Usability of Digital Cameras for Verifying Physically Based ...

Figure 26: We can define a mapping from spectral values to RGB values.
In the first case, we use XYZ values because they take the human observer
into account. To get the actual difference between image and real scene we con-
vert both the rendering result and the photograph first to XYZ color space and then
to L∗a∗b∗ color space and calculate the color difference ∆E∗ ab for each pixel. If
is within a certain error range we can consider two colors as visually equal.
∆E ∗ ab
However, doing those conversions brings up a severe problem: the different spec-
tral sensitivities of a human observer (XYZ space) and a (non-colorimetric, i.e.
common) digital camera. For the rendering, the original spectrum is multiplied
with the spectra of the color matching functions. For the photograph, the original
spectrum is multiplied with the spectra of the CCD sensors, which are different
from the spectra of the color matching functions. Two different original spectra
may cause the same XYZ value but different RGB values. Therefore, two colors
that are a metameric pair to a human observer will in general look different to
a digital camera. Thus, in this case a certain amount of error is introduced by
converting to XYZ space.
In the second case, we do not convert to XYZ space but to the camera’s RAW
RGB space directly. Therefore, we have to define a mapping from spectral values
to the RAW RGB values of the camera (see figure 26). For details on the mapping
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