Applications to Industrial Quality Inspection - 3D Computer Vision: Efficient Methods and Applications - page 256

Fig. 6.12 Three-dimensional reconstruction of the lateral surface of a sharpener, based on the
integration of shadow and shading features according to Sect. 5.2 .( a ) The sharpener. ( b ) Images
used for three-dimensional reconstruction. ( c ) Images rendered based on the reconstruction result.
( d ) Three-dimensional surface reconstruction. A surface defect of 0.5 mm depth is marked by an
ellipse

account photometric image information during the reconstruction process under
well-controlled conditions yields three-dimensional reconstruction results that are
at least comparable to those obtained with active scanning devices while requiring
significantly less complex and expensive instrumental configurations.

6.3.1 Inspection Based on Integration of Shadow and Shading
Fe a t u r e s

Wöhler and Hafezi ( 2005 ) regard the three-dimensional reconstruction of the lateral
surface of a sharpener consisting of aluminium. Due to the perpendicular view and
strongly oblique illumination, the specular components in the reflectance function
( 3.14 ) are negligible as cos θ r 1, such that Lambertian reflectance is a reasonable

approximation to the true reflectance behaviour. The result of the combined shadow
and shading approach according to Sect. 5.2 , relying on one shadow and one shad-
ing image, is shown in Fig. 6.12 . The surface displays a regularly striped structure
and a small surface defect, which are well visible in the reconstruction result. The
depth of the surface defect marked by an oval in Fig. 6.12 d of approximately 0 . 5mm
corresponds within 0 . 1 mm to the value derived by tactile measurement. This ex-
ample illustrates that even in the absence of accurate knowledge about the surface
reflectance the combined shadow and shading approach yields a quantitatively fairly
accurate three-dimensional reconstruction result.

6.3.2 Inspection of Surfaces with Non-uniform Albedo

An application in which the three-dimensional reconstruction approach must cope
with a non-uniform surface albedo is the inspection of uncoated steel sheet surfaces.