Applications to Industrial Quality Inspection - 3D Computer Vision: Efficient Methods and Applications - page 248

Fig. 6.5 Example of a high dynamic range intensity image ( left , grey values are displayed loga-
rithmically) and a polarisation angle image ( right , grey value map is scaled in degrees)

Fig. 6.6 Intensity images of the oil cap for pose 1 ( left ) and pose 2 ( right ). Grey values are dis-
played in logarithmic scale

same as in Sect. 5.6 (Baumer industrial CCD camera, 1032 × 776 pixel image size,

f 25 mm, object distance around 0 . 5 m). To increase the signal-to-noise ratio

of the intensity and polarisation data, the images were downscaled to 258

=

194
pixels, corresponding to a lateral resolution of about 0 . 4 mm per pixel. A depth
from defocus analysis was performed based on the full-resolution images acquired
at apertures of f / 8 and f / 2, respectively.
The system performs pose refinement with four initial poses differing by several
degrees in the rotation angles and a few millimetres in translation. The minimisa-
tion run yielding the lowest residual error is adopted as the pose estimation result.
Due to its complex shape, this object cannot be attached to the goniometer table in a
reproduceable manner; thus we determined the ground truth pose in this experiment
based on a stereoscopic bundle adjustment tool which exploits manually established
point correspondences between a calibrated stereo image pair and the CAD model
of the object. As in the first experiment, the goniometer was used to determine the
intensity and polarisation angle reflectance functions R I and R Φ . The light reflected
by the plastic surface of the oil cap is partially polarised by 10-20 %, such that the
polarisation angle can be used in addition to intensity, edges, and depth. The inten-
sity images of the two regarded poses are shown in Fig. 6.6 , illustrating that at some
places, especially near the right image border, the edges are not well defined, such
that the pose estimation algorithm must largely rely on intensity and polarisation in-
formation. The comparison to the ground truth is shown in Table 6.2 , demonstrating

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