Showing papers by "José A. Ferrari published in 2010"

Pulse-width modulation in defocused three-dimensional fringe projection.

Abstract: Shape measurements by fringe projection methods require high-quality sinusoidal fringes. We present a sinusoidal fringe generation technique that utilizes slightly defocused binary fringe projection. The proposed method is a spatial version of the well-known pulse-width modulation (PWM) technique of electrical engineering. PWM is easy to implement using off-the-shelf projectors, and it allows us to overcome the gamma problem (i.e., the nonlinear projector response) in the output light intensity. We will demonstrate that, with a small defocusing level--lower than with other techniques proposed in the literature--a high-quality sinusoidal pattern is obtained. Validation experiments using a commercial video projector are presented.

Orientation-selective edge detection and enhancement using the irradiance transport equation

Abstract: We present a method for orientation-selective edge detection and enhancement based on the irradiance transport equation. The proposed technique distinguishes the sign of the derivative of the intensity pattern along an arbitrarily selected direction. The method is based on the capacity of liquid-crystal displays to generate simultaneously a contrast reverted replica of the image displayed on it. When both images (the original one and its replica) are imagined across a slightly defocused plane, one obtains an image with enhanced first derivatives. Unlike most Fourier methods, the proposed technique works well with a low-coherence light source, and it does not require precise alignment. The proposed method does not involve numerical processing, and thus it could be potentially useful for processing large images in real-time applications. Validation experiments are presented.

Nondirectional edge enhancement by contrast-reverted low-pass Fourier filtering

Abstract: We present an image processing method for nondirectional edge extraction/enhancement. The method is based on the capability of twisted-nematic liquid-crystal displays (LCDs) to traduce the image information in changes of the state of polarization of the light, which allows us to generate simultaneously a "positive" and a "negative" (i.e., contrast-reversed) replica of the digital image displayed on the LCD. The negative image is low-pass filtered in a novel polarization-selective 4f optical processor. When the smoothed negative image is imagined together with the original image, an image with nondirectional edge enhancement is obtained. Unlike other Fourier methods presented in the literature, the proposed technique provides a simple way to control the relative amount of high frequencies present in the final image. The proposed method does not involve numerical processing, and, thus, it could be a useful tool for edge extraction/enhancement in large images in real-time applications. Validation experiments are presented.

Analog image contouring using a twisted-nematic liquid-crystal display.

Abstract: We present a novel image contouring method based on the polarization features of the twisted-nematic liquid-crystal displays (TN-LCDs). TN-LCDs are manufactured to work between a crossed polarizer-analyzer pair. When the analyzer is at 45 deg (instead of 90 deg) with respect to the polarizer, one obtains an optically processed image with pronounced outlines (dark contours) at middle intensity, i.e., the borders between illuminated and dark areas are enhanced. The proposed method is quite robust and does not require precise alignment or coherent illumination. Since it does not involve numerical processing, it could be useful for contouring large images in real-time, which presents potential applications in medical and biological imaging. Validation experiments are presented.

Application of DVD/CD pickup optics to microscopy and fringe projection

Abstract: We discuss a low-cost microscope that is constructed using commercial DVD and CD pickups and demonstrate its fine focus capacity and high resolution. Also, we demonstrate a fringe projection interferometer that uses the three-beam positioning system of a CD reader. We show that with a simple rotation of the grating placed inside the pickup head, an electrically controlled fringe-shift system is obtained, which can be used for 3D profilometry. Despite the low cost of these systems, they have a performance comparable to more complex systems.

Edge enhancement by unsharp masking using liquid-crystal displays

Abstract: A new method for real-time edge enhancement is presented. The method is based on the capacity of liquid-crystal displays to generate a contrast-reverted replica of the image displayed on it. When the negative replica is defocused and it is superposed to the original image (displayed in a second liquid-crystal display), one obtains an image with enhanced second derivatives. The proposed technique resembles the classical (photographic) unsharp masking method, but unlike the time-consuming photographic implementation, it can be applied in real-time. The proposed method is potentially interesting for image processing in areas such as microscopy, industrial edge inspection, etc. Validation experiments are presented.

Edge enhancement methods based on derivative operations

Abstract: Two methods are described for edge enhancement using optical derivative operations. The proposed methods are based on the polarization properties of the twisted-nematic liquid-crystal displays (LCDs). LCDs traduce the image information in changes of the state of polarization of the light, which allows us to generate simultaneously a "positive" and a "negative" (i.e., a contrast-reversed) replica of the digital image displayed on the LCD. In both methods negative and positive replicas are at the same time imagined across a plane. In first case when the negative replica has a lateral differential displacement relative to the original one, an image with enhanced first derivatives along a specific direction is obtained. In the case when the negative replica is low-pass filtered, one obtains the Laplacian of the original image. Unlike the usual Fourier (coherent) image processing, the technique proposed here works with incoherent illumination. Validation experiments are presented.