Applications to Industrial Quality Inspection - 3D Computer Vision: Efficient Methods and Applications - page 258

Fig. 6.14 Three-dimensional surface reconstruction of a steel sheet with a deformation. ( a )Two
shading images and one shadow image with their simulated counterparts. Illumination direction
is as indicated by the arrows .( b ) Albedo map computed according to ( 3.49 ). ( c ) Reconstructed
surface profile, shown with respect to the fitted reference plane. The depth of the deformation
corresponds to 0.36 mm

the iterative update rule equation ( 5.13 ) with the error term ( 3.43 ) for multiple light
sources. As the average inclination of the reconstructed part of the surface with re-
spect to the ground plane amounts to several degrees, a reference plane was fitted
to the reconstructed profile. In Fig. 6.14 c, this reference plane has been subtracted
from the reconstruction result. In the albedo map, variations appear that correspond
to dark and bright spots and lines. Again, intensity changes in the image which are
caused by shading effects have been separated from those due to variations of the
surface albedo.

6.3.3 Inspection Based on SfPR and SfPRD

The surface reconstruction algorithms described in Sect. 5.3 were applied by
d'Angelo and Wöhler ( 2005b , 2006 , 2008 ) to the raw forged iron surface of a con-
nection rod. This section describes their three-dimensional reconstruction results
and compares them to a ground truth cross section of the same surface, measured
with a scanning laser focus profilometer. The second inspected part is a slightly dam-
aged section of the raw surface of a flange also consisting of forged iron. Its surface
shows several small deformations. The depths of these deformations inferred from
the three-dimensional reconstruction results are compared to ground truth values
obtained by tactile measurement. The presentation in this section is adopted from
d'Angelo and Wöhler ( 2008 ).
A convergent stereo setup is utilised, consisting of two CCD cameras of 1032

×
776 pixel image resolution, equipped with lenses of 25 mm focal length. The base-