A Digital System For Surface Reconstruction

A digital photogrammetric system, STEREO, was developed to determine three dimensional coordinates of points of interest (POIs) defined with a grid on a textureless and smooth-surfaced specimen. Two CCD cameras were set up with unknown orientation and recorded digital images of a reference model and a specimen. Points on the model were selected as control or check points for calibrating or assessing the system. A new algorithm for edge-detection called local maximum convolution (LMC) helped extract the POIs from the stereo image pairs. The system then matched the extracted POIs and used a least squares “bundle” adjustment procedure to solve for the camera orientation parameters and the coordinates of the POIs. An experiment with STEREO found that the standard deviation of the residuals at the check points was approximately 24%, 49% and 56% of the pixel size in the X, Y and Z directions, respectively. The average of the absolute values of the residuals at the check points was approximately 19%, 36% and 49% of the pixel size in the X, Y and Z directions, respectively. With the graphical user interface, STEREO demonstrated a high degree of automation and its operation does not require special knowledge of photogrammetry, computers or image processing. A DIGITAL SYSTEM FOR SURFACE RECONSTRUCTION 2 Introduction To meet the needs of mechanics experiments, a linear image strain analysis (LISA) system was developed (Choi et al., 1991). This system measures the deformation of a specimen in two dimensions with high accuracy. Many experiments have been carried out with this system on various paper specimens. But, with LISA, the deformation of the specimen in the out-of-plane direction cannot be measured. A project started in 1991 to develop a system for reconstructing surfaces in three dimensions. Zhou et al. (1992) reported on the early results of this project. Since then, the system, now called STEREO, has been changed and improved substantially. This paper outlines the implementation of STEREO and the results of experiments with this system. STEREO has to meet several requirements imposed by mechanical experiments. First, the system needs to satisfy many experimental conditions and yield results speedily with little operator intervention. This requirement excludes the use of traditional photogrammetry, which often requires excessive time combined with particular skills and equipment. Second, the system should be accurate relative to its hardware limitations. The sensitive recording part of a CCD (charge coupled device) camera is a chip with an array of micro-sensors, each corresponding to a pixel in the output image. SL and SW, a CCD camera’s ability to measure coordinates in the image plane in the length and width directions, respectively, can be calculated: