Design and Detection Algorithm of White-Light Markers in Close-Range Photogrammetry

In the process of close-range photogrammetry, it is hard to stick markers directly on the surface of the object to be measured at times. Projecting optical markers is a good solution. However, there are some problems for the existing optical markers such as difficulty in automatic matching and splicing. Therefore, this paper proposes the design and detection algorithm of white-light markers. White-light marker is composed of collar coded targets and circular non-coded targets. Collar coded targets are designed in the middle of the white-light marker, which can enhance the brightness of the coded strip and improve the recognition accuracy. Circular non-coded targets are designed around the collar coded targets, which can realize matching of non-coded targets among multiple images. Based on Canny operator, feature edges are extracted from the images. According to the criterions of scale, form, etc., the edges of markers are screened out. Markers containing coded strip is decoded to determine the identity. Center of gravity method is used to make the center position of white-light makers come true. The designed white-light marker is applied to the deformation detection experiment of crane girder. Its deformation curve is basically similar with the curve of sticking markers, which verifies the feasibility of the deformation detection of crane girder based on white-light markers.