Silicon carbide mirrors are sought for a variety of aerospace applications. While optical polishing techniques are straight
forward for flat and spherical surfaces, material removal rates for this hard, brittle material are too low for affordable
processing of conventionally machined, ground, or as-produced surfaces. The problem is more severe for aspheres.
This paper reports on the use of picosecond pulsed laser ablation, combined with iterative metrology, to shape the SiC in
a manner that will reduce cost and lead time for mirror fabrication. The goal is to exploit relatively gentle, non-thermal
ablation to produce arbitrary surface shapes in SiC that are damage free and that minimize subsequent polishing time.
To apply the technology, detailed data must be developed to characterize laser-material interaction, the threshold for
ablation, and the dependence of the effective "tool shape" on laser operating parameters and firing patterns. An
algorithm can then be developed to calculate optimum laser guidance and firing commands for removal of the required
amount of material from the ceramic surface, with reference to metrology data previously collected on the mirror blank.
Recent results of machining quality, material removal rates, residual surface roughness, and suitability of surface for
subsequent polishing are reported for various types of SiC and paradigms of laser micromachining.
When an image of a 3-D scene is captured, only scene parts at the focus plane appear sharp. Scene parts in front of or
behind the focus plane appear blurred. In order to create an image where all scene parts appear sharp, it is necessary to
capture images of the scene at different focus levels and fuse the images. In this paper, first registration of multifocus
images is discussed and then an algorithm to fuse the registered images is described. The algorithm divides the image
domain into uniform blocks and for each block identifies the image with the highest contrast. The images selected in
this manner are then locally blended to create an image that has overall maximum contrast. Examples demonstrating
registration and fusion of multifocus images are given and discussed.
A method for detection, quantification, and visualization of brain shift in serial MR and CT images is presented. The method consists of three steps. It first establishes correspondence between a number of point landmarks in the images. It then uses the correspondences to determine a transformation function that warps one image to the geometry of the other. It finally uses the obtained transformation to create a vector flow that represents the local motion or deformation of one image with respect to the other. The method does not require the solution of a system of equations and, therefore, is especially effective when a large number of correspondences is needed to represent complex brain deformations.
An energy minimizing snake algorithm that runs over a grid is designed and used to reconstruct high resolution 3D human faces from pairs of stereo images. The accuracy of reconstructed 3D data from stereo depends highly on how well stereo correspondences are established during the feature matching step. Establishing stereo correspondences on human faces is often ill posed and hard to achieve because of uniform texture, slow changes in depth, occlusion, and lack of gradient. We designed an energy minimizing algorithm that accurately finds correspondences on face images despite the aforementioned characteristics. The algorithm helps establish stereo correspondences unambiguously by applying a coarse-to-fine energy minimizing snake in grid format and yields a high resolution reconstruction at nearly every point of the image. Initially, the grid is stabilized using matches at a few selected high confidence edge points. The grid then gradually and consistently spreads over the low gradient regions of the image to reveal the accurate depths of object points. The grid applies its internal energy to approximate mismatches in occluded and noisy regions and to maintain smoothness of the reconstructed surfaces. The grid works in such a way that with every increment in reconstruction resolution, less time is required to establish correspondences. The snake used the curvature of the grid and gradient of image regions to automatically select its energy parameters and approximate the unmatched points using matched points from previous iterations, which also accelerates the overall matching process. The algorithm has been applied for the reconstruction of 3D human faces, and experimental results demonstrate the effectiveness and accuracy of the reconstruction.
The 3-D regions obtained as a result of a volume image segmentation often need to be geometrically modeled and rendered. In this paper, use of rational Gaussian (RaG) surfaces in modeling and rendering 3-D regions is described. A new parametrization technique is introduced that morphs a sphere in a coarse-to-fine fashion to a 3-D region. Knowing parameters of points on the sphere, parameters of voxels on the region are determined. Having the parameters of the voxels, a RaG surface is then fitted to the voxels to obtain a smooth representation for the region. Examples of the proposed representation on various 3-D regions are presented.
A template-matching approach to registration of volumetric images is described. The process automatically selects about a dozen highly detailed and unique templates (cubic or spherical subvolumes) from the target volume and locates the templates in the reference volume. The centroids of the 'best' four correspondences are then used to determine the transformation matrix that resamples the target volume to overlay the reference volume. Different similarity measures used in template matching are discussed and preliminary results are presented. The proposed registration method produces a median error of 2.8 mm when registering Venderbilt image data sets, with average registration time of 2.5 minutes on a 400 MHz PC.
Interactive tools for segmenting 2-D and 3-D images are presented. These tools allow a user to quickly revise a segmentation result obtained from an automatic method. A thresholding technique is described that finds a unique threshold value for each homogeneous region in an image. The threshold value is found such that variance in the region is minimized under change in the threshold value. Curve- and surface-fitting methods are described that can accurately represent a region boundary in 2-D or 3-D with a parametric curve or a surface, respectively. A curve or a surface is optimized to minimize the number of control points representing a region with a prescribed accuracy. The optimized curve or surface is then revised by moving its control points interactively. Once a curve or a surface is found to accurately enclose a region of interest, it is quantized to produce the final 2-D region contour or 3-D region surface. These interactive tools can be used to revise unsatisfactory results obtained from any automatic segmentation method.
Given scattered measurements from a surface, a method for reconstruction of the surface using rational Gaussian surfaces is described. Rational Gaussian surfaces are globally defined parametric surfaces that adapt well to local data. Examples demonstrating reconstruction of 3- D scenes from scattered depth measurements obtained from stereo disparity, reconstruction of 3-D shapes from noisy range data, and segmentation of volumetric images for the extraction of generalized cylinders are described.
This course covers the basics of image registration and image fusion. It first details the steps involved in image registration, including feature selection, feature correspondence, image warping, and image resampling. It then covers applications of image registration in image fusion, change detection, and object recognition. Specific applications of image registration in medicine, remote sensing, industry, and the military are also discussed. Students attending this course become familiar with algorithms that register various types of images and learn applications of the algorithms in various disciplines. The course includes handouts prepared by the instructor.