|
The invention of digital camera stimulates photogrammetrists to pursue anautomatic system of photogrammetry. In close-range photogrammetry,automatically detecting, identifying and positioning targets are primary goalsto be reached in recent years. Currently proposed methods utilize targets ofregular shape, such as a round-shape target, which can be detected and locatedeasily in a digital image. However, a target like this kind can not beidentified among a number of others in an image. It is therefore difficult tofind the conjugate targets in some overlapped images. In this paper, two roundshape targets and two digits are composed to be a distinguishable target fromothers.By using the proposed image- processing techniques, a compound target isnot only detectable and locatable but also distinguishable by interpreting thetwo- digit subimage to obtain its identification number. There are 4 steps in the proposed method. First, a circular-targetdetector is used to search all possible targets, and locates the centersprecisely by using the least- squares matching technique. Second, the scale ofthe target is calculated by measuring the length between the centers of thetwo round-shape targets, so that the boundaries of the two digits can bedefined. Third, subimages of the digits are extracted and resampled to be thesame orientation and resolution as the provided templates of standard digits.Finally, the subimages of the digits are compared with the templates of thestandard digits to solve the identification number.
Three experiments were conducted to show the feasibility of the proposedwhite color background. All the targets were successfully and correctlyidentified in this experiment. The second experiment is designed to findtargets in different scales, so that several sets of targets attached in thevertical sides of a stairs were imaged and tested. Targets tend to bemisinterpreted when their scales are getting small. The third experiment isdesigned to identify targets in an oblique image. This experiment shows thatcorrect results can be obtained when the image angle is smaller than 30degrees.
|