Qican Zhang

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.

High-speed optical measurement for the drumhead vibration

A high-speed optical measurement for the vibrating drumhead is presented and verified by experiment. A projected sinusoidal fringe pattern on the measured drumhead is dynamically deformed with the membrane vibration and grabbed by a high-speed camera. The shape deformation of the drumhead at each sampling instant can be recovered from this sequence of obtained fringe patterns. The membrane vibration of Chinese drum has been measured with a high speed sampling rate of 1,000 frames/sec. and a standard deviation of 0.075 mm. The restored vibration of the drumhead is also presented in an animation.

An optical measurement of vortex shape at a free surface

We have proposed an optical method of vortex shape measurement based on Fourier transform profilometry (FTP) and verified it by experiment. The results of our experiment proposed in this paper show that FTP can efficiently reconstruct the vortex shape at a free surface and this method is suitable for wide use in studying such problems as liquid shear flow, wake of an object, flow behind a bluff body, and wetting angle.

Optical 3-D shape and deformation measurement of rotating blades using stroboscopic structured illumination

An optical method for shape and deformation measurement of a rotating blade, which is based on Fourier transform profilometry FTP and the stroboscopic effect, is proposed and has been verified by experiments. The integrated principles of the method, the process of 3-D phase unwrapping, and the detailed design of the synchronization con- trol unit are discussed. Experimental results on a commercial electric fan prove that FTP can reconstruct the shape of a rotating blade efficiently and reveal its deformation different times clearly by using stroboscopic structured illumination. This method can be used for studying high-speed motion with rapid periodic signals.

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.

Fourier transform profilometry based on a fringe pattern with two frequency components

A modified Fourier transform profilomtry (FTP) based on a fringe pattern with two frequency components is proposed. We discuss its principle, analyze the maximum measuring range and give an expression to describe the measurable slope of the height variation limitation of this method. The modified FTP provides us another approach to eliminate frequency overlapping. When the spectra distribution of a measured object is not spherical symmetry, we can avoid the frequency aliasing through projecting a fringe pattern with two frequency components, instead of increasing the density of the projected fringe and the resolution of CCD camera. The theoretical analysis and primary experiments verified the method.

Whole-field vibration analysis of a woofer's cone using a high-speed camera

In this paper we presented a method for whole-field three-dimensional (3D) shape measurement and vibration analysis of a vibrating woofers cone based on Fourier transform profilometry (FTP). A sequence of dynamic deformed fringe images can be grabbed by high-speed CCD camera and saved on disk rapidly. By Fourier transform, filtering, inverse Fourier transform and unwrapping these phase maps in 3D phase space, we can obtain the shape of the rapid vibrating woofers cone at different time. The results of our experiment indicate that the method, presented in this paper, can efficiently deal with the surface shape measurement for rapid motion object and will be a promising one with the development of high-speed frame grabber.

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.

A new color structured light coding method for three-dimensional measurement of isolated objects

A new coding method is proposed for measuring the three-dimensional (3D) shape of spatially isolated objects . Based on Gray Code and sinusoidal stripe, this coded fringe pattern whose RGB components comprise the multiplications of three different frames in Gray Code and sinusoidal stripes, appears to be digital color coded sinusoidal fringes. The digital color coded sinusoidal fringe pattern is created by software on a computer and then projected to the tested objects surface by a projector, and the image of the object is captured by a camera positioned at an angle different from that of the projector, then the image is processed, color Gray Codes are used to obtain the phase order and sinusoidal stripes are used to get the wrapped phase, then the wrapped phase can be unwrapped and the 3D surface information can be retrieved. With only one image, 3D shape of the isolated objects can be exactly reconstructed, thus the speed is limited only by the frame rate of camera. The principle of this technique is described and an actual measurement is presented, the result of the reconstructed shapes proves the correctness and feasibility of this new coding method.