Figure 4. (a) A meshed CAD model of the TP has been placed in the same georeferenced coordinate system as the images. The 3D world coordinates system is shown together with the local camera coordinate system. The x, y and z axis in the orthogonal basis uses the colors blue, red and black respectively. The optical axes of the camera is represented by the z-axis. The direction in which the camera is pointing is given by the green ray. The mapped paint defect/damage pixels are given by the red points near and on the tower. The green points represent the red points projected onto the TP. (b) This figure zooms in on a portion of the figure above. The red and green points are shown together with the blue normals to the surface given by the red points.
The camera in drones are often not of high quality and the sizes of the lenses are small. Distortions can therefore often be seen in drone images. The effect of radial and tangential lens distortion can be added to the equation that maps the 2D pixels paint damage to the 3D model. The dimensionless pixel values dlx and dly used in equation (1) are given by