Measuring Centers of Special Targets in Digital Still Images and Movie Frames: Approach and Evaluation

Digital cameras are undergoing explosive developments. As a result, high-resolution digital cameras have become very affordable, especially with the latest growth in smart phone technologies. Hence, the usage of existing measuring technologies and the development of new measuring technologies utilizing digital cameras is in high demand. This paper utilizes special black and white patterns, the so-called targets, to measure the relative displacement between two points. The paper introduces a new approach in measuring the location of their centers that is based on best fitting the transition zones to straight lines. The intersection of these lines produces the location of the center with sub-pixel accuracy. A special experimental rig was designed and built to evaluate the approach and compare the measurements to those obtained by a conventional position transducer connected to a data acquisition system. In the first part of the paper, the accuracy of the position transducer and the experimental setup is discussed. It is shown that the accuracy of the setup is much greater than the measurement expected from the digital images. Based on this, it was used as a reference system to evaluate the approach. In the second half of the paper, the approach is evaluated based on monitoring two targets. One of them is fixed and serves as a reference point, whereas another one is floating. The latter can move along the linear bearing system axis, which is orthogonal to the axis of the camera lens. The displacements of the floating target in respect to the fixed target were measured by a position transducer connected to a data acquisition system. The relative displacement of the floating target is captured by the digital camera and is based on the current location of the floating and fixed targets. This paper shows adequate accuracy of the approach and provides recommendations on the ways of keeping it at high accuracy for practical applications in experimental earthquake engineering.