Usability of Digital Cameras for Verifying Physically Based ...

Figure 27: The sensor plane marking on the camera was used as camera reference
point.
point on the camera (see figure 27). Based on the results of the tests we made
with the light cabinet (see section 6.2), we chose the values seen in figure 28 for
camera and chart positions. The size of the GretagMacbeth ColorChecker SG is
21.59 × 27.94cm. The camera is aiming at the center of the color chart.
A color chart should usually be measured under either a (45/0)-condition or
(0/45)-condition [WS82], i.e. either illuminated at an angle of 45 ± 5 ◦ and viewed
at 0 ± 5 ◦ or vice versa with respect to the center of the color chart. In our setup,
we are using a (0/45)-condition. We observed that the glossy reflection is still
very high at 45 ◦ , though. Therefore we are using an angle of 48 ◦ – which is still
within ±5 ◦ – because the glossy reflection decreases noticeably with increasing
angle (see figure 29).
6.1.2 Representative RGB Values
Figure 30 (a) shows a photograph of the ColorCheckerSG after correcting for dis-
tortion. As some of the methods described in section 7 are based on comparing the
RGB values of the patches to measured values manually, we have to extract RGB
values for all of the patches out of this photograph. Taking the RGB value of only
one random pixel of a patch is too imprecise, as we could accidently pick a dead
pixel (a defective pixel that remains unlit), a hot pixel (a pixel that stays on a solid
color, also known as stuck pixel) or a non-representative pixel (e.g. the maximum
value of all pixels in this patch). The first two cases are well known problems in
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