Image quality degradation of object-color metamer mismatching in digital camera color reproduction

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Metamer mismatching is a phenomenon that two objects, which are colorimetrically indistinguishable under one lighting condition, become distinguishable under another one. Due to the unavailability of spectral information, metamer mismatching introduces an inherent uncertainty into cameras' color reproduction. To investigate the degree of image quality degradation by the metamer mismatching, a large spectral reflectance database was collected in this study to search the object-color metamers sets (OCMSs) of the spectra in hyperspectral images. Then metamer-degraded images were constructed and compared with the ground truth images by directional statistics based color similarity index image quality assessment (DSCSI-IQA) metrics to evaluate the perceptual image degradation. The results indicate that the object-color metamer mismatching has only little impact on the image quality degradation, whereas the inappropriate selection of color correction matrices involved with the illumination metamerism is the primary factor for the accuracy decrease in the digital camera color reproduction.


Figure 1. Flowchart of the constructions of object-color metamer set (OCMS) and metamer-degraded images.

Figure 2. The spectral power distributions of 20 test illuminants. Blue solid line: 8 practical illuminants in Table 2. Yellow dashed line: the corresponding daylight series sources. Red dotted line: the corresponding 4-channel LED sources.

Figure 3. Spectral sensitivity functions of 3 test camera models. From left to right: Canon 60D DSLR, Nikon D80 DSLR, and PointGrey Grasshopper 50S5C industrial camera.

Figure 4. Degraded images and the corresponding color difference maps of "Painting" under the test illuminant F8 for Nikon D80 DSLR. (a) The ground truth image. (b) Metamer-degraded image with 0th percentile degree (i.e., root-polynomial color corrected image without the object-color metamer mismatching). (c) With 50th percentile degree. (d) With 95th percentile degree. (e) CAT02 color corrected image. (f) Inappropriate color corrected image by a daylight source-based color correction matrix.

Figure 5. DSCSI-IQA scores with different degrees of metamer mismatching for 3 test camera models.

Figure 6. DSCSI-IQA scores with different degrees of metamer mismatching for 8 practical test illuminants.



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