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