Aimed to provide a simple method with handy tools for acquiring the layout image of lining surface, this study develops an image mosaic technology which is able to assemble the image by means of photos taken from digital camera. The semi-controlled mosaic method associated with coordinate transformation is utilized to rectify the image distortion inherent from perspective projection. Then the layout image of lining surface can be obtained from successive mosaic of partial overlapping images, which are presented as 2D orthographic-like and are rectified from the photos captured the 3D tunnel lining surface. A series of experimental tunnel tests and field application are carried out to validate the proposed method and to examine its practicability.
To well identify exceptional events on lining structure of tunnels in operation, the layout image of lining surface is essential for inspection task. Conventionally approaches employ advancing scanning instruction to collect the image rapidly. And the exceptional events on lining surface can be made out through the image. An example is illustrated as Fig. 1. However, the equipments needed are costly, and sometimes additional pricey mobilization and demobilization are necessary. These troubles limit their serviceability for frequently preliminary inspection, especially for the application to the districts with lots of short tunnels at distances. The present work, aimed to provide a simple method with portably inexpensive tools for assembling the layout image of lining surface, develops an image mosaic technology. The semi-controlled mosaic method associated with coordinates transformation is used to rectify the image distortion inherent from perspective projection. Each image captured the 3D tunnel lining surface can be spread up into 2D orthographic-like one. Then the layout image of lining surface can be obtained from successive mosaic of the partial overlapping images. An experimental tunnel is built to examine the validity of the proposed method. The images mosaicked from photos taken in various attitudes are compared so as to discuss the effect of camera orientation. Further application is also evaluated by way of a railway tunnel case. And finally, the practicability of the proposed method is discussed.
Fig. 1. A layout image of tunnel lining surface produced by advancing scanning instrument and according exceptional events identified (Courtesy of United Geotech, Inc. ).(available in full paper)
2. IMAGE MOSAIC TECHNOLOGY
To mosaic images taken from camera, the warping, originated from perspective projection which founded the photography, needs to be rectified in advance . The fundamental concept of this study is to rectify the lining surface images by means of relating their 2D image coordinate to a system of 3D global coordinate describing the tunnel .
2.1. Coordinates transformation
The configuration of tunnel can be defined by appropriate equations. For instance, the lining surface of a tunnel with semi-circular cross-section in 3D global coordinate (X, Y, Z), Fig. 2, can be described as (mathematical equation available in full paper)
Where r and L represent the radius and length of the tunnel, respectively. For an arbitrary point CP1 with longitudinal distance Z1 away from the origin of the referred coordinate and making an angle ²1 measured from the horizontal, its coordinates can be given as X1, Y1, Z1) = (r cos ²1, r sin ²1, Z1).