement Measurement based on Phase Measuring Deflectometry

In the paper we present a new method to measure the movement displacement, which is easy to use, cheap and can reach micro‐resolution as well as large travel range. It is based on Phase Measuring Deflectometry (PMD) technique. The measurement system consists of standard sinusoidal fringe pattern, standard planar mirror and a CCD camera. The standard planar mirror is mounted on a translation object. The CCD camera will capture the standard sinusoidal fringe pattern via the mirror. And the phase distributions will be extracted by Fourier Transformation Technique, and then the displacement will be calculated by the known mapping table between the phase distributions and displacements. Both the computer simulation and experimental data confirm the feasibility of this method. It is promising to use in high‐precision displacement measurement.

Three-dimensional shape measurement of object in water using fringe projection and phase value tracking

In this paper, a practical method using phase tracking algorithm and ray tracing algorithm is proposed for measuring the 3D shape of an underwater object. A sinusoidal fringe is projected through the water to modulate the information of the tested objects 3D shape. Firstly, according to the binocular stereovision principle and regarding the projector as a special camera, the phase tracking algorithm is taken to identify the homologous points in the phase distribution of the deformed fringe captured by the camera and that of the fringe pattern projected by the projector. The relationship between those two coordinate systems will be fixed on. Secondly, the ray tracing algorithm is used to trace the propagation path of each ray and define the 3D coordinates of each point on the tested objects surface. Finally, the whole shape of the tested object can be reconstructed. Without any scanning device, the method proposed in this paper can speed up the measurement process and reduce the cost. An actual measurement for a model and a hand in the water was done, the prefect results of the reconstructed shapes prove this method correctly and feasibility.

Study on bi-color phase measurement deflectometry

Phase Measuring Deflectometry(PMD), which is aimed at testing specular free-form surfaces, has been developed in recent years. Normally, two sets of sinusoidal fringe patterns are needed, i.e. horizontal and vertical fringe patterns. So it will be time-consuming in PMD system while pursing high accuracy. Here we propose a Bi-color PMD system that will produce one frame fringe consisting of two interlaced RGB format base color fringe patterns, i.e. vertical pattern and horizontal pattern. The fringe patterns could be captured by color camera and the absolute phase will be got by phase-shift technique. It could lead to a faster measuring process and make PMD technique more useful.