Digital Photogrammetry for Automatic Close Range Measurement of Textureless and Featureless Objects

Diffraction grating based simultaneous projection of a large number of distinct and circular laser dots was adopted to solve correspondence and accuracy problems during automatic three dimensional measurement of a textureless and featureless object at close range. After rigid placement of the projector over the telescope of a geodimeter, accurate calibration and derivation of a camera model of the virtual active camera (the projector) was done without a control field and using a bundle adjustment procedure. While automatic detection and labelling of the actively placed target points over any given object remained a problem, algorithms were developed to locate and label these target points placed over a nearly flat and textureless surface. A conventional thresholding method was combined with least squares matching (LSM) to provide automatic subpixel image co-ordinate measurement of the target points placed over a textureless object but, considering the real object conditions, semi-automatic image co-ordinate measurement was done by LSM. The system provided an intersection parallax of 0.3 mm during a laboratory test scan of a planar object at 4.5 m distance.